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日本鉄鋼協会より俵論文賞を受賞

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

We investigated the stability of oxide nano particles in oxide dispersion-strengthened (ODS) steel, which is a promising candidate material for next-generation reactors, under neutron irradiation at high temperature to high doses. MA957, a 14Cr-ODS steel, was irradiated with Joyo in Japan Atomic Energy Agency under irradiation conditions of 130 dpa at 502 ºC, 154 dpa at 589 ºC, and 158 dpa at 709 ºC. Three-dimensional atom probe (3D-AP) and transmission electron microscope (TEM) observation were performed to characterize the oxide particles in the ODS steels. A high number density of Y-Ti-O particle was observed in the unirradiated and irradiated samples. Almost no change in the morphology of the oxide particles, i.e. average diameter, number density, and chemical composition, has been observed in the samples irradiated to 130 dpa at 502 ºC and to 154 dpa at 589 ºC. A slight decrease in number density was observed in the sample irradiated to 158 dpa at 709 ºC. The hardness of any of the irradiated samples was almost unchanged from that of the unirradiated sample. It was revealed that the oxide particles existed stable, and the strength of the material was sufficiently maintained even after being neutron irradiated to high dose of ∼160 dpa at high temperature up to 700 ºC.

DOI： 10.1016/j.jnucmat.2024.155252

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

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In boundary lubrication, the detachment of lubricant molecules from a solid surface is likely to occur due to the presence of high compressive normal stress combined with shear stress exerted on the solid–liquid interface. This phenomenon often results in a delamination behavior at the interface. We aim to investigate the nanoscale roughness effect on the oil film delamination from sliding iron surfaces with grain boundaries by coarse-grained molecular dynamics simulations. As a result, the oil film delamination was effectively suppressed in higher roughness. Two distinct mechanisms of delamination were found depending on surface roughness when the critical normal stress is exceeded. High roughness enhanced the ability to prevent complete slip but had negligible influence on partial slip.

Graphical Abstract

DOI： 10.1007/s11249-024-01872-2

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Other Link： https://link.springer.com/article/10.1007/s11249-024-01872-2/fulltext.html

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of Tribology  

A series of coarse-grained molecular dynamics simulations were conducted to investigate the temporal evolution of frictional behaviors of lubricants between sliding nanostructured iron surfaces. Grain boundary atoms were given a stronger interaction with the lubricant molecules. We varied the surface distance and interaction strength between grain boundary atoms and lubricants. It was found that, below the critical compressive stress, the oil film detached from the surface at first and then attached to it after several nanoseconds due to the localized molecular rearrangement within lubricants and the enhancement of interaction between lubricants and iron surfaces. The transition times required for oil film formation and de-lamination both increased as it approached the critical compressive stress. Larger interaction strength increased the delamination time but barely affected the formation time.

DOI： 10.1115/1.4064860

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of Alloys and Compounds  

The addition of Mn in Al–Fe alloys stabilizes the Al6Fe metastable phase (orthorhombic structure, oC28) owing to the partial occupation of Fe sublattice sites by Mn atoms (formation of Al6(Fe, Mn) phase), thereby achieving superior high-temperature strength of Al–Fe–Mn ternary alloys that are manufactured via laser powder bed fusion (L-PBF) process. This study aimed to fundamentally understand the thermal stability of the refined Al6(Fe, Mn) phase formed in the L-PBF fabricated Al–2.5%Fe–2%Mn alloy in terms of a thermodynamic approach and the kinetics of morphological changes in the Al6(Fe, Mn) phase at elevated temperatures ranging from 200 to 500 ℃. The L-PBF processed samples showed a high hardness of approximately 125 HV due to the formation of numerous dispersoids of the Al6(Fe, Mn) phase with sizes of several tens of nanometers in the α-Al matrix containing concentrated solute elements of Fe and Mn. The hardness and microstructure were almost unchanged even after exposure to a high temperature of 200 ℃ for an extended period of 1000 h. Upon exposure to 300 ℃, nucleation and growth of the Al6(Fe, Mn) phase occurred locally, particularly at grain boundaries in the α-Al matrix. Such a local growth contributed to a reduction in hardness upon thermal exposure. Simultaneously, numerous nanoscale precipitates were formed in the α-Al matrix, resulting in a suppressed reduction in hardness. Contrary to the result of the thermodynamic calculations, the θ-Al13Fe4 phase was scarcely found even after long-term exposure to 500 ℃. The θ-phase formation was accompanied by the dissolution of the refined Al6Fe metastable phase for strengthening, which significantly reduced the hardness during thermal exposure. The suppressed θ-phase formation could be associated with the thermodynamically stable Al6(Fe, Mn) phase developed during the L-PBF process, which predominantly contributed to the high microstructural stability of the L-PBF fabricated Al–Fe–Mn alloy at elevated temperatures.

DOI： 10.1016/j.jallcom.2024.174593

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Publishing type：Research paper (scientific journal)   Publisher：Materials Science and Engineering: A  

The upper limits for Sb, As, and Sn in Gr. 91 steel are set by the recent standardized Gr. 91 Type 2 specification. To clarify how the three impurities impact the microstructures and creep rupture strength of Gr. 91 welded joints, two types of steel with varying impurity concentrations were prepared. The first contained high impurity concentrations (Steel 1), while the other had low impurity concentrations (Steel 2). The results of the creep tests on the welded joints showed that Steel 1 had lower creep rupture strength compared to Steel 2, indicating premature failure caused by the impurities. Comparing the microstructures in the as-welded joints, it was observed that Steel 1 had more M23C6 particles in the fine-grained heat affected zone (FGHAZ) than Steel 2. This finding indicates that M23C6 dissolution during the welding process was hindered in Steel 1. It is believed that M23C6 dissolution was reduced due to the segregation of impurities at the grain boundaries or the interface between the matrix and M23C6 particles. Consequently, the microstructure in FGHAZ of Steel 1 showed lower creep resistance because the pinning force by M23C6 was decreased due to the reduction of re-precipitation on the grain boundaries during post weld heat treatment. The degradation in the creep rupture strength due to the impurities was attributed to the earlier progression of recovery process in FGHAZ during creep, leading to premature creep void formation.

DOI： 10.1016/j.msea.2024.146669

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Publishing type：Research paper (scientific journal)   Publisher：Materials Today Communications  

The investigation of heterogeneous deformation mechanisms in materials is important for understanding fracture mechanisms. In this study, uniaxial tensile deformation of polycrystalline α-titanium was numerically reproduced by three-dimensional crystal plasticity finite element analysis, and the following were revealed for the first time: (i) plate-like high-strain regions form inside the specimen, which are typically observed as band-like high-strain regions (high-strain bands: HSBs) on the surface of and cross-sections in α-titanium specimen, and (ii) the formation of HSBs also affects the work-hardening rate near the yielding point. The reasons for the difference in the formation mechanism of HSBs between the surface and inside were also discussed. The results obtained showed that active slip systems that contribute to the formation of HSBs may differ between the surface and inside of specimen, and strain in HSBs on the specimen surface is higher than that inside. The distributions of HSBs at the surface of and inside the specimen are determined by distributions of resolved shear stress. These results are important for understanding the differences in the fracture mechanisms between the surface and interior of materials.

DOI： 10.1016/j.mtcomm.2024.109230

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Materials Science and Engineering: A  

Plastic anisotropy in the yield stress of pearlitic steels evolved by drawing was studied. The tension and compression tests were performed in parallel and perpendicular directions to the drawing direction. The yield stress in the parallel tension and vertical compression tests increased with the increase in drawing strain. On the other hand, in the parallel compression, the yield stress decreased significantly at the small drawing strain and then increased at the larger strain. The difference between the yield stress at the tension and at the compression did not change largely after the start of the drawing. The observation of microstructure indicated no significant heterogeneity of the deformation microstructure and texture at the different locations in the radial direction of the drawn samples, meaning that the plastic anisotropy was brought by reasons other than the non-uniformity of the deformation condition. The measurement of the distribution of the local misorientation in ferrite suggested the large plastic deformation even at smaller strain by drawing. This result indicates that the residual stress in plastically deformed ferrite brings back stress, which can be the source of the plastic anisotropy.

DOI： 10.1016/j.msea.2024.146380

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：The Iron and Steel Institute of Japan  

<p>Microstructures of lath martensite have been studied intensively to understand their effect on the mechanical properties of steels. It is, however, said that the relation between microstructural factors and mechanical properties has not been clarified yet. The plastic deformation behavior of fully lath martensitic steels has become important because they are applied to automobile body structures such as bumper reinforcement. It is, therefore, important to understand the microstructural factors that control the work-hardening behavior of fully martensitic steels. Although we could not clarify differences in microstructural factors when manganese (Mn) concentrations of steels are altered, the work-hardening of 8 mass%Mn martensitic steel is much higher than that of 5 mass%Mn martensitic steel. It was found using the digital image correlation (DIC) method, that the strain concentration due to the in-lath-plane slip deformation is more developed in 5 mass%Mn martensitic steel than 8 mass%Mn martensitic steel. Transmission electron microscope (TEM) observations revealed the existence of two types of fine twins inside laths. Long twins that are parallel to the longitude of the lath are observed both in 5 mass%Mn and 8 mass%Mn martensitic steels. Short twins that partially cross the laths, on the other hand, can only be found in 8 mass%Mn martensitic steel. Since twin boundaries are high angle boundaries, the short twins are supposed to prevent the development of in-lath-plane slip deformation. This seems to be the mechanism of higher work-hardening behavior observed in 8 mass%Mn martensitic steel.</p>

DOI： 10.2355/tetsutohagane.tetsu-2023-068

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Scripta Materialia  

Ferromagnetism can be induced by introducing oxygen vacancies into metallic oxides. The oxygen deficiencies introduced into ZrO2 samples thus far have been present in unquantifiable concentrations and randomly scattered near the surface. We successfully produced room-temperature-ferromagnetic 9 μm-thick ZrO2−x films via a stepwise oxidation process consisting of annealing metallic Zr foil in air and under a controlled oxygen partial pressure. The ZrO2−x films are composed of a periodic structure with a spacing of 3–4 nm; this structure originated from the presence and absence of oxygen deficiencies. The unique structure in the ZrO2−x phase consisting of the periodic arrangements of oxygen deficiencies can be produced by the stepwise oxidation using Zr metal. Our proposed process should open the door for new applications of defect-driven ferromagnetic films of undoped magnetic oxides with a periodic oxygen-defective structure.

DOI： 10.1016/j.scriptamat.2023.115944

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

This paper discusses the applicability of Straalsund et al.’s technique for combining the Larson–Miller parameter (LMP) and life-fraction rule to form a single high-temperature strength equation for 9Cr-oxide-dispersion-strengthened (ODS) tempered martensitic steels (TMS). It uses the extensive dataset on creep rupture, tensile, and temperature-transient-to-burst tests of 9Cr-ODS TMS cladding tubes in the α-phase, α/γ-duplex, γ-phase matrices, which are accumulated by the Japan Atomic Energy Agency so far. The technique is adequately applicable to 9Cr-ODS TMS cladding tubes. A single high-temperature strength equation expressing the mechanical strength in different deformation and rupture modes (creep, tensile, temperature-transient-to-burst) is derived for 9Cr-ODS TMS cladding tubes. This equation can predict the rupture life of the cladding tubes under various stresses and temperatures over time. The applicable range of the high-temperature strength equation is specified in this study and the upper limit temperature for the equation is found to be 1200 °C. At temperatures higher than 1200 °C, the coarsening and aggregation of nanosized oxide particles and the γ to δ phase transformation are reported in previous studies. The high-temperature strength equation can be well applied to the creep and tensile strength in the α-phase matrix, the creep strength in the γ-phase matrix and the temperature-transient-to-burst strength in both phases except for the low equivalent stress (43 MPa) at temperatures exceeding 1050 °C. The mechanism of the notable consistency between creep and tensile strength in the α-phase matrix is discussed by analyzing the high-temperature deformation data in the light of existing deformation models. The study suggested the thermal activation process is dominant even for the high-temperature deformation modes exceeding yield stress in the 9Cr-ODS TMS with the fine-grained matrix.

