Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
titanium alloys strong and ductileChong, Y.*; Gholizadeh, R.*; Guo, B.*; 都留 智仁; Zhao, G.*; 吉田 周平*; 光原 昌寿*; Godfrey, A.*; 辻 伸泰*
Acta Materialia, 257, p.119165_1 - 119165_14, 2023/09
被引用回数：4 パーセンタイル：84.87(Materials Science, Multidisciplinary)チタン合金は、優れた歪み硬化能を有する反面、降伏強度が低いという問題を抱えている。ここでは、Ti-12Mo(wt.%)準安定チタン合金の降伏強度の向上における結晶粒の微細化と格子間物質の添加の寄与について検討した。その結果、結晶粒の微細化は材料を強化するどころか、この合金の極限引張強度を低下させることがわかった。この予想外の異常な挙動は、ひずみ誘起
マルテンサイト相変態が著しく促進されたことに起因しており、その場観察放射光X線回折分析により、この相が相よりはるかに軟らかいことが初めて明らかになった。また、酸素添加と結晶粒微細化の組み合わせにより、Ti-12Mo-0.3O(wt.%)合金において前例のない強度と延性の相乗効果が得られることが判明した。この三元合金における酸素溶質には2つの有利な点がある。第一に、溶質酸素は、微細な組織においても、歪みによるマルテンサイト相への変態を大きく抑制し、過剰なマルテンサイトによる軟化効果を回避することができる。次に、アトムプローブトモグラフィーで明らかになったように、酸素溶質が双晶境界に偏析しやすい。これにより、
変形双晶の成長が抑制され、より広範な双晶の核生成が促進される。本研究で得られた知見は、強靭な準安定チタン合金を設計するための費用対効果の高い根拠となり、この高強度対重量構造材料のさらなる普及に大きな意味を持つ。
Mao, W.; Gao, S.*; Gong, W.; Bai, Y.*; Harjo, S.; Park, M.-H.*; 柴田 曉伸*; 辻 伸泰*
Acta Materialia, 256, p.119139_1 - 119139_16, 2023/09
被引用回数：5 パーセンタイル：90.35(Materials Science, Multidisciplinary)変形誘起塑性(TRIP)鋼は、変形誘起マルテンサイト変態(DIMT)に伴う加工硬化率の向上により、強度と延性の優れた組み合わせを示す。TRIP鋼や合金の加工硬化挙動におけるDIMTの役割を定量的に評価することは、強度と延性の両立を可能にする先進材料を設計するための指針を与えるが、変形中に相組成が変化し続け、応力と塑性ひずみの両方が構成相間で動的に分配されるため、その評価は困難である。本研究では、Fe-24Ni-0.3C(wt.%)TRIPオーステナイト鋼の引張変形とその場中性子回折測定を行った。中性子回折測定による応力分割と相分割に基づく解析手法を提案し、試験片の引張流動応力と加工硬化率を、オーステナイト母相,変形誘起マルテンサイト、DIMT変態速度に関連する因子に分解し、試料の加工硬化挙動における各因子の役割を考察した。さらに、回折プロファイル解析により測定した転位密度を用いてオーステナイトとマルテンサイト間の塑性ひずみ分配を間接的に推定し、材料中のオーステナイトとマルテンサイト間の応力・ひずみ分配の全体像を構築した。その結果、変形誘起マルテンサイト変態速度とマルテンサイトが負担する相応力の両方が、材料の全体的な引張特性に重要な役割を果たしていることが示唆された。提案した分解解析法は、TRIP現象を示す多相合金の機械的挙動を調べるために広く適用できる可能性がある。
Mg and long-period stacking ordered phases in a Mg-Zn-Y alloy by hot-extrusion with low extrusion ratioHarjo, S.; Gong, W.; 相澤 一也; 川崎 卓郎; 山崎 倫昭*
Acta Materialia, 255, p.119029_1 - 119029_12, 2023/08
被引用回数：6 パーセンタイル：95.52(Materials Science, Multidisciplinary)An as-cast sample and two hot-extruded samples with different extrusion ratios (R) of Mg
Zn
Y
alloy containing the HCP matrix (Mg) and the long-period stacking ordered phase (LPSO) of about 25-vol%, were used in tensile deformation in situ neutron diffraction experiments, to elucidate the effects of uniquely different microstructural evolutions in Mg and LPSO with varying the R value to the mechanical properties. Mg behaved as the soft phase and LPSO as the hard phase, and hot-extrusion improved the strength of both. At the R value of 5.0, a bimodal microstructure was created in Mg, increasing largely the yield strength of Mg. With increasing the R value to 12.5, the bimodal microstructure of Mg collapsed and the yield strength of Mg decreased. However, the strength of LPSO increased monotonously with increasing the R value due to the developments of kink bands and texture.
