| 1. |
H. Akamine, A. Heima, Y. Soejima, M. Mitsuhara, T. Inamura, M. Nishida, Where and when are dislocations induced by thermal cycling in Ti–Ni shape memory alloys?, Acta Materialia, 244, 118588, 2023.01, 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.. |
| 2. |
S. Zhu, H. Akamine, Y. Nagahata, T. Tojigamori, H. Miki, Y. Zhang, T. Tokunaga, S. Iikubo, Microstructure control and its observation of rapid solidification Cu–La alloy for the development of fluoride-ion batteries, Journal of Alloys and Compounds, 930, 167447, 930, 167447, 2023.01. |
| 3. |
Francesco Della Porta, Akira Heima, Yuri Shinohara, Hiroshi Akamine, Minoru Nishida, Tomonari Inamura, Triplet condition: A new condition of supercompatibility between martensitic phases, Journal of the Mechanics and Physics of Solids, https://doi.org/10.1016/j.jmps.2022.105050, 169, 105050, 2022.09. |
| 4. |
Mitsuhiro Matsuda, Kodai Kiwaki, Hiroshi Akamine, Minoru Nishida, Effect of Hf on the microstructure and martensitic transformation behavior in Ti–Pd–Hf alloy, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2022.165491, 917, 165491, 917, 165491, 2022.05. |
| 5. |
Masaru Itakura, Koichi Sugiyama, Hiroshi Akamine, Minoru Nishida, Michihide Nakamura, Tetsuhiko Mizoguchi, Takahiko Iriyama, Suppression mechanism of abnormal grain growth by Zr addition in pressless processed Nd-Fe-B sintered magnets, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2021.161244, 887, 161244, 2021.12, Microstructures of pressless processed (PLP) Nd-Fe-B sintered magnets with and without Zr addition were investigated by scanning electron microscopy and scanning/transmission electron microscopy. In the Zr-free magnet, huge polygonal abnormal grains (AGs) about 100 times larger than the normal grain size were generated by PLP sintering at 975 °C. Electron backscattered diffraction analysis revealed that the PLP sintered magnet had a biaxial oriented texture of fine Nd2Fe14B grains with the orientation of the AGs significantly deviated from the biaxial oriented grains. By adding a small amount of Zr, fine plate-like Zr-B precipitates with a long period stacking ordered structure derived from the ZrB2 phase formed on the growth surfaces of Nd2Fe14B grains with a crystallographic orientation relationship. There were two Nd-rich grain boundary (GB) phases, namely, orthorhombic Nd3(Co, Cu) and metastable CaF2-NdO2, which became liquid during sintering. The boron consumption associated with the formation of Zr-B precipitates increased the Nd-rich GB phases, resulting in a homogeneous distribution of the liquid phase around the Nd2Fe14B grains. Therefore, the addition of Zr effectively suppressed the abnormal grain growth during PLP sintering owing to the pinning effect of the Zr-B precipitates and the formation of a homogeneous liquid phase distribution.. |
| 6. |
H. Akamine, M. Mitsuhara, M. Nishida, V. Samaee, D. Schryvers, G. Tsukamoto, T. Kunieda, H. Fujii, Precipitation behaviors in Ti-2.3 wt.%Cu alloy during isothermal and two-step aging, Metallurgical Materials Transactions A, https://doi.org/10.1007/s11661-021-06265-x, 52, 7, 2760-2772, 2021.07. |
| 7. |
Y. Soejima, A. Heima, H. Akamine, T. Inamura, M. Nishida, Comparison of in situ SEM and TEM observations of thermoelastic martensitic transformation in TiNi shape memory alloy, Materials Transactions, 10.2320/matertrans.MT-M2020219, 61, 11, 2107-2114, 2020.09, ©2020 The Japan Institute of Metals and Materials. To quantitatively understand the thin foil effect in in situ observations of the B2 to B19A transformation in TiNi alloy, the microstructure of the B19A martensite in thin foil and bulk specimens was compared. The transformation temperatures decreased with decreasing specimen thickness. There were large habit plane variants more than several tens of micrometers in size in the area of the specimen less than 10μm thick. The critical thicknesses for reproducing the transformation behavior in the bulk material was about 20μm based on the self-accommodation morphology and 4 μm based on the twin width ratio of the (011) type II twin.. |