DOI： 10.1016/j.jnucmat.2024.155008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Iron and Steel Institute of Japan  

<p>Microstructures of lath martensite have been studied intensively to understand their effect on the mechanical properties of steels. It is, however, said that the relation between microstructural factors and mechanical properties has not been clarified yet. The plastic deformation behavior of fully lath martensitic steels has become important because they are applied to automobile body structures such as bumper reinforcement. It is, therefore, important to understand the microstructural factors that control the work-hardening behavior of fully martensitic steels. Although we could not clarify differences in microstructural factors when manganese (Mn) concentrations of steels are altered, the work-hardening of 8 mass%Mn martensitic steel is much higher than that of 5 mass%Mn martensitic steel. It was found using the digital image correlation (DIC) method, that the strain concentration due to the in-lath-plane slip deformation is more developed in 5 mass%Mn martensitic steel than 8 mass%Mn martensitic steel. Transmission electron microscope (TEM) observations revealed the existence of two types of fine twins inside laths. Long twins that are parallel to the longitude of the lath are observed both in 5 mass%Mn and 8 mass%Mn martensitic steels. Short twins that partially cross the laths, on the other hand, can only be found in 8 mass%Mn martensitic steel. Since twin boundaries are high angle boundaries, the short twins are supposed to prevent the development of in-lath-plane slip deformation. This seems to be the mechanism of higher work-hardening behavior observed in 8 mass%Mn martensitic steel.</p>

DOI： 10.2355/isijinternational.isijint-2024-208

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Environmental Nanotechnology, Monitoring and Management  

Electrical Furnace Steel slag (EAF) was converted into steel slag nanocomposite (SSNC) by solvothermal preparation technique and assisted by H2O2 for Fenton and photo-Fenton-like degradation of six different dyes in both suspension and spin-coated modes. The physico-chemical properties were evaluated using various characterization tools. The result reveals that the EAF-SSNC exhibited nanocomposite behaviour. As a substrate for studying the advance oxidataion processes, methylene blue (MB) was first used. The kinetic study followed the pseudo-first-order reaction kinetics. The effect of operating factors was studied using the response surface methodology. Based on the optimum parameters, five reusability test was performed for both suspension and spin-coated mode, where 92.89% of MB was degraded in the fifth cycle for suspension mode and 88.21% for spin-coated mode. The photodegradation efficiency of methylene blue, methyl red, rhodamine-B, Congo red, acid blue-25, and methyl orange were 85.84%, 74.62%, 70.53%, 58.66%, 45.21%, and 40.38%, respectively, at a neutral pH. The effect of scavenger and total organic carbon (TOC) were analyzed. Moreover, the degradation pathway was identified using an LC-mass spectrometer. This work showed that slag nanocomposite is effective for dye degradation and lowering the overall cost of the Fenton process at a neutral pH.

DOI： 10.1016/j.enmm.2023.100836

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

The Japan Atomic Energy Agency has been developing 9Cr-oxide dispersion strengthened (ODS) steel as a fuel cladding material for sodium-cooled fast reactors (SFRs). Previous studies have formulated the creep rupture equation for 650°C–850°C. However, little data have been obtained above 850°C, and no equation has been formulated. This study conducted creep tests to evaluate creep strength at 700°C–1000°C. Two creep test methods, the internal pressure and ring creep tests under development, were used, and the validation of the ring creep test method was conducted. The results showed that 9Cr-ODS steel undergoes almost no strength change due to the matrix’s phase transformation, and a single equation can express a creep rupture strength from 700°C to 1000°C. In validating the ring creep test method, analysis clarified the effect of stress concentration on the specimen. Plastic deformation occurs at high initial stress and may lead to early rupture. The results will be essential for future creep testing and evaluation of neutron-irradiated 9Cr-ODS steel.

DOI： 10.1080/00223131.2023.2269178

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Publishing type：Research paper (scientific journal)   Publisher：IWA Publishing  

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<p></p> This study converted slag from the steelmaking industry into an Fe2O3-rich nanocomposite using solvothermal technique for photodegradation of pharmaceutical wastewater in an immobilized mode. The nanocomposite was characterized using XRF, SEM, EDX, TEM, FTIR, XRD, and UV–Vis spectrometer. The XRF analysis result reveals a significant increase in the weight percent of Fe2O3 and SiO2, with a decrease in CaO content. The SEM images revealed the spherical and heterogeneous nature of the nanocomposite in shape and structure, while the FTIR confirms the increase in the vibration band of Si–O–Si and Fe–O with a reduction in the wide stretch mode of Ca–O. The XRD result illustrated the crystalline peak of Fe2O3 with a nanoparticle crystal size of 15.17 nm. The slag nanocomposite was used for the photodegradation of paracetamol. The optimum operating parameters were obtained using response surface methodology at an R2 value of 0.99 and p-value &amp;lt; 0.05. The degradation efficiency obtained at the optimum value was 96.96%. The degradation efficiency of the fifth repeated cycle of the immobilized nanocomposite was 77.89%. The degradation mechanism revealed that OH• radical was the major species of the degradation process. This work showed that slag nanocomposite might be effectively used for pharmaceutical wastewater treatment.

DOI： 10.2166/wpt.2023.152

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Language：English   Publisher：The Japan Institute of Metals and Materials  

<p>Early-stage dislocation structures inside dislocation channels of rapid-cooled and tensile-deformed aluminum single crystals were investigated by using the bright field imaging mode in a scanning transmission electron microscope (STEM-BF). Inside dislocation channels, arrays of the prismatic dislocation loops originated from dislocations of the primary slip system, i.e., (1 1 1)[<span style="text-decoration: overline;">1</span> 0 1], were mainly formed. Dislocations of the primary coplanar slip systems such as (1 1 1)[0 1 <span style="text-decoration: overline;">1</span>] and (1 1 1)[<span style="text-decoration: overline;">1</span> 1 0] were activated owing to internal stresses caused by the primary dislocations pile-up inside the cleared channels. The activated primary coplanar dislocations left the dislocation loops elongating along the edge dislocation directions behind them. Inter-dislocation-loop interactions occur especially at the arrays of the prismatic dislocation loops originated from dislocations of the primary slip systems and produce “butterfly shape” dislocation loops. As the “butterfly shape” dislocation loops have “sessile” junctions, they should act as “obstacles” against the following multiplications and glides of the dislocations. When the interactions proceed, “tangled structures” would be formed around the arrays of the prismatic dislocation loops originated from dislocations of the primary slip system.</p><p> </p><p>This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. <b>87</b> (2023) 67–80.</p>

DOI： 10.2320/matertrans.mt-m2023032

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Metastable β titanium alloys possess excellent strain-hardening capability, but suffer from a low yield strength. As a result, numerous attempts have been made to strengthen this important structural material in the last decade. Here, we explore the contributions of grain refinement and interstitial additions in raising the yield strength of a Ti-12Mo (wt.%) metastable β titanium alloy. Surprisingly, rather than strengthening the material, grain refinement actually lowers the ultimate tensile strength in this alloy. This unexpected and anomalous behavior is attributed to a significant enhancement in strain-induced α’’ martensite phase transformation, where in-situ synchrotron X-ray diffraction analysis reveals that this phase is much softer than the parent β phase. Instead, a combination of both oxygen addition and grain refinement is found to realize an unprecedented strength-ductility synergy in a Ti-12Mo-0.3O (wt.%) alloy. The advantageous effect of oxygen solutes in this ternary alloy is twofold. Firstly, solute oxygen largely suppresses strain-induced transformation to the α’’ martensite phase, even in a fine-grained microstructure, thus avoiding the softening effect of excessive amounts of α’’ martensite. Secondly, oxygen solutes readily segregate to twin boundaries, as revealed by atom probe tomography. This restricts the growth of {332}<113> deformation twins, thereby promoting more extensive twin nucleation, leading to enhanced microstructural refinement. The insights from our work provide a cost-effective rationale for the design of strong yet tough metastable β titanium alloys, with significant implications for more widespread use of this high strength-to-weight structural material.

DOI： 10.1016/j.actamat.2023.119165

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Acta Materialia  

Mg–Zn–Y alloys that contain an Mg-based long-period stacking ordered (LPSO) phase exhibit excellent mechanical properties because of the kink bands formed by plastic deformation. Such “kink-band strengthening” has attracted significant attention. Herein, we conducted deformation tests on directionally solidified single-phase LPSO Mg85Zn6Y9 alloys to investigate the kink-band strengthening mechanism. High-angular resolution electron backscatter diffraction was performed to detect misorientation between the matrices on either side of the kink bands. The misorientations corresponded to the magnitudes of the Frank vectors of the disclinations around the kink, and closely matched the estimations from geometric analysis, which supported the existence of disclinations. Moreover, the Frank vector of the kink bands increased after shear deformation, which indicates that shear deformation introduced new disclinations around the kinks. Scanning electron microscopy demonstrated that the kinks clearly obstructed basal ⟨a⟩ slip. Furthermore, scanning transmission electron microscopy of a sheared kink band revealed dislocation pile-ups on both sides of the kink/matrix interface, as well as the formation of secondary kinks that stabilized the structure against shear deformation (i.e., self-accommodation) and bend contours caused by elastic stress fields. The results suggest that disclinations behave as long-range obstacles to dislocation motion (e.g., by reducing the increase in kink-band self-energy or by the elastic stress field of disclinations). We believe that this study will play a key role in identifying the factors responsible for kink-band strengthening in LPSO-phase Mg alloys and in understanding the phenomena underlying the strengthening mechanism.

DOI： 10.1016/j.actamat.2023.118785

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Publishing type：Research paper (scientific journal)   Publisher：Journal of Nuclear Science and Technology  

The Japan Atomic Energy Agency (JAEA) has been developing 9Cr-oxide dispersion strengthened (ODS) tempered martensitic steel (TMS) as a candidate material for use in the fuel cladding tubes of sodium-cooled fast reactors (SFRs). The creep property is essential for the fuel cladding tube of SFR; the reliable prediction of in-reactor creep-rupture strength is critical for implementing the 9Cr-ODS TMS cladding tube in the SFR. This study investigated the quantitative correlation between the creep properties of 9Cr-ODS TMS at 700°C and the dispersions of nanosized oxides by analyzing the creep data and the material’s nanostructure. The possibility of deriving a formula for estimating the in-reactor creep properties of 9Cr-ODS TMSs based on an analysis of the nanostructure of neutron-irradiated 9Cr-ODS TMSs was also discussed. The creep properties of 9Cr-ODS TMS at 700°C closely correlated with the dispersion of nanosized oxide particles. The correlation between creep-rupture lives and nanosized oxide particle dispersion in 9Cr-ODS TMS was determined using existing creep models. The elucidation of correlation between the stress exponent of secondary creep rate and the nanostructure is essential to enhance future modeling reliability and formulation.