neutron diffractionGong, W.; Harjo, S.; 友田 陽*; 諸岡 聡; 川崎 卓郎; 柴田 曉伸*; 辻 伸泰*
Acta Materialia, 250, p.118860_1 - 118860_16, 2023/05
被引用回数：2 パーセンタイル：74.65(Materials Science, Multidisciplinary)Martensitic transformation is accompanied by the generation of microscale and macroscale internal stresses during cooling below the martensitic transformation start temperature. These internal stresses have been determined through X-ray or neutron diffraction, but the reported results are not consistent, probably because the measured lattice parameter is influenced not only by the internal stress but also by several factors, including solute elements and crystal defects. Therefore, neutron diffraction combined with dilatometry measurements during martensitic transformation and subsequent cyclic tempering were performed for an Fe-18Ni alloy. The phase strains calculated by lattice parameter variations show that a hydrostatic compressive strain in austenite and a tensile strain in martensite arose as the martensitic transformation progressed during continuous cooling or isothermal holding. However, the phase stresses of austenite and martensite estimated from these strains failed to hold stress balance law when dense crystal defects involved in the processes. After these crystal defects were removed by appropriate tempering, the stress balance law held well. Meanwhile, the phase stresses of austenite and martensite were changed to opposite, revealing their true identity. Various crystal defects in austenite and martensite, introduced by plastic accommodation, were suggested to affect their lattice parameters and then their phase stresses.
Kwon, H.*; 志賀 基之; 君塚 肇*; 小田 卓司*
Acta Materialia, 247, p.118739_1 - 118739_11, 2023/04
被引用回数：3 パーセンタイル：74.65(Materials Science, Multidisciplinary)機械学習によるモーメントテンソルポテンシャルを用いた経路積分シミュレーションから、体心立方格子金属(Nb, Fe, W)中の希薄水素の拡散係数を密度汎関数理論レベルの精度で推定した。この計算結果は、精度が高いと考えられるいくつかの実験結果と大いに一致した。また、実験結果と矛盾なく同位体効果を再現した。
Kwon, H.*; Sathiyamoorthi, P.*; Gangaraju, M. K.*; Zargaran, A.*; Wang, J.*; Heo, Y.-U.*; Harjo, S.; Gong, W.; Lee, B.-J.*; Kim, H. S.*
Acta Materialia, 248, p.118810_1 - 118810_12, 2023/04
被引用回数：13 パーセンタイル：99.28(Materials Science, Multidisciplinary)Maraging steels, known for ultrahigh strength and good fracture toughness, derive their superior properties from lath martensite structure with high-density nanoprecipitates. In this work, we designed a novel Fe-based medium-entropy alloy with a chemical composition of Fe
Co
Ni
Mo
in atomic% by utilizing the characteristics of the maraging steels. By a single-step aging of only 10 min at 650 
C, the alloy showed microstructures consisting of a very high number density of (Fe, Co, Ni)
Mo
-type nanoprecipitates in lath martensite structure and reverted FCC phase, which led to ultrahigh yield strength higher than 2 GPa. This work demonstrates a novel direction to produce strong and ductile materials by expanding the horizons of material design with the aid of high-entropy concept and overcoming the limits of conventional materials.
