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論文

Temperature-dependent hardening contributions in CrFeCoNi high-entropy alloy

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

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 $$sim$$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 $$sim$$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.

論文

High temperature reaction of multiple eutectic-component system; The Case of solid metallic Zr and molten stainless steel-B$$_{4}$$C

墨田 岳大; 小畠 雅明; 高野 公秀; 池田 篤史

Materialia, 20, p.101197_1 - 101197_11, 2021/12

The eutectic melting, one of the fundamental phenomena in high temperature reactions involving liquid phases, is a primitive but important subject for both scientific and industrial fields associated with metallurgy. The present study aims at revealing the formation and reaction mechanisms of the multiple eutectic-component system consisting of solid metallic Zr and molten stainless steel- boron carbide (SS-B$$_{4}$$C) by combining multiple analytical methods (i.e. powder X-ray diffraction (PXRD), scanning electron microscopy- energy dispersive X-ray spectroscopy (SEM-EDS), and dynamic secondary ion mass spectrometry (D-SIMS)) with thermodynamical consideration. The results indicate that the solidified Zr-SS-B$$_{4}$$C mixture is composed of major phases of (Fe,Cr)$$_{2}$$Zr, (Fe,Cr,Ni)$$_{2}$$Zr, (Ni,Fe)Zr$$_{2}$$, ZrB$$_{2}$$, and a minor phase of ZrC. The results also reveal that the eutectic melting between solid metallic Zr and molten SS-B$$_{4}$$C can be described as the combination of diffusion kinetics and thermodynamic stability. That is, the initial formation of ZrC and ZrB$$_{2}$$ layers at the reaction interface significantly retards the diffusion of other SS-B$$_{4}$$C components (i.e. Cr, Fe, and Ni) into solid metallic Zr.

論文

Density functional theory study of solute cluster growth processes in Mg-Y-Zn LPSO alloys

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:2 パーセンタイル:70.42(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。

論文

Enhancement of fatigue resistance by overload-induced deformation twinning in a CoCrFeMnNi high-entropy alloy

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:13 パーセンタイル:92.23(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.

論文

Continuous and discontinuous yielding behaviors in ferrite-cementite steels

Wang, Y.*; 友田 陽*; 大村 孝仁*; Gong, W.*; Harjo, S.; 田中 雅彦*

Acta Materialia, 196, p.565 - 575, 2020/09

 被引用回数:6 パーセンタイル:69.06(Materials Science, Multidisciplinary)

The continuous and discontinuous yielding behaviors in ferrite-cementite steels were complementarily investigated via nano- and macroscale deformation examinations. The results obtained by electron microscopy, synchrotron X-ray, and neutron diffractions indicate that the ferrite-cementite interface of the heat-treated specimen is semi-coherent with a high internal stress field, whereas that of the recrystallized one is incoherent with a low internal stress field. Moreover, coherency strain, which depends on the total area of the ferrite-cementite interface, and thermal strain, which is governed by temperature, are the two factors that influence peak broadening. The nanoindentation tests revealed that the critical loads are significantly lower near the semi-coherent interface than those near the incoherent interface and the ferrite grain boundary; this suggests that dislocations are easily emitted from the semi-coherent interface.

論文

Neutron diffraction monitoring of ductile cast iron under cyclic tension-compression

Harjo, S.; 窪田 哲*; Gong, W.*; 川崎 卓郎; Gao, S.*

Acta Materialia, 196, p.584 - 594, 2020/09

 被引用回数:3 パーセンタイル:59.43(Materials Science, Multidisciplinary)

To understand work hardening behavior during low-cycle loading, ductile cast iron containing spheroidal graphite, pearlite, and ferrite matrix was investigated in an in situ neutron diffraction study of up to four cycles of tensile-compressive loading with applied strains of $$pm$$0.01. The amplitudes of applied stress, Bauschinger stress, and Bauschinger strain were found to increase with increasing cycle number, indicating work hardening as cyclic loading progressed. Absolute values of ferrite lattice strain at maximum and minimum applied strains increased with increasing cycle number, indicating an increase in ferrite strength. Consequently, the stress contribution to the strength from ferrite increased as cyclic loading progressed. The increase in ferrite strength, caused by dislocation accumulation in ferrite during cyclic loading, played an important role in the work hardening of the ductile cast iron.