DOI： 10.1080/00223131.2022.2096147

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Materials Science and Engineering: A  

In textured α-Ti, band-like high-strain regions at approximately 45° to the loading direction are commonly observed under tensile deformation. Herein, such high-strain regions were called high-strain bands (HSB), and the reasons for the formation mechanism of HSBs were investigated by a uniaxial tensile test and crystal plasticity finite element (CPFE) analyses. First, we conducted a uniaxial tensile test of commercially pure titanium (CP–Ti) with a TD-split texture, where aggregates of (0001) split and incline in the transverse direction, and changes in strain distributions with deformation were observed by digital image correlation. The strain distributions showed that HSBs of approximately 45° to the loading direction were formed from the initial stage of deformation. Second, a geometric model including crystal orientation information was constructed from the crystal orientation map of the CP-Ti specimen obtained by electron back-scattered diffraction, and the cause of the formation of HSBs was investigated by CPFE analysis. The stress–strain relationship and strain distributions obtained by CPFE analysis correlated well with those obtained experimentally, and the HSBs were successfully reproduced. Finally, CPFE analyses were conducted when the loading conditions, critical resolved shear stress (CRSS), or elastic constants were changed. These results show that the HSBs formed from the initial stage of deformation and the distributions formed by elastic deformation changed depending on the elastic anisotropy. However, the distribution of HSBs after plastic deformation was unchanged by elastic anisotropy. This may be because the directions of easy deformation by elastic and plastic deformation coincided. Elastic anisotropy did not affect the distributions of HSBs formed by plastic deformation, and elastic and plastic deformations were predominantly determined by elastic constants and CRSS, respectively. Curved HSBs were also observed when plastic deformation-resistant regions occupied a large area in the specimen. In this case, the distributions of HSBs, caused by elastic deformation did not coincide with those of the plastic deformation.

DOI： 10.1016/j.msea.2023.144670

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Interstitial oxygen embrittles titanium, particularly at cryogenic temperatures, which necessitates a stringent control of oxygen content in fabricating titanium and its alloys. Here, we propose a structural strategy, via grain refinement, to alleviate this problem. Compared to a coarse-grained counterpart that is extremely brittle at 77 K, the uniform elongation of an ultrafine-grained (UFG) microstructure (grain size ~ 2.0 µm) in Ti-0.3wt.%O is successfully increased by an order of magnitude, maintaining an ultrahigh yield strength inherent to the UFG microstructure. This unique strength-ductility synergy in UFG Ti-0.3wt.%O is achieved via the combined effects of diluted grain boundary segregation of oxygen that helps to improve the grain boundary cohesive energy and enhanced <c + a> dislocation activities that contribute to the excellent strain hardening ability. The present strategy will not only boost the potential applications of high strength Ti-O alloys at low temperatures, but can also be applied to other alloy systems, where interstitial solution hardening results into an undesirable loss of ductility.

DOI： 10.1038/s41467-023-36030-0

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Other Link： https://www.nature.com/articles/s41467-023-36030-0

Language：Japanese   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.62.14

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Materials  

In this study, pure titanium equivalent to Grade 1 was subjected to tensile tests at strain rates ranging from 10−6 to 100 s−1 to investigate the relationship between its mechanical properties and its twinning and slip. Deformation properties and microstructures of samples having average grain sizes of 210 μm (Ti-210), 30 μm (Ti-30), and 5 μm (Ti-5) were evaluated. With increasing strain rates, the 0.2% proof stress and ultimate tensile strength increased for all samples; the fracture strain increased for Ti-210, decreased for Ti-5, and changed negligibly for Ti-30. Comparing high (100 s−1) and low (10−6 s−1) strain rates, twinning occurred more frequently in Ti-30 and Ti-210 at high strain rates, but the frequency did not change in Ti-5. The frequency of 1st order pyramidal slip tended to be higher in Ti-30 and Ti-5 at low strain rates. The higher ductility exhibited by Ti-210 at high strain rates was attributed to the high frequency of twinning. In contrast, the higher ductility of Ti-5 at low strain rates was attributed to the activity of the 1st order pyramidal slip.

DOI： 10.3390/ma16020529

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Systematic experiments were performed to determine where and when dislocations are induced during the thermal cycling of Ti–Ni shape memory alloys. The decrease in transformation temperatures upon thermal cycling became less pronounced with increasing Ni concentration, which was supported by the dislocation densities estimated from scanning transmission electron microscopy observations. The arrangement of transformation-induced dislocations after the 1st thermal cycle reproduced the self-accommodation structure of B19ʹ martensite. Three-dimensional visualization revealed that the complex spiral appearance of the dislocations after the 1st thermal cycle was attributable to the duplicated projection of relatively simple elongated dislocation loops aligned in the depth direction parallel to 〈111〉B2 along {110}B2. No dislocations were observed along the twin boundary in the initially transformed martensite, whereas there were numerous dislocations parallel to the trace of the {110}B2 planes in the martensite formed upon the 2nd thermal cycle. The results suggest that the transformation-induced dislocation loops were generated during the reverse transformation rather than the forward transformation. The possible source site of the transformation-induced dislocations is also discussed.

DOI： 10.1016/j.actamat.2022.118588

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Authorship：Lead author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Iron and Steel Institute of Japan  

<p>Oxide Dispersion Strengthened (ODS) ferritic steel, a candidate material for fast reactor fuel cladding, has low thermal expansion, good thermal conductivity, and excellent resistance to irradiation damage and high temperature strength. The origin of the excellent high-temperature strength lies in the dispersion of fine oxides. In this study, creep tests at 700°C or 750°C, which are close to the operating temperatures of fast reactors, and high-temperature tensile tests at 900°C to 1350°C, which simulate accident conditions, were conducted on 9Cr ODS ferritic steels, M11 and MP23, and 12Cr ODS ferritic steel, F14, to confirm the growth behavior of oxides. In the M11 and F14 creep test samples, there was little oxide growth or decrease in number density from the initial state, indicating that dispersion strengthening by oxides was effective during deformation. After creep deformation of F14, the development of dislocation substructures such as dislocation walls and subgrain boundaries was hardly observed, and mobile dislocations were homogeneously distributed in the grains. The dislocation density increased with increasing stress during the creep test. In the high-temperature ring tensile tests of MP23 and F14, the strength of both steels decreased at higher temperatures. In MP23, elongation decreased with increasing test temperature from 900°C to 1100°C, but increased at 1200°C, decreased drastically at 1250°C, and increased again at 1300°C. In F14, elongation decreased with increasing temperature. It was inferred that the formation of the δ-ferrite phase was responsible for this complex change in mechanical properties of MP23 from 1200 to 1300°C.</p>

DOI： 10.2355/tetsutohagane.tetsu-2022-080

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Iron and Steel Institute of Japan  

<p>The creep deformation behavior and microstructure of a N-containing steel expected to exhibit high creep strength and excellent oxidation resistance were investigated. Even for steel with a high W content, it was possible to form a martensitic microstructure by adding a sufficient amount of N. Comparison of the microstructures of the N-containing steel and a C-containing steel confirmed that the two steels have the same crystal orientation relationship. The N-containing steel precipitated with the Laves phase as a strengthening phase displayed a higher creep strength than conventional steel under relatively high stress. However, the superiority of the creep strength of the N-containing steel relative to the conventional steel decreased under low stress. The stress exponent of the N-containing steel was different from those of the C-containing steel and the conventional steel. This deference considered to be ascribed to the difference of variation behavior of dislocation density during creep deformation.</p>

DOI： 10.2355/tetsutohagane.tetsu-2022-065

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Language：Japanese   Publisher：The Iron and Steel Institute of Japan  

DOI： 10.2355/tetsutohagane.109.149

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Publisher：28th IFHTSE 2023 Congress  

Lath martensitic steel is expected to be applied to automobile body structures. It is necessary to balance the strength and ductility to clarify the relation between microstructural factors and mechanical properties. Manganese is known to increase the work-hardening rate of lath martensitic steels. However, the reason for this has not been clarified yet. In this study, low-carbon lath martensitic steels with different manganese concentrations were observed by microscopy, served tensile tests, and strain distribution observations by the microscale digital image correlation analysis. The work-hardening rate of martensitic steel containing 8 mass% manganese was higher than that of martensitic steels containing 3 and 5 mass% manganese. When the macroscopic nominal strain was 0.01, in-lath slips were activated in martensitic steel containing 5 mass% manganese. On the other hand, out-of-lath slips as well as the in-lath slips were activated in martensitic steel containing 8 mass% manganese. Twins, called long twins, whose longitudes are the same as the longitude of laths exist in both martensitic steels containing 5 and 8 mass% manganese. Short twins that partially cross the lath were only observed in martensitic steel containing 8 mass% manganese. Because twin boundaries are high-angle boundaries, the short twins are supposed to prevent the development of in-lath slip deformation and promote out-of-lath slip deformation. This seems to be the mechanism of the higher work-hardening behavior observed in martensitic steel containing 8 mass% manganese.

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Publisher：Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals  

Early–stage dislocation structures inside dislocation channels of rapid–cooled and tensile–deformed aluminum single crystals were investigated by using the bright field imaging mode in a scanning transmission electron microscope (STEM–BF). Inside dislocation channels, arrays of the prismatic dislocation loops originated from dislocations of the primary slip system, i.e., ð1 1 1Þ½1- 0 1], were mainly formed. Dislocations of the primary coplanar slip systems such as ð1 1 1Þ½0 1 1-] and ð1 1 1Þ½1- 1 0] were activated owing to internal stresses caused by the primary dislocations pile–up inside the cleared channels. The activated primary coplanar dislocations left the dislocation loops elongating along the edge dislocation directions behind them. Inter–dislocation–loop interactions occur especially at the arrays of the prismatic dislocation loops originated from dislocations of the primary slip systems and produce “butterfly shape” dislocation loops. As the “butterfly shape” dislocation loops have “sessile” junctions, they should act as “obstacles” against the following multiplications and glides of the dislocations. When the interactions proceed, “tangled structures” would be formed around the arrays of the prismatic dislocation loops originated from dislocations of the primary slip system.

DOI： 10.2320/jinstmet.J2022030

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Language：Japanese   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.61.543

DOI： 10.2320/matertrans.mt-md2022017_references_DOI_TuQpzV6CsZfx6a9NZMIjDoLQAJM

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PLANNED-18H05481/

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Materials Science and Engineering: A  

In this study, the criteria for {112‾2} compression twinning in commercially pure titanium (CP–Ti) were investigated by uniaxial tensile tests, crystal plasticity finite element (CPFE) analyses, and slip operation factor (SOF) calculations. First, the aggregates of the [0001] axes of CP-Ti were inclined in the rolling direction (RD), implying its RD-split texture. The development of the crystal orientation distribution with deformation was observed by electron back-scattered diffraction (EBSD). Active slip systems were identified by kernel average misorientation (KAM) and intergranular misorientation axis (IGMA) analyses. The dominant slip system was prismatic <a>, whereas the non-prismatic <a> slip systems were activated near the grain boundary. Active twin systems were also identified by the rotation angles of the [0001] axes between the twin and matrix. The dominant active twin system was the {112‾2} compression twin, although a uniaxial tensile load was applied. Second, the positions of {112‾2} twinning were predicted by CPFE analysis using the resolved shear stress (RSS) criterion while considering plastic deformation. SOF analysis was also employed for the prediction. The CPFE and SOF analyses yielded almost the same level of prediction accuracy. However, these calculations do not completely predict the twinning positions. Finally, the criteria for {112‾2} twinning were discussed, and it was revealed that hydrostatic pressure and RSS are possible criteria for {112‾2} twinning in the continuum model.