Woo, W.*; Kim, Y. S.*; Chae, H. B.*; Lee, S. Y.*; Jeong, J. S.*; Lee, C. M.*; Won, J. W.*; Na, Y. S.*; 川崎 卓郎; Harjo, S.; et al.
Acta Materialia, 246, p.118699_1 - 118699_13, 2023/03
被引用回数：11 パーセンタイル：98.19(Materials Science, Multidisciplinary)In situ neutron diffraction experiments have been performed under loading in cast-wrought (CW) and additively manufactured (AM) equiatomic CoCrNi medium-entropy alloys. The diffraction line profile analysis correlated the faulting-embedded crystal structure to the dislocation density, stacking/twin fault probability, and stacking fault energy as a function of strain. The results showed the initial dislocation density of 1.8
10
m
in CW and 1.310
m in AM. It significantly increased up to 1.310
m- in CW and 1.710 m in AM near fracture. The dislocation density contributed to the flow stress of 470 MPa in CW and 600 MPa in AM, respectively. Meanwhile, the twin fault probability of CW (2.7%) was about two times higher than AM (1.3%) and the stacking fault probability showed the similar tendency. The twinning provided strengthening of 360 MPa in CW and 180 MPa in AM. Such a favorable strengthening via deformation twinning in CW and dislocation slip in AM was attributed to the stacking fault energy. It was estimated as 18.6 mJ/m
in CW and 37.5 mJ/m in AM by the strain field of dislocations incorporated model. Dense dislocations, deformation twinning, and atomic-scale stacking structure were examined by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM).
Lam, T.-N.*; Chin, H.-H.*; Zhang, X.*; Feng, R.*; Wang, H.*; Chiang, C.-Y.*; Lee, S. Y.*; 川崎 卓郎; Harjo, S.; Liaw, P. K.*; et al.
Acta Materialia, 245, p.118585_1 - 118585_9, 2023/02
被引用回数：8 パーセンタイル：80.32(Materials Science, Multidisciplinary)The present study investigates the crystallographic-texture effects on the improved fatigue resistance in the CoCrFeMnNi high-entropy alloys (HEAs) with the full-size geometry of the ASTM Standards E647-99. We exploited X-ray nano-diffraction mapping to characterize the crystal-deformation levels ahead of the crack tip after stress unloading under both constant- and tensile overloaded-fatigue conditions. The crack-tip blunting-induced much higher deformation level was concentrated surrounding the crack-tip which delays the fatigue-crack growth immediately after a tensile overload. The predominant deformation texture orientation in the Paris regime was investigated, using electron backscatter diffraction and orientation distribution function analyses. The twinning formation-driven shear deformation gave rise to the development of the Goss-type texture within the plastic deformation regime under a tensile-overloaded-fatigue condition, which was attributed to enhance the crack deflection and thus the tensile induced crack-growth-retardation period in the CoCrFeMnNi HEA.
neutron diffraction澤口 孝宏*; 友田 陽*; 吉中 奎貴*; Harjo, S.
Acta Materialia, 242, p.118494_1 - 118494_14, 2023/01
被引用回数：4 パーセンタイル：45.58(Materials Science, Multidisciplinary)Fe-Mn-Si-based alloys, show superior resistance to plastic fatigue compared to the conventional steels, which is ascribed to the reversible back-and-forth movement of Shockley partial dislocations associated with a reversible martensitic transformation between the face-centered cubic 
-austenite and hexagonal close-packed 
-martensite. The purpose of this study was to gather evidence of the reversible martensitic transformation using in situ neutron diffraction under cyclic loading. Three Fe-30Mn-Si-Al alloys with different Gibbs free energy differences at 298 K were studied to unravel the effect of phase stability on the degree of reversibility.