論文

${it In situ}$ WB-STEM observation of dislocation loop behavior in reactor pressure vessel steel during post-irradiation annealing

Du, Y.*; 吉田 健太*; 嶋田 雄介*; 外山 健*; 井上 耕治*; 荒河 一渡*; 鈴土 知明; Milan, K. J.*; Gerard, R.*; 大貫 惣明*; et al.

Materialia, 12, p.100778_1 - 100778_10, 2020/08

長期に原子炉圧力容器の健全性を確保するためには、照射が材料に及ぼす影響を理解する必要がある。本研究では我々が新規開発したWB-STEMを用いて、中性子照射された原子炉圧力容器試験片を焼鈍中、照射誘起転位ループの観察を行った。焼鈍温度を上げると$$<100>$$ループの割合が増加していることが確認された。また、2つの$$frac{1}{2}$$$$<111>$$ループが衝突して$$<100>$$ループになる現象の観察に初めて成功した。転位に転位ループがデコレートする現象も観察され、分子動力学シミュレーションによってそのメカニズムが説明することができた。

論文

On the phase transformation and dynamic stress-strain partitioning of ferrous medium-entropy alloy using experimentation and finite element method

Bae, J. W.*; Jung, J.*; Kim, J. G.*; Park, J. M.*; Harjo, S.; 川崎 卓郎; Woo, W.*; Kim, H. S.*

Materialia, 9, p.100619_1 - 100619_15, 2020/03

In the present study, an integrated experimental-numerical analysis on ferrous medium-entropy alloy (FMEA) was conducted to understand the micromechanical response of the constituent phases in the FMEA at -137$$^{circ}$$C. The initial face-centered cubic (FCC) single phase microstructure of the FMEA was transformed to body-centered cubic (BCC) martensite during tensile deformation at -137$$^{circ}$$C, resulting in improved low-temperature mechanical properties. The microstructure evolution due to deformation-induced phase transformation mechanism and strain partitioning behavior was analyzed using ${it ex situ}$ electron backscatter diffraction. The mechanical responses related to the stress partitioning between constituent phases and deformation-induced transformation rate were measured using ${it in situ}$ neutron diffraction in combination with the nanoindentation analysis.

論文

Role of retained austenite in low alloy steel at low temperature monitored by neutron diffraction

山下 享介; 諸岡 聡; Harjo, S.; 川崎 卓郎; 古賀 紀光*; 梅澤 修*

Scripta Materialia, 177, p.6 - 10, 2020/03

 被引用回数:13 パーセンタイル:94.74(Nanoscience & Nanotechnology)

${it In situ}$ neutron diffraction measurements during tensile tests at low temperatures of a low alloy steel containing retained austenite have been performed. Evolutions of phase fractions and phase stresses were analyzed and discussed with the progress of deformation. The role of austenite in the steel during deformation at low temperatures was observed not to directly in the contribution to the strengths but in the improvement of the elongation by transformation of austenite to martensite -and in the increasing of the work-hardening rate by an increase in the phase fraction of martensite and the work hardening of martensite.

論文

Study on formation mechanism of {332}$$<$$113$$>$$ deformation twinning in metastable $$beta$$-type Ti alloy focusing on stress-induced $$alpha"$$ martensite phase

Cho, K.*; 森岡 亮太*; Harjo, S.; 川崎 卓郎; 安田 弘行*

Scripta Materialia, 177, p.106 - 111, 2020/03

 被引用回数:12 パーセンタイル:92.23(Nanoscience & Nanotechnology)

Deformation behavior of Ti-12Mo alloy which is primarily deformed by {332}$$<$$113$$>$$ deformation twinning was studied by in situ neutron diffraction measurements and deformation structure analyses, focusing on the formation of stress-induced $$alpha"$$ martensite phase. We directly observed for the first time that the $$alpha"$$ phase was formed at the initial stage of the plastic deformation and the volume fraction of the phase increased with increasing plastic deformation. This stress-induced $$alpha"$$ martensite phase acts as a nucleation site for the {332}$$<$$113$$>$$ twins. Further increase in stress results in a replacement of the $$alpha"$$ phase by the {332}$$<$$113$$>$$ twins.

論文

Real time observation of martensite transformation for a 0.4C low alloyed steel by neutron diffraction

Wang, Y.*; 友田 陽*; 大村 孝仁*; 諸岡 聡; Gong, W.*; Harjo, S.