DOI： 10.1016/j.msea.2022.143302

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Iron and Steel Institute of Japan  

The effects of the neodymium content on the creep properties and microstructures of 9Cr-3Co-3W-Nd-B steel were investigated. Neodymium had a mild effect on the creep rupture strength at contents up to 0.056 mass%. This suggested that the effects of neodymium compounds and solid-dissolved neodymium were minimal in the microstructures and uniformly creep-deformed parts after normalizing and tempering heat treatment. On the other hand, the reduction of area after creep rupture was improved by the addition of neodymium. Creep rupture occurred at the prior austenite grain boundaries on steel without neodymium. Therefore, neodymium conclusively adhered to the segregated sulfur at the prior austenite grain boundaries to suppress the formation of creep cracks.

DOI： 10.2355/isijinternational.isijint-2022-057

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Language：Japanese   Publisher：The Japan Institute of Light Metals  

<p>The mechanical properties of JISAC8A alloys (Al-12%Si-1%Mg-1%Ni-1%Cu alloy) fabricated by laser-based powder bed fusion (LB) or electron-based powder bed fusion (EB) or gravity casting (CAST) were studied by means of tensile tests at room temperature and 300°C. These alloys were under two types of heat-treatment sequences. One was a T6 treatment, which was corresponding to the heat treatment at 500°C for 2h and 170°C for 4 h, followed by the annealing at 300°C for 10 h. Another was directly annealed at 300°C for 10 h after the fabrication. In the annealed specimens without T6 treatment, the LB alloy showed the highest strength and best elongation at room temperature and 300°C, because eutectic Si particles and Al-Ni-Cu-Fe compounds formed very fine network structure in Al matrix. In the annealed specimens with T6 treatment, the tensile strength at room temperature did not depend on the fabrication methods but the LB alloy showed the best elongation because of finer and isotropic shape of secondary phase particles. The annealed CAST alloy showed the highest strength at 300°C, and the annealed LB alloy showed the lowest strength at 300°C.</p>

DOI： 10.2464/jilm.72.206

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Materials Science and Engineering: A  

It is possible that strain localization in polycrystalline α-Ti leads to the fracture, and it is crucial to evaluate the local slip activities for individual slip systems depending on their textures and loading conditions. In this study, the effects of textures and strain rates on local slip activities were investigated using the crystal plasticity finite element method. For the analysis, microstructural models of α-Ti with the following three textures were employed: aggregates of (0001) axes are (i) splitting in rolling direction (RD-split texture), (ii) splitting in transverse direction (TD-split texture), and (iii) aligned in normal direction (basal texture). For each texture model, two variations in the crystal orientation distributions were considered, namely, small and large scatterings of crystal orientations in the (0001) axes by normal random numbers. The differences in the strain rate sensitivities of the critical resolved shear stresses (CRSSs) among slip systems were also considered. Tensile loading was applied by a forced displacement in the RD with two strain rate conditions of 1.0 × 10−4 s−1 and 1.0 × 10−1 s−1. Local non-prismatic slips were easier to operate in the models with basal and RD-split textures than with the TD-split texture. The slip strains for non-prismatic slip systems were higher at higher strain rates, while activities in the prismatic slips decreased with an increase in strain rates. The mechanism of the exchange of slip system activities can be explained by strain redistribution between hard and soft regions and changes in CRSS as a function of strain rates.

DOI： 10.1016/j.msea.2022.143133

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Language：Japanese  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Institute of Metals and Materials  

Tangled dislocation structures inside the dislocation channels of rapid-cooled and tensile deformed aluminum single crystals were investigated by using BF-STEM. Inside the dislocation channels, arrays of the prismatic dislocation loops belonging to the primary slip system, i.e., (1 1 1)[<span style="text-decoration: overline;">1</span> 0 1], were mainly formed. Dislocations of the primary coplanar slip systems such as (1 1 1)[0 1 <span style="text-decoration: overline;">1</span>] and (1 1 1)[<span style="text-decoration: overline;">1</span> 1 0] were activated due to the internal stresses caused by the primary dislocations pile-up inside the cleared channels. The activated primary coplanar dislocations leave the dislocation loops elongated along the edge dislocation directions behind them. Inter-dislocation-loop interactions take place especially at the arrays of the prismatic dislocation loops of the primary slip systems and produce “butterfly shape” dislocation loops. Since the “butterfly shape” dislocation loops have “sessile” junctions, they should act as “obstacles” against the following multiplications and glides of the dislocations. As the interactions proceed, the arrays are stabilized and grow as “tangles”.

DOI： 10.2320/matertrans.mt-m2021230

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Materials Science and Engineering: A  

To clarify the effect of twinning deformation on the work-hardening behavior of commercially pure titanium, the changes in the grain size and texture due to twinning deformation were investigated, along with the grain-size dependence of the work-hardening behavior. In specimens with an average grain size of 10 μm or less, twinning deformation was inactive, and the instantaneous work-hardening exponent (n = d(lnσ)/d (lnε)) was constant at approximately 0.2. For specimens in which twinning deformation occurred, the average grain size decreased and the texture changed with increasing strain due to twinning deformation, which resulted in an increase in the instantaneous n value. In the grain-size range at which twinning deformation did not occur, the smaller the grain size, the higher the rate of increase in the dislocation density and the greater the activity of ⟨c + a⟩-type dislocations, resulting in an increased strain hardening rate. Furthermore, the flow stress could be approximated using only the Bailey–Hirsh equation. The promotion of work hardening by twinning deformation can be mostly explained by the increase in the dislocation density increment rate, increase in the fraction of ⟨c + a⟩ dislocation due to grain refinement, and change in the Taylor factor due to the change in the texture.

DOI： 10.1016/j.msea.2022.142907

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Language：Japanese   Publisher：The Japan Institute of Metals and Materials  

Recent Studies on TEM/STEM Tomography

DOI： 10.2320/materia.61.84

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Other Link： https://www.jstage.jst.go.jp/article/materia/61/2/61_84/_pdf

Language：Japanese   Publisher：Interdisciplinary Graduate School of Engineering Sciences, Kyushu University  

Effects of O and Al Addition on T winning Deformation in Titanium

DOI： 10.15017/4763153

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Language：Japanese   Publisher：The Iron and Steel Institute of Japan  

<p>The effects of the neodymium content on the creep properties and metallographic structure of 9Cr-3Co-3W-Nd-B steel were investigated. Neodymium had a mild effect on the creep rupture strength at contents up to 0.056 mass%. This suggested that the effects of neodymium compounds and solid-dissolved neodymium were minimal in the microstructures and uniformly creep-deformed parts after normalizing and tempering heat treatment. On the other hand, the reduction of area after creep rupture was improved by the addition of neodymium. Creep rupture occurred at the prior austenite grain boundaries on steel without neodymium. Therefore, neodymium conclusively adhered to the segregated sulfur at the prior austenite grain boundaries to suppress the formation of creep cracks.</p>

DOI： 10.2355/tetsutohagane.tetsu-2021-103

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In order to clarify the effects of grain boundary, grain size and deformation temperature on twinning deformation in polycrystalline commercially pure titanium, active twinning systems during compression deformation at temperatures from 25 °C to 800 °C investigated using SEM/EBSD techniques. Four twinning systems were confirmed to operate under compression at 10% strain in a deformation temperature dependent manner: {112̅1} and {112̅2} twins were observed only at relatively low temperatures (e.g. ≦ 400 °C), while, {101̅1} twins were observed at relatively high temperatures (e.g. ≧ 400 °C), and {101̅2} twins formed at every tested temperature (25 °C–800 °C). Our results suggested that the effects of strain concentration at grain boundaries on twinning differ based on the active twinning systems, which are affected by: 1) the difference in the formation temperature ranges between single-crystal and polycrystalline specimens, 2) the ratio of the number of deformation twins touching grain boundaries to the total number of deformation twins, and 3) grain size dependence.

DOI： 10.1016/j.jallcom.2021.161154

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Scanning transmission electron microscopy (STEM) is suitable for visualizing the inside of a relatively thick specimen than the conventional transmission electron microscopy, whose resolution is limited by the chromatic aberration of image forming lenses, and thus, the STEM mode has been employed frequently for computed electron tomography based three-dimensional (3D) structural characterization and combined with analytical methods such as annular dark field imaging or spectroscopies. However, the image quality of STEM is severely suffered by noise or artifacts especially when rapid imaging, in the order of millisecond per frame or faster, is pursued. Here we demonstrate a deep-learning-assisted rapid STEM tomography, which visualizes 3D dislocation arrangement only within five-second acquisition of all the tilt-series images even in a 300 nm thick steel specimen. The developed method offers a new platform for various in situ or operando 3D microanalyses in which dealing with relatively thick specimens or covering media like liquid cells are required.

DOI： 10.1038/s41598-021-99914-5

Language：Japanese  

Dislocation Motion and Microstructure Evolution Deformed during Creep at Room Temperature in Ti-6Al-4V Alloy

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DOI： 10.2320/matertrans.mt-m2021021

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Time evolution of precipitates related to age-hardening in Ti–2.3 wt pct Cu alloys was investigated by electron microscopy. In isothermal aging at 723 K, the hardness increases continuously owing to precipitation strengthening, whereas in two-step aging where the aging temperature is switched from 673 K to 873 K after 100 hours, the hardness is found to drastically drop after the aging temperature switches. In isothermal aging, metastable and stable precipitates are independently nucleated, whereas characteristic V-shaped clusters of precipitates are observed during the two-step aging. It is revealed by atomic-scale observations that the V-shaped clusters are composed of metastable and stable precipitates and each type of precipitate has a different orientation relationship with the α phase: (10 3 ¯) //(0001) and [0 1 ¯ 0] //[11 2 ¯ 0] for the metastable, and (201)//(1 1 ¯ 03) and [0 1 ¯ 0] //[11 2 ¯ 0] for the stable precipitates, respectively. The drop in hardness during two-step aging can be explained by a synergistic effect of decreased precipitation strengthening and solid solution strengthening. α α α α

DOI： 10.1007/s11661-021-06265-x

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A 3-μm-thick film of Magnéli Ti O was successfully fabricated on the surface of Ti metal foil by a stepwise oxidation process involving annealing at 973 K in air followed by heating to 1173 K under a low oxygen partial pressure. Transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy showed that the Magnéli Ti O phase consisted of a period of four layers of equiaxial grains several hundreds of nanometers in size. The black Magnéli Ti O thin film formed by our process absorbed light in the visible and near-infrared regions. 4 7 4 7 4 7

DOI： 10.1016/j.scriptamat.2021.113829

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A quantitative evaluation method of a serration deformation behavior by a dead-weight tensile test combined with a strain measurement by a DIC method is proposed. This method applies to binary solid solution aluminum alloys and a ternary cluster strengthened aluminum alloy. In the dead-weight tensile test, the serrated flow was measured as a stress-strain curve with a stepped-shape divided into two stages of a stress rising phase and a strain burst phase. By adopting this tensile test, the serration deformation behavior can be measured with an extremely reproducibility. It was confirmed that an elastic deformation occurs predominantly in the stress rising phase, and this deformation behavior is discussed in relation to the concentration of solid solution elements. Propagation of PLC bands has occurred in the strain burst phase. An activation volume obtained from the strain rate in the PLC bands was in good agreement with a calculated value based on a distribution interval assuming a regular square distribution of solute elements. For the ternary alloy strengthened by clusters, it is also shown that this method can be applied to the qualitative estimation of the change in the residual amount of the solid solution element in the matrix during natural aging.

