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Naeem, M.*; He, H.*; Harjo, S.; 川崎 卓郎; Lin, W.*; Kai, J.-J.*; Wu, Z.*; Lan, S.*; Wang, X.-L.*
Acta Materialia, 221, p.117371_1 - 117371_18, 2021/12
被引用回数：31 パーセンタイル：94.71(Materials Science, Multidisciplinary)We studied the deformation behavior of CrFeCoNi high-entropy alloy by in situ neutron diffraction at room temperature, intermediate low temperature of 140 K, low temperatures of 40 K (no serrated deformation) and 25 K (with massive serrations). The contributions from different deformation mechanisms to the yield strength and strain hardening have been estimated. The athermal contributions to the yield strength were 
183 MPa at all temperatures, while the Peierls stress increased significantly at low temperatures (from 148 MPa at room temperature to 493 MPa at 25 K). Dislocations contributed to 94% strain hardening at room temperature. Although the dislocation strengthening remained the major hardening mechanism at very low temperatures, the planar faults contribution increased steadily from 6% at room temperature to 28% at 25 K.
土田 紀之*; Harjo, S.
Metals, 11(12), p.2053_1 - 2053_16, 2021/12
被引用回数：2 パーセンタイル：16.07(Materials Science, Multidisciplinary)To improve uniform elongation (U.El) due to the TRIP effect, it is important to control deformation-induced martensitic transformation (DIMT) until the late part of deformation. In the present study, tensile tests with decreasing deformation temperatures were conducted to achieve continuous DIMT up to the late part of deformation. As a result, the U.El was improved by approximately 1.5 times compared with that in the tensile test conducted at 296 K. The enhancement of the U.El in the temperature change test was discussed with the use of neutron diffraction experiments.
Kim, Y. S.*; Chae, H.*; Woo, W.*; Kim, D.-K.*; Lee, D.-H.*; Harjo, S.; 川崎 卓郎; Lee, S. Y.*
Materials Science & Engineering A, 828, p.142059_1 - 142059_10, 2021/11
被引用回数：16 パーセンタイル：79.15(Nanoscience & Nanotechnology)CoCrNi medium entropy alloy (MEA) and stainless steel 316L (SS316L) were manufactured by direct energy deposition of additive manufacturing (DED-AM). Exceptional mechanical properties of DED CoCrNi at 210K were achieved by the activities of a multiple deformation scheme that changed from dislocation slip to twinning-induced plasticity followed by transformation-induced plasticity. While SS316L at room temperature has micro-twins, CoCrNi at 210K exhibited nano-twins, resulting from lower stacking fault energy. Moreover, transformed hexagonal close-packed (HCP) phases were found near the face-centered cubic (FCC) {111} grain boundaries, where remarkable stacking faults and severe lattice distortion were measured.
Naeem, M.*; Zhou, H.*; He, H.*; Harjo, S.; 川崎 卓郎; Lan, S.*; Wu, Z.*; Zhu, Y.*; Wang, X.-L.*
Applied Physics Letters, 119(13), p.131901_1 - 131901_7, 2021/09
被引用回数：9 パーセンタイル：64.95(Physics, Applied)We investigated the in situ deformation behavior of the CrCoNi medium-entropy alloy at a cryogenic temperature of 140 K and compared it with deformation at room temperature. The sample exhibited higher strength and larger ductility at the cryogenic temperature. The CrCoNi alloy remained single-phase face-centered cubic at room temperature, while deformation at 140 K resulted in a martensitic transformation to the hexagonal close-packed structure. The phase transformation, an additional deformation mechanism to stacking faults, twinning, and dis- location slip, resulted in a higher work hardening at cryogenic temperature. The study addresses the structure metastability in the CrCoNi alloy, which led to the formation of epsilon-martensite from the intrinsic stacking faults.
He, H.*; Naeem, M.*; Zhang, F.*; Zhao, Y.*; Harjo, S.; 川崎 卓郎; Wang, B.*; Wu, X.*; Lan, S.*; Wu, Z.*; et al.
Nano Letters, 21(3), p.1419 - 1426, 2021/02
被引用回数：41 パーセンタイル：95.42(Chemistry, Multidisciplinary)In CrCoNi, a so-called medium-entropy alloy, an fcc-to-hcp phase transformation has long been anticipated. Here, we report an in situ loading study with neutron diffraction, which revealed a bulk fcc-to-hcp phase transformation in CrCoNi at 15 K under tensile loading. By correlating deformation characteristics of the fcc phase with the development of the hcp phase, it is shown that the nucleation of the hcp phase was triggered by intrinsic stacking faults. The confirmation of a bulk phase transformation adds to the myriads of deformation mechanisms available in CrCoNi, which together underpin the unusually large ductility at low temperatures.
Ahadi, A.*; Khaledialidusti, R.*; 川崎 卓郎; Harjo, S.; Barnoush, A.*; 土谷 浩一*
Acta Materialia, 173, p.281 - 291, 2019/07
被引用回数：21 パーセンタイル：76.12(Materials Science, Multidisciplinary)The temperature-dependent elasticity of the B19' NiTi is unknown today. To gain insights into the lattice-level temperature-dependent elasticity of the B19' crystal, we present results of 
neutron diffraction experiments performed on polycrystalline martensitic specimens in the temperature range of 300 down to 50 K. The experimental results are compared with the density functional theory molecular dynamics (DFT-MD) and Quasi Harmonic Approximation (QHA) calculations. The results confirm that the temperature-dependent Young's modulus (TDYM) of the B19' crystal is strongly anisotropic. The experimental results along with the first-principles calculations confirm that the Elinvar and Invar are two confluent properties in NiTi SMAs and can be tailored by texturing B19' crystallographic orientations.
Pt福田 隆*; 山口 貴司*; 掛下 知行*; Harjo, S.; 中本 建志*
Journal of Applied Physics, 126(2), p.025107_1 - 025107_6, 2019/07
被引用回数：1 パーセンタイル：4.3(Physics, Applied)We have studied the structure change of an ordered FePt (degree of order 0.75) under a compressive stress applied in the [001] direction by neutron diffraction. In the absence of the stress, the alloy exhibits a weak first order martensitic transformation at 90 K from the L1
-type cubic structure to the L6
-type tetragonal structure. Under the compressive stress of 100 MPa, the first order nature of the thermally-induced martensitic transformation was undetectable in the temperature range of between 70 K and 270 K. The first order nature of the stress-induced martensitic transformation was also undetectable in the stress range of between 6 MPa and 300 MPa when tested at 120 K and higher temperatures. Under these conditions, the lattice parameters change continuously both in the cooling process and in the stress-applying process. A significant stress-induced softening of lattice was observed between 120 K and 265 K but not at 93 K and 295 K.