phase to phase in zirconium alloy revealed by in-situ neutron diffraction during high temperature deformationGuo, B.*; Mao, W.; Chong, Y.*; 柴田 曉伸*; Harjo, S.; Gong, W.; Chen, H.*; Jonas, J. J.*; 辻 伸泰*
Acta Materialia, 242, p.118427_1 - 118427_11, 2023/01
被引用回数：5 パーセンタイル：64.46(Materials Science, Multidisciplinary)Dynamic transformation from alpha (HCP) to beta (BCC) phase in a zirconium alloy was revealed by the use of in-situ neutron diffraction during hot compression. The dynamic transformation was unexpectedly detected during isothermal compression at temperatures of 900C and 950C (alpha + beta two-phase region) and strain rates of 0.01 s
and 0.001 s, even though equilibrium two-phase states were achieved prior to the hot compression. Dynamic transformation was accompanied by diffusion of Sn from beta to alpha phase, which resulted in changes of lattice parameters and a characteristic microstructure of alpha grains. The details of dynamic transformation are discussed using the evolution of lattice constants.
Chong, Y.*; 都留 智仁; Guo, B.*; Gholizadeh, R.*; 井上 耕治*; 辻 伸泰*
Acta Materialia, 240, p.118356_1 - 118356_15, 2022/11
被引用回数：15 パーセンタイル：92.67(Materials Science, Multidisciplinary)本研究では、室温でのチタンの引張特性と変形挙動に及ぼす窒素含有量と結晶粒径の影響を体系的に調査した。巨大ひずみ加工と焼鈍により、超高降伏強度(1.04GPa)と大きな均一伸び(10%)の前例のない相乗効果を組み合わせた、完全再結晶微細構造を備えた超微細粒(UFG)Ti-0.3wt.%N合金が得られた。Ti-0.3wt.%N合金の硬化およびひずみ硬化メカニズムは、変形下部構造の観察と第一原理計算によって包括的に研究された。UFG Ti-0.3wt.%Nの優れた強度/延性バランスへの窒素の寄与は2倍であることが明らかになった。粒子内の窒素原子は、八面体から六面体のサイトへの窒素のシャッフルにより、角柱面上の
転位の運動を強く妨げ、純Tiの6倍の摩擦応力の増加を引き起こした。さらに、Ti-0.3wt.%N合金の柱面と錐面の間の積層欠陥エネルギー差が大幅に減少することで、
転位の活性化が容易になり、ひずみ硬化率の向上に寄与した。我々の実験的および理論的計算研究は、延性を大幅に犠牲にすることなく手頃な価格の高強度チタンの設計に関する知見を与える。
neutron diffraction and electron microscopy studyZheng, R.*; Gong, W.; Du, J.-P.*; Gao, S.*; Liu, M.*; Li, G.*; 川崎 卓郎; Harjo, S.; Ma, C.*; 尾方 成信*; et al.
Acta Materialia, 238, p.118243_1 - 118243_15, 2022/10
被引用回数：17 パーセンタイル：93.62(Materials Science, Multidisciplinary)Grain refinement can lead to the strengthening of metallic materials according to the Hall-Petch relationship. However, our recent results suggested that grain boundary sliding is the dominant deformation mode in bulk ultrafine grained (UFG) pure Mg at room temperature, leading to softening. Here, for the first time, we report that the Hall-Petch strengthening can be regained in bulk UFG pure Mg at cryogenic temperature. At 77K, the UFG pure Mg with a mean grain size of 0.6 
m exhibited ultrahigh tensile yield strength and ultimate tensile strength of 309 MPa and 380 MPa, respectively. Combined neutron diffraction and electron microscopy investigation indicated that residual dislocation structures and deformation twins hardly formed in the UFG specimen during tensile test at 298K. In contrast, fast accumulation of lattice defects and remarkable reorientation were evident at 77K, suggesting that the grain-boundary-mediated process was suppressed and the plastic deformation was dominated by dislocation slip and deformation twinning. In addition, all the pure Mg specimens exhibited pronounced strain hardening at 77 K, which was mainly attributed to the suppressed grain boundary sliding and dynamic recovery. The mean dislocation density and relative fractions of dislocations with various Burgers vectors of the UFG specimen deformed at 77K were determined quantitatively from neutron diffraction data.