DOI： 10.1016/j.msea.2021.141277

Language：English   Publishing type：Research paper (scientific journal)  

In this study, we conducted a quantitative evaluation of dislocation density by scanning electron microscopy electron channeling contrast imaging for α grains of a Ti-6Al-4V alloy deformed at room temperature. The depth of visibility of dislocations is experimentally measured as 140 to 160 nm by a serial sectioning observation. This result is compared with the theoretical value and applied to evaluate dislocation density. These factors confirm that the theoretically calculated value of the depth of visibility, at 5 to 6 times the extinction distance, is valid for the hexagonal close-packed Ti alloy.

DOI： 10.1093/jmicro/dfaa060

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In this study, the effect of grain boundary on the frictional behavior under the oil-lubricated condition was investigated by varying the grain size (crystallite size) of the pure Fe samples prepared via physical vapor deposition. A high fraction of grain boundary in the sample yielded a low friction coefficient μ in the case of a lubrication oil that formed a chemisorbed film on the surface of the material. Thick absorbed layer formed on the surface was observed only in the sample with high fraction of grain boundary. These results indicate that a disordered structure in the vicinity of grain boundary preferentially forms chemisorbed films and reduces μ by protecting the surface of the material.

DOI： 10.1016/j.triboint.2020.106781

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Thermoelastic martensitic transformation (MT) plays a vital role in shape memory effects and superelasticity of various alloys. To understand the self-accommodation mechanism of the MT of Zr–Co–Pd alloys, which demonstrate typical MT from B2 to B33 structure, the configuration of the alloy’s martensite variants and their crystallographic relationship were investigated by electron backscatter diffraction, transmission electron microscopy, and high-angle annular dark-field scanning transmission electron microscopy. Results suggest that two habit plane variants (HPVs) bounded by {021} compound twinning are formed around each < 100> axes of the B2 parent phase. It is determined that the minimum self-accommodation unit, to relax the strain energy accompanying MT, is a pair of trapezoid- or triangle-shaped HPVs. No lattice invariant shear exists in the martensite variants with a trapezoid- or triangle-shape. In the latter stage of MT, each HPV pair may contact and impinge on {110} plane of B2 parent phase, which is shear and shuffling plane on MT. The combination of the four variants forms the roof-type morphology. B33 B2 B2

DOI： 10.1007/s10853-020-05599-y

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Slip operation factor (SOF), which considers the effect of mechanical interactions between plastically “soft” and “hard” regions, is a function of the Schmid factor (SF) and critical resolved shear stress (CRSS). It is used as an indicator to efficiently predict the ease of slip operation of metal materials at the grain level. While SOF could predict strain distributions, it was not compliant to the prediction of those for individual slip systems. Additionally, the contribution of secondary slip systems was disregarded in the SOF. In this study, the SOF was first extended to adapt to individual slip systems. This extended and generalized form of the SOF was called SOFS. Next, the contribution of secondary slip systems was considered in SOF, where the modified version of the SOF was called MSOF. Polycrystalline α-Ti models were built from crystal orientation maps obtained by electron back-scattered diffraction (EBSD), and the strain distributions under uniaxial tensile loading were predicted using the SOF, SOFS, and MSOF. The results obtained were compared with those obtained via crystal plasticity finite element (CPFE) analysis. The distributions obtained by the SOFS were similar to slip strain distributions for individual slip systems when single slips were dominant and the deformation was slight. The MSOF also successfully predicted the strain distributions with higher accuracy than that offered by the SOF.

DOI： 10.1016/j.mtcomm.2021.102041

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Dislocation structures inside the cleared dislocation channels in rapid-cooled and tensile-deformed aluminum single crystals were investigated by using transmission electron microscope (TEM). The present study especially focused on the dislocation structures at their early formation stage. In their very beginning stage, arrays of prismatic dislocation loops of the primary slip system were essentially formed elongating along [1- 2 1] direction and each prismatic loop stacked to [1- 0 1]. With the progress of plastic deformation, the number of the prismatic loops composing the array increased and produced tangled structures with dislocations of the primary coplanar slip system. The tangled structures may act as strong obstacles against the following primary dislocations and become a triggering factor for the creation of the cell structure.

DOI： 10.2320/matertrans.MT-M2020255

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In this study, the relationship between changes in the magnetic properties and creep strength with the addition of 3 or 6 mass% Co was investigated for ferritic steel containing 15 mass% Cr. Co addition up to 6 mass% hardly contributed to solid solution strengthening or precipitation strengthening at room temperature. However, in the range of 650 to 750°C, the steel with the larger amount of Co exhibited higher creep strength, which is explained by a reduction in the diffusion rate associated with a change in magnetic properties by Co addition. An increase of the volume magnetization of the steel with increasing Co content in the range from room temperature to about 800°C was confirmed. Comparing the difference in volume magnetization and the ratio of the creep strain rate for steels with different amounts of Co, a clear correlation was found. That is, at the temperature at which the difference in volume magnetization reached a maximum, the peak of the creep strain rate ratio was also observed. This result is explained as follows. In a low temperature region where the magnetization is large or in a high temperature region above the Curie point of both steels, the steels exhibit no significant difference in the creep strength. However, at a temperature where one steel loses its ferromagnetism but the other steel retains it, a significant difference in the creep strength is observed.

DOI： 10.2355/isijinternational.ISIJINT-2020-315

Language：Japanese   Publishing type：Research paper (scientific journal)  

The deformation behavior and microstructure evolution of Ti-6Al-4V alloy under room temperature creep was investigated using mechanical test and scanning electron microscope observation with electron back-scatter diffraction method. The alloys were creep deformed and ruptured under initial stresses of 874 MPa, 889 MPa and 904 MPa at room temperature. The rapid stress change test revealed that creep deformation was controlled by the viscous slip motion of dislocations. The stress exponent was estimated as 59. The strain rate of acceleration creep region calculated by the Norton's law with the high stress exponent was inconsistent with that measured by the experimental creep test. Using the slip trace analysis, it was found that single dislocation slip in basal and prism were mainly activated in the early stage of creep, and multiple slips were often observed as the deformation progresses. Especially, the multiple slip including 1st pyramidal slip believed to be effective for suppressing strain rate acceleration in creep. In addition, the work hardening behavior during creep showed a strain rate dependence, indicating that the lower the strain rate is, the more work hardening occurs.

DOI： 10.2464/JILM.70.405

Language：English   Publishing type：Research paper (scientific journal)  

The combination of in-situ and three-dimensional (3D) in transmission electron microscopy (TEM) is one of the emerging topics of recent advanced electron microscopy research. However, to date, there have been only handful examples of in-situ 3D TEM for material deformation dynamics. In this article, firstly, the authors briefly review technical developments in fast tilt-series dataset acquisition, which is a crucial technique for in-situ electron tomography (ET). Secondly, the authors showcase a recent successful example of in-situ specimen-straining and ET system development and its applications to the deformation dynamics of crystalline materials. The system is designed and developed to explore, in real-time and at sub-microscopic levels, the internal behavior of polycrystalline materials subjected to external stresses, and not specifically targeted for atomic resolution (although it may be possible). Technical challenges toward the in-situ ET observation of 3D dislocation dynamics are discussed for commercial structural crystalline materials, including some of the early studies on in-situ ET imaging and 3D modeling of dislocation dynamics. A short summary of standing technical issues and a proposed guideline for further development in the 3D imaging method for dislocation dynamics are then discussed.

DOI： 10.1016/j.cossms.2020.100850

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the pseudo three-dimensional features of kink bands formed by the compression deformation of a directionally solidified Mg Zn Y alloy. The kink bands of interest included ridge kinks with clear surface relief and gentle kinks with almost no surface relief. Observation of surface undulation by laser microscopy and electron backscatter diffraction with serial sectioning by mechanical polishing revealed the pseudo three-dimensional morphology and crystal rotation of these kink bands. The features of the observed kink bands are explained by a pseudo three-dimensional arrangement of disclination lines. 85 6 9

DOI： 10.1016/j.mtla.2020.100716

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The formation process of the kink bands in a directionally solidified, polycrystalline, long-period stacking ordered (LPSO) Mg Zn Y phase during compression deformation was investigated. In-situ compression observations by scanning electron microscopy and electron backscattered diffraction analysis revealed that regions with localized crystal orientation rotation appeared before ridge kinks formed as surface relief. In this paper, it is called as pre-kink The observed pre-kinks were composed of two separate regions rotated in opposite directions to each other and distinguished from one another by the three boundaries. A transmission electron microscopy observation revealed that the boundaries of the pre-kink are sub-boundaries consisting of edge dislocation array, and both its ends are terminated within the matrix. These characteristics match the reported that of regular ridge kink boundaries. In addition, while pre-kinks do not possess the ability of complete reversibility, its boundaries are capable of moving elastically. 85 6 9

DOI： 10.1016/j.actamat.2020.04.051

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The effect of lattice defects on the tribological behavior under tricresyl phosphate (TCP) added poly-α-olefin (PAO) lubrication was investigated in the nanostructured steels produced by heavy plastic deformation processes. In surface-nanostructured SUJ2-bearing steel, tribological behavior with high friction coefficient was observed in ball-on-disk tests when compared to non-deformed steel. In addition, a similar phenomenon was observed in ultra-low carbon (ULC) steel with a high density of lattice defects (grain boundary, dislocation and so on). By increasing the density of lattice defects, a higher friction coefficient was observed. The reason for the tribological behavior with high friction coefficient seems to be that the compound film of Fe-O-P system formed in the ball-on-disk test was worn down.

DOI： 10.2355/isijinternational.ISIJINT-2019-707

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Uniaxial tension experiments and electron backscatter diffraction were performed on a bimodal Ti-6Al-4V alloy to study in situ the deformation behavior of primary hcp-Ti (α_p). It was found that the strain could be accommodated by the activation of slip systems and by grain rotations. The prismatic slip was the primary slip mode of the α_p. From the analysis of kernel average misorientation and geometrically necessary dislocation, it was shown that the dislocations mainly distributed in the vicinity of grain and sub-grain boundaries, and part of the dislocations distributed around slip lines. It was the dislocation activities that led to the formation of the low angle grain boundary and its transformation to the high angle grain boundary. It's important that tracking of deformation heterogeneities with significance to performance. By analyzing the rotation angle, average rotation rate, and rotation path of grains, it was shown that grain rotation heterogeneity occurred during the deformation. From the observation of the loading direction, grain rotation paths kept with the texture evolution direction of all α_p. The grains activated in the basal slip gradually rotated to the ⟨101¯1⟩ pole and enhanced the intensity of the ⟨101¯1⟩ texture. Meanwhile, the grains activated in prismatic or 1st-order pyramidal slip rotated to the ⟨101¯0⟩ pole and enhanced the intensity of the ⟨101¯0⟩ texture. Grain rotation and texture evolution are related to mechanical properties.