| 8. |
Fumiyoshi Yamashita, Yasunori Ide, Hiroshi Akamine, Kouji Ishikawa, Minoru Nishida, Effect of C/O Ratio on Phase Change and Stability of Inclusions in Ti–Ni Alloys Fabricated by a Commercial Production Process, Shape Memory and Superelasticity, 10.1007/s40830-020-00302-1, 6, 3, 354-364, 2020.09, © 2020, ASM International. The effects of carbon/oxygen concentrations in Ti-51 at.% Ni on the morphology and phase changes of inclusions in a commercial wire manufacturing process were investigated. Whereas cast material fabricated with a carbon to oxygen mass concentration ratio (C/O ratio) of 1.0–1.5 were found to contain single-phase Ti(C,O), when hot working was performed, some of the Ti(C,O) exhibited a phase change to Ti2Ni(O), and a mixed phase structure was found in specimens with a C/O ratio of less than 1.5. On the other hand, single-phase Ti(C,O) was found to remain in the wire produced from the specimens with a C/O ratio of 1.5.. |
| 9. |
Hiroshi Akamine, Yohei Soejima, Tadaaki Nakamura, Sahar Farjami, Takashi Fukuda, Tomoyuki Kakeshita, Minoru Nishida, Isothermal martensitic transformations in an aged Ni-rich Ti-Ni alloy containing coherent Ti3Ni4 particles, Materials Transactions, 2020.03, The time dependence nature of B2 (ordered cubic) to R (trigonal) and R to B19A (monoclinic) martensitic transformations in an aged Ni-rich Ti-Ni alloy was investigated by electrical resistivity measurements. Both B2 to R and R to B19A transformations exhibit time dependence nature in the aged alloy containing coherent lenticular Ti3Ni4 precipitates with an average diameter of about 110 nm. However, the successive B2-R-B19A isothermal transformation is not detected by electrical resistivity measurements. The isothermal R-B19A transformation is visualized by in situ scanning electron microscopy (SEM) observations.. |
| 10. |
Hiroshi Akamine, So Okumura, Sahar Farjami, Yasukazu Murakami, Minoru Nishida, Imaging of surface spin textures on bulk crystals by scanning electron microscopy, Scientific Reports, 10.1038/srep37265, 6, 2016.11, © The Author(s) 2016. Direct observation of magnetic microstructures is vital for advancing spintronics and other technologies. Here we report a method for imaging surface domain structures on bulk samples by scanning electron microscopy (SEM). Complex magnetic domains, referred to as the maze state in CoPt/FePt alloys, were observed at a spatial resolution of less than 100 nm by using an in-lens annular detector. The method allows for imaging almost all the domain walls in the mazy structure, whereas the visualisation of the domain walls with the classical SEM method was limited. Our method provides a simple way to analyse surface domain structures in the bulk state that can be used in combination with SEM functions such as orientation or composition analysis. Thus, the method extends applications of SEM-based magnetic imaging, and is promising for resolving various problems at the forefront of fields including physics, magnetics, materials science, engineering, and chemistry.. |
| 11. |
H. Akamine, K.H.W. van den Bos, N. Gauquelin, S. Farjami, S. Van Aert, D. Schryvers, M. Nishida, Determination of the atomic width of an APB in ordered CoPt using quantified HAADF-STEM, Journal of Alloys and Compounds, 10.1016/j.jallcom.2015.04.205, 644, 570-574, 2015.09. |
| 12. |
Hiroshi Akamine, Sahar Farjami, Hidetsugu Sakaguchi, Minoru Nishida, Phase-field simulation of magnetic field-induced preferential variant selection in A1-L10 disorder-order transformation, International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, PTM 2015
PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, 425-426, 2015.06, Near-equiatomic Co-Pt, Fe-Pd and Fe-Pt alloys undergo cubic-tetragonal disorder-order transformation and form three orientation variants. It has been reported that magnetic fieldinduced preferential nucleation of one variant in the initial stage of ordering introduces preferential growth of the same variant in the later magnetic field-free stage [1]. In this study, the effect of preferential nucleation on microstructure evolution in the later stage is studied by phasefield method. The results indicate that the increase in fraction of preferential variant depends on nucleation ratio between the variants, and the stress field originated from the macroscopic shape deformation is the most possible factor affecting preferential growth in the later stage.. |
| 13. |
Hidetsugu Sakaguchi, Hiroshi Akamine, Domain wall dynamics in ginzburg-landau-type equations with conservative quantities, Journal of the Physical Society of Japan, 10.7566/JPSJ.83.064006, 83, 6, 2014.06, © 2014 The Physical Society of Japan. In the Ginzburg-Landau equation, there are domain walls connecting two metastable states. The dynamics of domain walls has been intensively studied, but there remain still unsolved but crucial problems even for a single domain. We study the domain wall dynamics in three different Ginzburg-Landau-type equations satisfying conservation laws. In a modified ø4 model satisfying the law of energy conservation and the Lorentz invariance, the motion of a domain wall is accelerated and the velocity approaches its maximum. In a one-dimensional model of eutectic growth, the order parameter is conserved and a domain wall connecting a metastable uniform state and a spatially periodic pattern appears. We try to find a selection rule for the wavelength of a spatially periodic pattern. In a model equation for martensitic transformation, a domain wall connecting a uniform metastable state and a zigzag structure appears which propagates at a high velocity.. |
| 14. |
Hiroshi Akamine, Sahar Farjami, Masatoshi Mitsuhara, Minoru Nishida, Takashi Fukuda, Tomoyuki Kakeshita, Electron microscopy study of preferential variant selection in CoPt alloy ordered under a magnetic field, Materials Transactions, 10.2320/matertrans.MAW201313, 54, 9, 1715-1718, 2013.08, Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) observations were carried out to investigate microstructure formation and variant selection process in L10-type ordered CoPt alloy upon a two-step ordering heat-treatment. The first step corresponds to nucleation process carried out under a magnetic field of 10 T and the second step represents growth process without magnetic field. After the first step of ordering, ordered domains of about 5 nm in size were observed and fraction of the preferred variant with the c-axis parallel to applied magnetic field was slightly higher than that of the other two variants. Formation of tweed microstructure along {011}L10 was confirmed at the initial stage of ordering. This structure is considered to be derived from the periodic alignment of interface between two ordered variants with twin relation. At the early stage of the second step of ordering, numerous micro-twins were formed through tweed microstructure and the volume fraction of the preferred variant was increased accompanying with modulation of twins, while that of other two variants was decreased. After the second step of ordering, the twins were vanished and single variant was obtained.. |
| 15. |
K. Morimoto, N. Katayama, H. Akamine, M. Itakura, Coercivity enhancement of anisotropic Dy-free Nd-Fe-B powders by conventional HDDR process, Journal of Magnetism and Magnetic Materials, 10.1016/j.jmmm.2012.06.003, 324, 22, 3723-3726, 2012.11, Coercivity enhancement of Dy-free Nd-Fe-Co-B-Ga-Zr powders was studied using the conventional hydrogenation-decomposition-desorption-recombination (HDDR) process. It was found that the addition of Al together with the proper Nd content and the slow hydrogen desorption of the HDDR treatment can induce high coercivity in the powder. For example, the 14.0 at% Nd-2.0 at% Al powder exhibits H cJ of 1560 kA/m, B r of 1.22 T, and (BH) max of 257 kJ/m 3. The high coercivity inducement of the powder is thought to be attributed to the formation of Nd-rich phase, which continuously surrounds fine Nd 2Fe 14B grains. © 2012 Elsevier B.V.. |