Ramadhan, R. S.*; Glaser, D.*; 祖山 均*; Kockelmann, W.*; 篠原 武尚; Pirling, T.*; Fitzpatrick, M. E.*; Tremsin, A. S.*
Acta Materialia, 239, p.118259_1 - 118259_12, 2022/10
被引用回数：3 パーセンタイル：45.58(Materials Science, Multidisciplinary)Mechanical surface treatment technologies such as laser peening and cavitation peening require detailed characterization, including residual stress analysis, to optimize their processing parameters. Recent developments at neutron facilities allow non-destructive 2-dimensional residual strain mapping through Bragg edge imaging. The present work highlights the application of Bragg edge neutron imaging for the study of mechanical surface treatments, through determination of lattice spacing distributions by energy-resolved radiography. Through three different examples, the unique capabilities of the method are demonstrated, particularly for providing near surface residual strain maps within samples with complex geometries with relatively high spatial resolution. By providing a comparison with X-ray diffraction and neutron diffraction results, the present work emphasizes the potential of Bragg edge neutron imaging as a tool for surface treatment research.
榊 浩司*; Kim, H.*; Majzoub, E. H.*; 町田 晃彦*; 綿貫 徹*; 池田 一貴*; 大友 季哉*; 水野 正隆*; 松村 大樹; 中村 優美子*
Acta Materialia, 234, p.118055_1 - 118055_10, 2022/08
被引用回数：9 パーセンタイル：83.6(Materials Science, Multidisciplinary)Local structure in the di-hydride phases of V-Ti-Cr solid solution alloys were investigated using synchrotron X-ray and neutron total scattering experiments. Both Rietveld refinement and pair distribution function (PDF) refinement of the X-ray scattering data indicated that the crystal structure of the metal lattice was a face centered cubic (FCC) structure and no difference between their local structure and average structure was observed. However, the CaF structure model did not reproduce the first peak corresponding to the metal-hydrogen correlation in neutron PDF patterns. When special quasi-random structure (SQS) models are applied for the refinements, the whole neutron PDF patterns were reproduced. Distribution of interatomic distances between hydrogen and metal atoms in the relaxed SQS models showed that interatomic distance of hydrogen with Cr was shorter than that with V and that with Ti was longer than that with V, independently of the chemical compositions.
Liu, M.*; Gong, W.; Zheng, R.*; Li, J.*; Zhang, Z.*; Gao, S.*; Ma, C.*; 辻 伸泰*
Acta Materialia, 226, p.117629_1 - 117629_13, 2022/03
被引用回数：45 パーセンタイル：99.49(Materials Science, Multidisciplinary)One hopeful path to realize good comprehensive mechanical properties in metallic materials is to accomplish homogeneous nanocrystalline (NC) or ultrafine grained (UFG) structure with low dislocation density. In this work, high pressure torsion deformation followed by appropriate annealing was performed on 316 stainless steel (SS). For the first time, we successfully obtained NC/UFG 316 SS having uniform microstructures with various average grain sizes ranging from 46 nm to 2.54 m and low dislocation densities. Among the series, an un-precedentedly high yield strength (2.34 GPa) was achieved at the smallest grain size of 46 nm, in which dislocation scarcity induced hardening accounting for 57% of the strength. On the other hand, exceptional strength-ductility synergy with high yield strength (900 MPa) and large uniform elongation (27%) was obtained in the fully recrystallized specimen having the grain size of 0.38 m. The high yield stress and scarcity of dislocation sources in recrystallized UFGs activated stacking faults and deformation twins nucleating from grain boundaries during straining, and their interaction with dislocations allowed for sustainable strain hardening, which also agreed with the plaston concept recently proposed. The multiple deformation modes activated, together with the effective strengthening mechanisms, were responsible for the outstanding comprehensive mechanical performance of the material.