DOI： 10.1016/j.matchar.2020.110282

Language：English   Publishing type：Research paper (scientific journal)  

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) enable the visualization of three-dimensional (3D) microstructures ranging from atomic to micrometer scales using 3D reconstruction techniques based on computed tomography algorithms. This 3D microscopy method is called electron tomography (ET) and has been utilized in the fields of materials science and engineering for more than two decades. Although atomic resolution is one of the current topics in ET research, the development and deployment of intermediate-resolution (non-atomic-resolution) ET imaging methods have garnered considerable attention from researchers. This research trend is probably not irrelevant due to the fact that the spatial resolution and functionality of 3D imaging methods of scanning electron microscopy (SEM) and X-ray microscopy have come to overlap with those of ET. In other words, there may be multiple ways to carry out 3D visualization using different microscopy methods for nanometer-scale objects in materials. From the above standpoint, this review paper aims to (i) describe the current status and issues of intermediate-resolution ET with regard to enhancing the effectiveness of TEM/STEM imaging and (ii) discuss promising applications of state-of-the-art intermediate-resolution ET for materials research with a particular focus on diffraction contrast ET for crystalline microstructures (superlattice domains and dislocations) including a demonstration of in situ dislocation tomography.

DOI： 10.1093/jmicro/dfaa002

Language：English   Publishing type：Research paper (scientific journal)  

Friction property of the case hardening steel subjected to excess vacuum carburizing and subsequent severe plastic deformation and induction hardening was evaluated by the traction test. The purpose of this study is to clarify the effect of fine microstructure on the friction property, focusing on the interaction between the fine microstructure and the lubricating oil additives. The vacuum carburizing treatment is performed at the hyper-eutectoid composition of 1.0 mass% C. Subsequently, the carburized surface was formed the white layer by the surface-nanostructured wearing (SNW) process, and the specimen having the initial microstructure was subjected to induction hardening. The microstructure of the condition with SNW was finer compared to that with SNW-less. According to the traction test, traction coefficient (ì) in the specimen having the fine microstructure on the rolling contact surface decreased. Therefore, it was found that the decrease of ì could be achieved by the application of high-density lattice defects (grain boundaries in this study). After the test, the rolling contact surface of the specimen with fine microstructure became smooth, and the surface showed high reactivity with the lubricating oil additives and formed the compound film of Fe-O-P system having a fine, spherical morphology. The surface roughness was improved by the presence of the wear particles on the surface. Therefore, it was thought that the ì was decreased because the transition to a mild friction condition was caused due to the dispersion of the contact pressure.

DOI： 10.2355/tetsutohagane.TETSU-2019-082

Language：English   Publishing type：Research paper (scientific journal)  

An indicator to predict the slip operation at the crystal-grain level, namely slip operation factor (SOF), was established as a function of the Schmid factor (SF) and critical resolved shear stress (CRSS). Plastically "soft" and "hard" regions were estimated by the Schmid factor values normalized by the CRSS - the normalized Schmid factor (NSF). The effect of the interaction among the regions was incorporated into SOF. A microstructural map of α-titanium (α-Ti) was obtained by the electron backscatter diffraction patterns. Several spatial distributions of SOF were calculated based on the map by changing the interaction range among the regions. The distributions were compared with those of the strains obtained by the crystal plasticity finite element (CPFE) analysis. Good agreement between the distributions was found near the macroscopic yield point when the interaction range was appropriate, although some significant differences between the distributions were also noticed after the yielding point. The prediction accuracy by SOF was higher than that by SF and NSF. The reasons for the high accuracy revealed by the SOF analysis and the differences between the distributions indicated by the CPFE analysis were also investigated.

DOI： 10.1016/j.ijplas.2019.12.001

Language：English   Publishing type：Research paper (scientific journal)  

Microstructural characterization of martensite with long period stacking order (LPSO) structure in a Hf-Co-Pd alloy was investigated by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). LPSO structure with six stacking sequences; 6O (orthorhombic) (Space group: Immm), were newly discovered. Based on the electron diffraction experiments, the lattice parameters of LPSO phase were estimated to be a = 0.33 nm, b = 0.45nm and c = 1.53 nm. The formation mechanism of martensite with LPSO structure and the crystallographic orientation relationship between parent B2 phase and 6O martensite are also discussed.

DOI： 10.2320/matertrans.MT-MJ2019002

Language：Japanese  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese  

Language：English   Publishing type：Research paper (scientific journal)  

The effect of Fe addition on the high-temperature mechanical properties of heat-resistant aluminum alloys produced by selective laser melting (SLM) was investigated in relation to the alloy microstructures. Fe is generally detrimental to the properties of cast aluminum alloys; however, we found that Fe-containing alloys produced by SLM had improved high-temperature strength and good ductility. Microstructural observations revealed that the increase in the high-temperature strength of the alloys was due to the dispersion of fine rod-shaped Fe-Si-Ni particles unique to the SLM material instead of the cell-like structure of eutectic Si.

DOI： 10.3390/met9040468

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Synopsis: The effect of lattice defects on the tribological behavior for high friction coefficient under tricresyl phosphate (TCP) added poly-α-olefin (PAO) lubrication was investigated in the nanostructured steels produced by heavy plastic deformation processes. In the surface-nanostructured SUJ2 bearing steel, the tribological behavior with high friction coefficient was observed in the ball-on-disk tests in comparison with the non-deformed steel. In addition, the similar phenomenon was observed in the ultra-low carbon (ULC) steel with high-density of lattice defects (grain boundary, dislocation and so on). By increasing the density of lattice defects, higher friction coefficient was shown. The reason of the tribo-logical behavior with high friction coefficient seems that the compound film of Fe-O-P system formed in the ball-on-disk test was worn.

DOI： 10.2355/tetsutohagane.TETSU-2018-043

Language：Japanese  

Long-period Stacking Ordered Structure of Martensite in Zr-Co Based Alloys

DOI： 10.2320/materia.58.90

Language：English   Publishing type：Research paper (scientific journal)  

Inhomogeneous deformation of a single ¡-¢ colony in a Ti6Al4V alloy under uniaxial tensile conditions was numerically simulated using a crystal plasticity finite element (CPFE) method, and we predicted density changes in geometrically necessary dislocations (GNDs) depending on the vanadium concentration in the ¢ phase (V_¢). The geometric model for the CPFE analysis was obtained by converting data from electron back-scatter diffraction patterns into data for the geometric model for CPFE analysis, using a data conversion procedure previously developed by the authors. The results of the image-based crystal plasticity analysis indicated that smaller V_¢ induced greater stress in the ¡ phase and smaller stress in the ¢ phase close to the ¡-¢ interfaces in the initial stages of deformation because of the elastically softer ¢ phase with lower V_¢. This resulted in greater strain gradients and greater GND density close to the interfaces in the initial stages of deformation within the single ¡-¢ colony when the ¢ phase plastically does not deform.

DOI： 10.2320/matertrans.M2019016

Language：Japanese  

Language：English   Publishing type：Research paper (scientific journal)  

This study proposes a design guideline for polycrystal Ni-based model alloys with high ductility and 100-MPa creep rupture strength beyond 800°C and 10⁵h. These alloys are strengthened by both the precipitation of fine γ′ particles inside the grain and the Laves phase at the grain boundary. For investigating the damage mechanism, transformation from the non-equilibrium Laves phase to the σ phase at the grain boundary and formation of the equilibrium needle-like Laves phase inside the grain are promoted by increasing the Fe concentration. The rupture time of Fe-free alloys significantly increases because of the equilibrium Laves phase at the grain boundary owing to a suitable Mo equivalent. In particular, W addition can help achieve high-temperature creep strength. The precipitate-free zone (PFZ) is predominantly formed by prior migration at the grain boundary without precipitation. Creep rupture occurs at the precipitation/matrix interface in the PFZ. Therefore, transformation control from the Laves to the σ phase at the grain boundary suppresses creep degradation. Consequently, a Ni-based alloy with strength >100 MPa and rupture elongation >20% at 800°C and 10⁵h is fabricated using Larson–Miller parameter conversion, and the alloy design guideline’s validity is confirmed.

DOI： 10.1080/14786435.2018.1524161

Language：English   Publishing type：Research paper (scientific journal)  

A ZrN contact on a Ge substrate can alleviate the intrinsic Fermi-level pinning (FLP) position toward conduction band edge, which is induced by an amorphous interlayer (a-IL) containing nitrogen atoms at the interfaces. Since the a-IL could be retained on the Ge surface, we demonstrated a wide range Schottky barrier height (SBH) control for metal/a-IL/Ge contacts. The sputtering power for ZrN affects the SBH, pinning factor (S), and effective charge neutral level. A high S value of 0.26 was achieved, which is comparable to that of metal/Si contacts. A model was proposed for explaining the mechanism of this effective FLP alleviation.

DOI： 10.1088/1361-6641/aae4bd

Language：Japanese  

Crystal plasticity analysis on microscopic deformation of Ti-6Al-4V alloy depending on the vanadium content in β phase

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Multilayer hexagonal boron nitride (h-BN) is an ideal insulator for two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, because h-BN screens out influences from surroundings, allowing one to observe intrinsic physical properties of the 2D materials. However, the synthesis of large and uniform multilayer h-BN is still very challenging because it is difficult to control the segregation process of B and N atoms from metal catalysts during chemical vapor deposition (CVD) growth. Here, we demonstrate CVD growth of multilayer h-BN with high uniformity by using the Ni-Fe alloy film and borazine (B
₃
H
₆
N
₃
) as catalyst and precursor, respectively. Combining Ni and Fe metals tunes the solubilities of B and N atoms and, at the same time, allows one to engineer the metal crystallinity, which stimulates the uniform segregation of multilayer h-BN. Furthermore, we demonstrate that triangular WS
₂
grains grown on the h-BN show photoluminescence stronger than that grown on a bare SiO
₂
substrate. The PL line width of WS
₂
/h-BN (the minimum and mean widths are 24 and 43 meV, respectively) is much narrower than those of WS
₂
/SiO
₂
(44 and 67 meV), indicating the effectiveness of our CVD-grown multilayer h-BN as an insulating layer. Large-area, multilayer h-BN realized in this work will provide an excellent platform for developing practical applications of 2D materials.

DOI： 10.1021/acsnano.8b03055

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

The antiphase boundary (APB)-like structure of B33 martensite in a ZrCoPd alloy was investigated by means of conventional transmission electron microscopy and high-Angle annular dark-field scanning transmission electron microscopy. The APB-like structure had atomic shifts along both the c-Axis on the (010)_B33 basal plane and the b-Axis on the (001) _B33 plane. The displacement vector of the APB-like structure could be expressed as R = (0, 1/4, =)_B33 The formation mechanism of the APB-like structure was also elucidated.

