||S. Yamasaki, T. Miike, M. Mitsuhara, H. Nakashima, R. Akiyoshi, T. Nakamura and S. Kimura, Quantitative analysis of the Portevin–Le Chatelier effect by combining digital image correlation and dead-weight-type tensile test, Materials Science and Engineering A, 2021.06.
||Shigeto Yamasaki, Tsubasa Tokuzumi Wansong Li, Masatoshi Mitsuhara, Koji Hagihara, Toshiyuki Fujii, Hideharu Nakashima, Kink Formation Process in Long-Period Stacking Ordered Mg-Zn-Y Alloy, Acta Materialia, https://doi.org/10.1016/j.actamat.2020.04.051, 195, 15, 25-34, 2020.08, The formation process of the kink bands in a directionally solidified, polycrystalline, long-period stacking ordered (LPSO) Mg85Zn6Y9 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..
||Shigeto Yamasaki, Masatoshi Mitsuhara, Ken Ichi Ikeda, Satoshi Hata, Hideharu Nakashima, Low-stress creep deformation in long-term aged ferritic heat-resistant steel, Materials Transactions, 10.2320/matertrans.M2013427, 55, 5, 842-849, 2014.05, The transition of the creep deformation mechanism in the low-stress region of Grade P92 high Cr ferritic heat-resistant steel was investigated by a helicoid spring creep test. Specifically, the effect of variation in the microstructure of steel on creep deformation behavior was evaluated by subjecting samples to thermal aging for 1000, 3000, 5000, and 10000 h at 700°C over a wide stress range. In addition, stress exponents were determined from the stress dependence of the minimum strain rate in the creep curves up to 270 ks. The transition of the creep mechanism was indicated when the stress exponent decreased from 4 in the high-stress region to 1 in the low-stress region below 40 MPa. A quantitative evaluation of the microstructure of a tempered martensite sample, including the determination of the amount of dissolved Mo and W, dispersion state of the precipitates, and length of the grain boundaries per unit area, was also carried out. Furthermore, the change in the minimum strain rate was evaluated as a function of the microstructural changes that accompanied thermal aging. It was found that the change in the strain rate was the most affected by the fineness of the martensitic lath structure in the high-stress region and by the dispersion density of M23C6 precipitates in the low-stress region. Based on these results, it was concluded that the microstructural parameter that most affects creep deformation behavior differs depending on the stress region due to the difference in the creep mechanism..
||S. Yamasaki, M. Mitsuhara, K. Ikeda, S. Hata, H. Nakashima, 3D visualization of dislocation arrangement using scanning electron microscope serial sectioning method, Scripta Materialia, 10.1016/j.scriptamat.2015.02.001, 101, 80-83, 2015.05, We performed the three-dimensional visualization of dislocations through serial sectioning and use of SEM electron channeling contrast (ECC) images for a crept nickel-based alloy. We successfully reconstructed a volume of approximately 7.5 μm3, including dislocation arrangements, by performing calculations based on the continuous tomograms of ECC images. By incorporating the information on crystal orientation obtained by the electron back-scattered diffraction, we verified that the three-dimensional arrangement of dislocations, such as slip plane, was accurately reflected in the three-dimensional volume..
||Shigeto Yamasaki, Masatoshi Mitsuhara, Hideharu Nakashima, Mitsuharu Yonemura, Evaluation of local creep strain in face-centred cubic heatresistant alloys using electron backscattered diffraction analysis, isij international, 10.2355/isijinternational.ISIJINT-2016-712, 57, 5, 851-856, 2017.01, 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..
||Shigeto Yamasaki, Masatoshi Mitsuhara, Hideharu Nakashima, Deformation microstructure and fracture behavior in creep-exposed Alloy 617, Materials Transactions, 10.2320/matertrans.M2016407, 58, 3, 442-449, 2017.01, 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..
||Shigeto Yamasaki, Masatoshi Mitsuhara, Hideharu Nakashima, Development of high-chromium ferritic heat-resistant steels with high nitrogen content, isij international, 10.2355/isijinternational.ISIJINT-2017-758, 58, 6, 1146-1154, 2018.01, 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 Cr2N 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..
||Shigeto Yamasaki, Tomo Okuhira, Masatoshi Mitsuhara, Hideharu Nakashima, Jun Kusui, Mitsuru Adachi, Effect of Fe addition on heat-resistant aluminum alloys produced by selective laser melting, Metals, 10.3390/met9040468, 9, 4, 2019.04, 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..