岡田 和歩*; 柴田 曉伸*; Gong, W.; 辻 伸泰*
Acta Materialia, 225, p.117549_1 - 117549_13, 2022/02
被引用回数：15 パーセンタイル：93.62(Materials Science, Multidisciplinary)In this study, the deformation microstructure of hydrogen-charged ferritic-pearlitic 2Mn-0.1C steel was characterized using SEM-BSE, SEM-EBSD, TEM, and neutron diffraction. The microscopic mechanism of hydrogen-related quasi-cleavage fracture along the 
planes was also discussed. It was found that hydrogen increased the relative velocity of screw dislocations to edge dislocations, leading to a tangled dislocation morphology, even at the initial stage of deformation (strain = 0.03). In addition, the density of screw dislocations at the later stage of deformation (strain = 0.20) increased in the presence of hydrogen. Based on the experimental results, it is proposed that a high density of vacancies accumulated along slip planes by jog-dragging of screw dislocations, and coalescence of the accumulated vacancies led to the hydrogen-related quasi-cleavage fracture along the {011} slip planes.
Wei, D.*; Wang, L.*; Zhang, Y.*; Gong, W.; 都留 智仁; Lobzenko, I.; Jiang, J.*; Harjo, S.; 川崎 卓郎; Bae, J. W.*; et al.
Acta Materialia, 225, p.117571_1 - 117571_16, 2022/02
被引用回数：59 パーセンタイル：99.75(Materials Science, Multidisciplinary)Recently-developed high-entropy alloys (HEAs) containing multiple principal metallic elements have ex-tended the compositional space of solid solutions and the range of their mechanical properties. Here we show that the realm of possibilities can be further expanded through substituting the constituent metals with metalloids, which are desirable for tailoring strength/ductility because they have chemical interactions and atomic sizes distinctly different from the host metallic elements. Specifically, the metalloid substitution increases local lattice distortion and short-range chemical inhomogeneities to elevate strength, and in the meantime reduces the stacking fault energy to discourage dynamic recovery and encourage defect accumulation via partial-dislocation-mediated activities. These impart potent dislocation storage to improve the strain hardening capability, which is essential for sustaining large tensile elongation. As such, metalloid substitution into HEAs evades the normally expected strength-ductility trade-off, enabling an unusual synergy of high tensile strength and extraordinary ductility for these single-phase solid solutions.
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：94.34(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.
板倉 充洋; 山口 正剛; 江草 大佑*; 阿部 英司*
Acta Materialia, 203, p.116491_1 - 116491_9, 2021/01
被引用回数：23 パーセンタイル：88.82(Materials Science, Multidisciplinary)シンクロ型長周期積層(LPSO)合金において溶質原子クラスタはキンク変形やキンク強化といった特異な塑性に中心的役割を果たす。強化機構を解明にはその原子構造を決定することが必須であるが、クラスタに含まれる格子間原子に関して不明な点が残っていた。本研究では第一原理計算によりクラスタが形成される過程を詳細に調べ、その成長過程で中心付近にある原子が自発的に格子間原子となり、同時に空孔が形成されることを示した。これにより実験で不明であった格子間原子の種類と割合が決定された。更に自発的な空孔形成はこれまで金属材料では知られていない現象であり、これによってLPSO構造の形成が加速されていることを初めて示した。
Lam, T.-N.*; Lee, S. Y.*; Tsou, N.-T.*; Chou, H.-S.*; Lai, B.-H.*; Chang, Y.-J.*; Feng, R.*; 川崎 卓郎; Harjo, S.; Liaw, P. K.*; et al.
Acta Materialia, 201, p.412 - 424, 2020/12
被引用回数：33 パーセンタイル：91.53(Materials Science, Multidisciplinary)We examined fatigue-crack-growth behaviors of CoCrFeMnNi high-entropy alloys (HEAs) under as-fatigued and tensile-overloaded conditions using neutron-diffraction measurements coupled with diffraction peak-profile analyses. We applied both high-resolution transmission electron microscopy (HRTEM) and neutron-diffraction strain mapping for the complementary microstructure examinations. Immediately after a single tensile overload, the crack-growth-retardation period was obtained by enhancing the fatigue resistance, as compared to the as-fatigued condition. The combined mechanisms of the overload-induced larger plastic deformation, the enlarged compressive residual stresses and plastic-zone size, the crack-tip blunting ahead of the crack tip, and deformation twinning governed the pronounced macroscopic crack-growth-retardation behavior following the tensile overload.