DOI： 10.2320/matertrans.M2018146

Language：Japanese   Publishing type：Research paper (other academic)  

Rolling Contact Fatigue Behavior in Surface Nanostructured Steels

DOI： 10.1299/jsmemm.2018.OS1420

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the interaction of femtosecond (fs) laser and hard-to-process semiconductor material 4H-SiC at near-threshold fluence, fs laser was repeatedly irradiated to SiC surface at different scanning velocities and scan times. The evolutions of surface morphologies were observed and discussed according to Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM) and Transmission Electron Microscope (TEM). Discontinuous zones were ablated in SiC surface after laser irradiation at near-threshold fluence 1.1 J/cm². High spatial frequency rippled structures substantially shorter than the wavelength of incident fs laser were fabricated. The width of the ablated zones increased with lower scanning velocities and more scan times. The mechanism was discussed. Incubation effect occurred in the subsurface of SiC triggered inhomogeneous energy deposition accumulation, which was responsible for the discontinuous ablated zones. Moreover, an amorphous layer with a thickness of about 30 nm was observed in 4H-SiC surface where no ablation was induced after repeated irradiation. This was discussed and explained from the aspects of molecular dynamics simulations of fs laser irradiation to semiconductor materials.

DOI： 10.1149/2.0421712jss

Language：English   Publishing type：Research paper (scientific journal)  

New ferritic heat-resistant steels with high nitrogen content were prototyped and their microstructures and mechanical properties at high temperature were evaluated. The addition of 0.3 mass% N into ferritic steels was achieved without the formation of blowholes by applying pressurized melting methods under an atmosphere of up to 4.0 MPa. The high-nitrogen ferritic heat-resistant steels contained several kinds of nitrides within the lath martensitic structure. V-rich coarse particles were identified as crystallized MN. Fine VN or Cr
₂
N particles were precipitated on the martensitic grain boundaries such as prior-austenite grain boundary, packet boundary, block boundary and lath boundary depending on the V content. The martensitic structure of the high-nitrogen steels contained a hierarchical microstructure including martensitic laths, blocks, packets, and prior-austenitic grains. These martensitic structures satisfied the Kurdjumov–Sachs relationship as with conventional carbon steel. The creep strengths of the prototyped steels were comparable with those of Gr. 91 steel, albeit lower than those of Gr. 92. Additional precipitates other than nitrides are required for further strengthening of the developed steels.

DOI： 10.2355/isijinternational.ISIJINT-2017-758

Language：Japanese   Publishing type：Research paper (scientific journal)  

213 The function of β-phase within a lamellar microstructure deforming in Ti-6Al-4V alloy

DOI： 10.1299/jsmehokkaido.2017.55.29

Language：Japanese   Publishing type：Research paper (scientific journal)  

Global concerns over the environmental impact and health effects of the lead-based solders have led to the development of lead-free solder alloys. Further improvements in the reliability of lead-free solder alloys at high temperatures are required for downsizing of electronic components in vehicles. In this work, tensile and creep tests and microstructure analysis were carried out to determine the effect of Bi or Sb addition on high-temperature deformation behavior in Sn-Cu-Ni solder alloys. The addition of Bi or Sb increased the strength of the Sn-Cu-Ni solder alloys. The stress exponent was estimated to be ≥3, indicating that the high- Temperature deformation was controlled by dislocation creep. Furthermore, in both the alloys, the stress exponent observed in the low stress region was nearly equal to 3 and discontinuously increased to ≥7 in the high stress region. For Sb addition, the solute atmosphere drag mechanism was observed in the low stress region.

DOI： 10.2320/jinstmet.J2017025

Language：Japanese   Publishing type：Research paper (scientific journal)  

Creep properties and microstructures for a polycrystalline Ni-based heat-resistant alloy whose grain boundaries were covered by dense intergranular intermetallics were investigated. Creep tests were carries out at 850°C and 80-130 MPa. The creep strength of this alloy was higher than the Alloy617 and HR6W, and equal to the Alloy740, which are pre-existing candidate materials for steam pipes of A-USC power plant. The retardation of acceleration of creep rate was observed characteristically in the creep curves. This retardation behavior was deeply related to the superior creep strength of this alloy. The spherical Ni3Al (γ') particles were distributed uniformly in the grain interior, whose coarsening behavior was monotonically dependent on the creep time. The intermetallics of Laves phase and s pahse were formed densely at grain boundary. High coverage ratio of the intergranular intermetallics was maintained until the later stage of acceleration creep region. Therefore, it suggested that the retardation of creep acceleration was not caused by the precipitates behavior of intragranular γ' particles and intergranular intermetallics, though both the precipitates were understandably effective against the creep strengthening. The plate-like Laves phase was formed in the grain interior during creep. The evolution of volume fraction of intragranular Laves phase depended on not creep time but creep strain. From the results of SEM/EBSD analyses and TEM observations, it revealed that the intragranular Laves phase enhanced the work-hardenability due to the constraint on plasticity and originated the retardation of creep acceleration.

DOI： 10.2355/tetsutohagane.TETSU-2016-107

Language：English   Publishing type：Research paper (scientific journal)  

The microscopic factors causing tensile failure of an α/β laminated Ti-6Al-4V alloy were investigated through in situ scanning electron microscopy and sampling moiré at an ambient temperature. Specifically, multiscale in situ microscopic observations were conducted to extract the most crucial factor of the failure. Slip localization in the vicinity of an intergranular α-sheet was clarified to be the primary factor that causes failure of the Ti-6Al-4V alloy. In addition, no relationship between interfacial strain localization and macroscopic shear localization at 45 degrees against the tensile direction was observed.

DOI： 10.1016/j.matchar.2017.04.010

Language：English   Publishing type：Research paper (scientific journal)  

Creep strain in SUS347HTB austenitic heat-resistant steel and Ni-based heat-resistant alloys was evaluated by electron backscattered diffraction (EBSD). Localized crystallographic misorientations in the crept samples were quantified by using misorientation indicators such as kernel average misorientation and grain reference orientation deviation. In most crept samples, the misorientation indicators increased with creep deformation. However, this trend was not observed for alloys with dense dispersions. We proposed a method to extract and evaluate data only near the grain boundary from the total EBSD data. For Ni-based alloys, the misorientation indicators tended to increase preferentially near grain boundaries. Conversely, there was no substantial difference between the misorientation indicators near grain boundaries and the intergranular region for SUS347HTB. Consequently, although it is necessary to limit the region for evaluating the misorientation indicators according to the dispersion density of the reinforcing phase in the materials, the misorientation indicators, such as kernel average misorientation or grain reference orientation deviation, are useful for evaluating the creep strain in face-centred cubic heat-resistant alloys.

DOI： 10.2355/isijinternational.ISIJINT-2016-712

Language：English   Publishing type：Research paper (scientific journal)  

The causes of the change in creep rupture ductility with the creep test temperature in Alloy 617 were investigated. The rupture ductility in the creep test was low at 700°C, whereas it was high at 800°C. Although the rupture ductility depended on the creep test temperature, creep fracture occurred due to cavity formation at the grain boundaries under all the creep conditions. In the sample crept at 800°C, subgrains developed with creep deformation. However, the crept sample at 700°C fractured before the subgrain formation. Although the work hardening due to the creep deformation occurred at 700°C, the work hardening in the sample crept at 800°C was small. The deformation of the grains was suppressed by the work hardening and by γ particle dispersion strengthening at 700°C. The difference in the strength in the crystal grains that resulted from the microstructure formed during creep caused the difference in the growth of the cavities.

DOI： 10.2320/matertrans.M2016407

Language：English   Publishing type：Research paper (scientific journal)  

Nanosilver paste is a promising material for power device interconnects. Interconnects are fabricated from nanosilver paste through a sintering process that drives off solvents and dispersants and fuses the silver particles. The integrity of the resulting interconnect is affected by the silver microstructure. This paper explored how sintering temperature, atmosphere, and time influenced microstructure as revealed by transmission electron microscopy and 3-D imaging via dual-beam serial sectioning. Nanosilver paste was sintered in combinations of the following parameters: A sintering atmosphere of air or nitrogen; temperatures of 120 °C or 255 °C; and sintering times of 5, 10, or 30 min. For the 255 °C temperature, oxygen in air facilitated removal of organic solvent and dispersant molecules and led to a microstructure with a coarser ligament network than samples sintered at the same temperature and times in nitrogen. The coarser ligament network was characterized by thick connected ligaments, large connected pores, and few isolated pores; this microstructure has been correlatedwith improved mechanical strength. Details of both 2-D and 3-D ligament network morphology, grain morphology, grain size, and the associated grain boundaries are discussed.

DOI： 10.1109/TED.2016.2639363

Language：Japanese   Publishing type：Research paper (scientific journal)  

New ferritic heat resistant steels with high nitrogen content were developed and these microstructure and the mechanical properties at high temperature were evaluated. 0.3 mass% N could be added into ferritic steels without blow holes by applying pressurized melting methods with pressurized atmosphere up to 4.0 MPa. The high nitrogen ferritic heat resistant steels contained several kind of nitrides within the lath martensitic structure. V-rich coarse particles were identified as crystallized VN. Fine VN or Cr₂N particles were precipitated on the martensitic grain boundaries depending on the amount of V content. The martensitic structure in the high nitrogen steels contained a hierarchical structure of martensitic lath, block, packet and prior austenitic grain. These martensitic structure satisfied the K-S relationship as with the conventional carbon steel. The creep strength of the developed steels were comparable to Gr.91 steel though weaker than Gr.92. It is required additional precipitates other than nitrides for further strengthening of the developed steels.

DOI： 10.2355/tetsutohagane.TETSU-2016-057

Language：English   Publishing type：Research paper (scientific journal)  

Creep behaviors and microstructures for two Ni-based heat-resistant alloys with different carbon contents were investigated. The chemical compositions of the alloys were Ni-20Cr-15Co-6Mo-1Ti-2Al-2Nb-0.004 and 0.021C (mass%). The 0.004C and 0.021C alloys are referred to as the low-and high-C alloys, respectively. After solid-solution treatment at 1373 K for 1 h and isothermal annealing at 1023 K for 32 h, fine Ni3Al (γ) particles were formed in the grain interior of both alloys. The average diameter and number density of γ particles were similar in both alloys. M23C6 carbides were formed on grain boundaries after the isothermal annealing. Coverage ratios with the carbides in the high-C alloy were higher than that in the low-C alloys. Creep tests were performed at 1123 K and 130 MPa. The rupture time for the high-C alloy was longer than that for the low-C alloy, though both minimum creep rates were similar. In the high-C alloy, the creep strain was stored uniformly in the grain interior and the formation of a precipitate-free zone during the creep deformation was suppressed. Therefore, intergranular carbides with a high coverage ratio decreased the creep rate in the acceleration region.

DOI： 10.2320/matertrans.M2016291

Language：English   Publishing type：Research paper (scientific journal)  

Multicomponent nano-agents were designed and built via a core-shell approach to enhance their surface enhanced Raman scattering (SERS) signals. These nano-agents had 36 nm �12 nm gold nanorod cores coated by 4 nm thick silver shell films and a subsequent thin bifunctional thiolated polyethylene glycol (HS-PEG-COOH) layer. Ambient time-lapsed SERS signal measurements of these functionalized nanorods taken over a two-week period indicated no signal degradation, suggesting that large portions of the silver shells remained in pure metallic form. The morphology of the nanorods was characterized by transmission electron microscopy (TEM) and ultra-high resolution scanning TEM. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were utilized to assess the oxidation states of the silver shells covered by HS-PEG-COOH. The binding energies of Ag 3d XPS spectra yielded very small chemical shifts with oxidation; however, the AES peak shapes gave meaningful information about the extent of oxidation undergone by the nano-agent. While the silver shells without HS-PEG-COOH coatings oxidized significantly, the silver shells with HS-PEG-COOH remained predominantly metallic. In fact, six month-old samples still retained mostly metallic silver shells. These findings further demonstrate the stability and longevity of the nanostructures, indicating their significant potential as plasmonically active agents for highly sensitive detection in various biological systems, including cancer cells, tissues, or even organisms.

DOI： 10.1088/1361-6528/28/2/025704

Language：English   Publishing type：Research paper (scientific journal)  

Ta-inserted SmCo₅/Fe nanocomposite thick film magnets were synthesized by high-speed pulsed laser deposition followed by pulse annealing. The microstructures of the film magnets were characterized by high-resolution scanning electron microscopy and scanning transmission electron microscopy. The as-deposited thick film possessed a multilayered Sm-Co/Ta/α-Fe/Ta structure with amorphous Sm-Co layers and [110]-oriented crystalline α-Fe layers. After pulse annealing, many fine grains of Laves phase TaCo₂ were formed, and then the multilayered structure was converted to a granular nanocomposite thick film magnet composed of fine crystalline grains of Sm(Co, Fe)₅, α-(Fe, Co), and TaCo₂. The volume fractions and grain sizes of hard magnetic Sm(Co, Fe)₅, soft magnetic α-(Fe, Co), and TaCo₂ were controlled by the thicknesses of the Ta layer, producing a nanocomposite thick film magnet with good exchange coupling.

DOI： 10.2320/matertrans.M2017035

Language：Japanese   Publishing type：Research paper (scientific journal)  

In recent years, it has become necessary to develop lead substitutes, such as lead-free solder alloys, because of increased environmental concerns regarding the use of leaded materials. In addition, electronic components that use lead-free solder alloys will need to be smaller and usable at higher operating temperatures in next-generation semiconductor devices. Therefore, lead-free solder alloys must be made more reliable. In this work, tin-copper-nickel( Sn-Cu-Ni) solder alloys, Sn-Cu solder alloys, and Sn-Ni solder alloys, as well as 99.96 mass% pure Sn, were subjected to tensile testing. The results showed the effects of adding Cu and Ni to Sn on the high-temperature deformation behavior of the Sn-Cu-Ni solder alloys. For each alloy and Sn, the stress exponent was estimated to be <5. This result indicated that, in each sample, the high-temperature deformation was controlled by dislocation creep. Furthermore, the creep activation energy was dependent on stress, and was affected to the greatest extent when adding Cu.

DOI： 10.2320/jinstmet.J2016069

Language：English   Publishing type：Research paper (scientific journal)  

The multi-scale complexity of lath martensitic microstructures requires scale-bridging analyses to better understand the deformation mechanisms activated therein. In this study, plasticity in lath martensite is investigated by multi-field mapping of deformation-induced microstructure, topography, and strain evolution at different spatial resolution vs. field-of-view combinations. These investigations reveal site-specific initiation of dislocation activity within laths, as well as significant plastic accommodation in the vicinity of high angle block and packet boundaries. The observation of interface plasticity raises several questions regarding the role of thin inter-lath austenite films. Thus, accompanying transmission electron microscopy and synchrotron x-ray diffraction experiments are carried out to investigate the stability of these films to mechanical loading, and to discuss alternative boundary sliding mechanisms to explain the observed interface strain localization.

DOI： 10.1016/j.actamat.2016.09.006

Language：Japanese  

EBSD‐FEMデータ変換インターフェースの構築とTi‐6Al‐4V合金の結晶塑性解析への適用

Language：English   Publishing type：Research paper (scientific journal)  

The microstructure of an AlN template after high-temperature annealing was investigated by transmission electron microscopy (TEM). The AlN template was prepared by depositing an AlN layer of about 200nm thickness on a sapphire (0001) substrate by metal-organic vapor phase epitaxy. The AlN template was annealed under (N₂ + CO) atmosphere at 1500-1650 °C. TEM characterization was conducted to investigate the microstructural evolution, revealing that the postannealed AlN has a two-layer structure, the upper and lower layers of which exhibit Al and N polarities, respectively. It has been confirmed that postannealing is an effective treatment for controlling the microstructure.

DOI： 10.7567/APEX.9.065502

Language：English   Publishing type：Research paper (scientific journal)  

An aluminium nitride (AlN) buffer layer with 200 nm thickness was grown on (0001) sapphire substrate using the metal-organic vapour phase epitaxy (MOVPE) method in a low-pressure furnace, followed by a clean-up treatment of sapphire substrate at 1100°C. Thereafter, the AlN buffer layer was annealed at a high temperature in the range of 1500°C to 1700°C for 2 hours under the atmosphere of N₂+CO. The objective of this research is to determine the microstructure changes with different annealing temperatures. Cross-sectional TEM has revealed that, after annealing at 1500°C, two types of defects remained in the AlN buffer layer: inverted cone shape domains and threading dislocations. The former domains were observed in an image taken with diffraction of g=0002, but not in an image with g=1010. The morphology and the diffraction condition for the image contrast strongly, suggesting that the domains are inversion domains. The threading dislocations were invisible in the image taken with the diffraction of g=0002, revealing that they were a-Type dislocations. However, after annealing at 1600oC, the inversion domains coalesced with each other to give a two-layer structure divided by a single inversion domain boundary at the centre of the AlN buffer layer. The density of threading dislocation was roughly estimated to be 5×109 cm^-2 after annealing at 1500°C, and to be reduced to 5×10⁸ cm^-2 after annealing at 1600°C. These experimental results validate the fact that the annealing temperature around 1600°C is high enough to remove the defects by the diffusion process. Therefore, it was discovered that high temperature annealing is an effective treatment to alter the microstructure of AlN thin films and remove defects by the diffusion process. Annealing at high temperature is recommended to increase the emission efficiency for fabrication of optoelectronic devices.

DOI： 10.15282/jmes.10.1.2016.14.0182

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1007/s11661-016-3344-7

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.2320/matertrans.MB201505

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1080/09500839.2016.1154200

Language：English   Publishing type：Research paper (scientific journal)  

Anthropogenic emissions of greenhouse gases are one of the possible reasons for the global warming. Coal consumption is predicted to be increased over several decades due to increasing demands in some countries. In the medium-term energy strategy, coal-fired power plants thus need to be developed. For this purpose, developments of new heat-resistant alloys that can stand severe steam conditions in advanced power plants are essential. A potential alloy was studied, which revealed that hardness of the alloy is related to the distribution of precipitates. The development of the alloy is also vital to make the power plants more economically competitive.

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2015.10.017

Language：English   Publishing type：Research paper (scientific journal)  

In this paper we introduce an approach for precise orientation mapping of crystalline specimens by means of transmission electron microscopy. We show that local orientation values can be reconstructed from experimental dark-field image data acquired at different specimen tilts and multiple Bragg reflections. By using the suggested method it is also possible to determine the orientation of the tilt axis with respect to the image or diffraction pattern. The method has been implemented to automatically acquire the necessary data and then map crystal orientation for a given region of interest. We have applied this technique to a specimen prepared from a Ni-based super-alloy CMSX-4. The functionality and limitations of our method are discussed and compared to those of other techniques available.

DOI： 10.1016/j.ultramic.2015.07.003

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.4930573

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.scriptamat.2015.02.001

Language：Japanese  

Three-Dimensional Observation of Lattice Defects Using Electron Tomography
Three-dimensionalization, i.e., direct representation or fabrication of three-dimensional (3D) objects, is now a key technology in various science and engineering fields. Electron microscopy, a vital nanoscale characterization tool, is no exception, thus various imaging methods have been developed to extend its imaging capabilities from conventional two dimensions to three dimensions. In this article, we focus on electron tomography (ET), which is a typical 3D imaging method using transmission electron microscopy (TEM) or scanning transmission electron microscopy（STEM）, and overview the current status and future prospects of ET and an application of ET to 3D imaging of dislocations in crystalline materials as an practical example.

DOI： 10.5940/jcrsj.57.276

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DOI： 10.1080/14786435.2015.1021400

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DOI： 10.1080/09500839.2014.995739

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The antiphase boundary (APB)-like structure of both 9R and B19 martensites in the Ti-Pd-Fe alloy was investigated by means of transmission electron microscopy. Some APB-like structures with curved and wide contrasts along the (0 0 1)_9Rbasal plane are observed in 9R martensitic plates. The atomic displacement on the APB-like structure reflects the atomic movement stemming from the microdomains formed as a pre-martensitic transformation. The displacement vector of the APB-like structure in the B19 martensite can be expressed as R = 〈1/3 0 -1/2〉_B19. The density of APB-like contrasts increases by the substitution of Fe for Pd in Ti-Pd-Fe alloy.

DOI： 10.1016/j.jallcom.2014.08.258

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The effect of molybdenum (Mo) on softening behavior of cold-rolled high manganese austenitic stainless steels was investigated. The high temperature hardness of cold-rolled Mo-free steel was drastically decreased by prolonged annealing at 873 K. On the other hand, there is not remarkable degradation of high temperature hardness in the steels containing more than 1 mass% Mo. Fully recrystallized structure is observed in Mo-free steel after annealing at 873 K for 1440 ks. Since deformation structures were not disappeared in the steels containing more than 1 mass% Mo even after annealing, the addition of Mo retarded the recrystallization in the work-hardening steels. After annealing, fine particles of M C type carbide precipitated at the grain boundaries in the steel containing Mo. The amount of Mo in M C carbides was increased with increasing Mo content. On the other hand, the size of these carbides slightly decreased with increasing Mo content. These fine carbides strongly prevented the grain boundary movement (migration), therefore recrystallizaiton in the work-hardening steels was retarded. Moreover, the growth of M C carbides at the grain boundary was consistent with Ostwald ripening equation substituted Mo for Cr. This result suggested that the growth rate of M C was controlled by the diffusion of Mo. 23 6 23 6 23 6 23 6

DOI： 10.2355/tetsutohagane.100.406

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In order to clarify crack propagation behavior of alumina (α-Al O ), intergranular and transgranular fractures of alumina were investigated by electron microscopy techniques. For the intergranular fractures cracked grain boundaries were investigated by scanning electron microscopic electron back-scattered diffraction (SEM-EBSD) analysis. It was revealed that the intergranular fracture behavior of polycrystalline alumina with random distribution of crystallographic orientations is independent on grain boundary characters. For the transgranular fractures, fracture surfaces formed in a single-crystalline alumina were observed by electron tomography combined with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and high resolution TEM (HR-TEM). Three kinds of fracture surfaces were observed: even fracture surfaces, even fracture surfaces with step-terraces and fracture surfaces composed of the even fracture surface and the curved one. The even fracture surfaces mostly consist of cleavage planes parallel to {1 over(1, ̄) 0 2} and {1 over(1, ̄) 0 4}. On the other hand, the even fracture surfaces with step-terraces consist of the cleavage planes and basal planes (0 0 0 1). For the fracture surfaces composed of the even fracture surface and the curved one, both the even and curved shapes the even parts were formed by the fine cleavage planes and basal planes, whose intervals were inhomogeneous. It was indicated that the fracture surfaces in alumina are composed of the cleavage planes or basal plane at an atomic scale. © 2009 Elsevier Ltd. All rights reserved. 2 3

DOI： 10.1016/j.ijfatigue.2009.06.006

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DOI： 10.2320/matertrans.E2008002

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Material property of tungsten coated reduced-activation ferritic/martensitic steel

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