Acta Materialia, 184, p.30 - 40, 2020/02

 被引用回数:10 パーセンタイル:90.5(Materials Science, Multidisciplinary)

A high-intensity and high-resolution neutron diffractometer with a thermomechanically controlled processing simulator was employed in-situ to investigate martensite transformation behavior with and without ausforming for a medium-carbon low-alloy steel. Serial neutron diffraction profiles have revealed that the transformation behavior could be successfully monitored during quenching with and without the ausforming process. The lattice parameter and the full width at half maximum of austenite peaks significantly decreases and increases upon martensite transformation, respectively. After ausforming, the data reveal that lattice parameters are larger in austenite whereas smaller in martensite compared with those in the non-ausformed case, which is ascribed to the introduced dislocations. The cutting-edge operant quantitative measurements with neutron diffraction for steel production is demonstrated.

論文

Neutron diffraction study on full-shape Japanese sword

Harjo, S.; 川崎 卓郎; Grazzi, F.*; 篠原 武尚; 田中 眞奈子*

Materialia, 7, p.100377_1 - 100377_9, 2019/09

A mapping measurement using pulsed neutron diffraction with time-of-flight method is performed on a full-shape Japanese sword made in Keicho era (1596-1615) to elucidate the manufacturing process. The obtained diffraction patterns are analyzed by the Rietveld refinement and a line profile analysis. The constituent phases in the area closer to the back of the blade (ridge) are found to be ferrite and cementite, composing pearlite, while the area close to the edge is composed by martensite and austenite. The distributions of constituent phases are well explained with the distributions of dislocation density and crystallite size. The carbon contents and the residual macroscopic stresses are estimated from the obtained phase fractions and lattice parameters.

論文

Neutron diffraction study of temperature-dependent elasticity of B19' NiTi; Elinvar effect and elastic softening

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:10 パーセンタイル:70.55(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 ${it in situ}$ 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.

論文

Nature of dynamic ferrite transformation revealed by ${it in-situ}$ neutron diffraction analysis during thermomechanical processing

柴田 曉伸*; 竹田 泰成*; Park, N.*; Zhao, L.*; Harjo, S.; 川崎 卓郎; Gong, W.*; 辻 伸泰*

Scripta Materialia, 165, p.44 - 49, 2019/05

 被引用回数:15 パーセンタイル:88.34(Nanoscience & Nanotechnology)

Nowadays, a new concept of process utilizing dynamic ferrite transformation, which can achieve ultrafine-grained structure with a mean grain size of approximately 1$$mu$$m, has been proposed. This paper reports transformation mode of dynamic ferrite transformation and formation mechanism of ultrafine-grained structure revealed by our novel technique of ${it in-situ}$ neutron diffraction analysis during thermomechanical processing. Dynamic ferrite transformation occurs in a diffusional manner, whose partitioning behavior changes from para- to ortho-equilibrium with the progress of transformation. Moreover, we propose that dynamic recrystallization of dynamically-transformed ferrite is the main mechanism for the formation of ultrafine-grained structure.

論文

Mechanism of hardening and damage initiation in oxygen embrittlement of body-centred-cubic niobium

Yang, P.-J.*; Li, Q.-J.*; 都留 智仁; 尾方 成信*; Zhang, J.-W.*; Sheng, H.-W.*; Shan, Z.-W.*; Sha, G.*; Han, W.-Z.*; Li, J.*; et al.

Acta Materialia, 168, p.331 - 342, 2019/04

 被引用回数:17 パーセンタイル:90.93(Materials Science, Multidisciplinary)

ニオブなどの体心立方構造をもつ金属材料は低濃度の酸素固溶によって脆化しやすいことが知られているが、酸素誘起の硬化や損傷の機構は明らかになっていない。我々は、実験、および第一原理計算と分子動力学計算を用いて詳細な機構を検討した。その結果、酸素の格子間原子はらせん転位と引力相互作用を生じ、それによって転位運動でクロスキンクが生成され、同時に多くの空孔が生成されることを明らかにした。これらの空孔はさらに酸素と転位の三体間の硬化によって転位の運動を阻害することで、著しい硬化を生じることを明らかにした。

論文

Reversible elastocaloric effect at ultra-low temperatures in nanocrystalline shape memory alloys

Ahadi, A.*; 川崎 卓郎; Harjo, S.; Ko, W.-S.*; Sun, Q. P.*; 土谷 浩一*

Acta Materialia, 165, p.109 - 117, 2019/02

 被引用回数:19 パーセンタイル:91.93(Materials Science, Multidisciplinary)

We report emergence of an unprecedented elastocaloric effect with a decrease of temperature, well below the temperature range of martensitic phase transformation, in a model nanocrystalline NiTi that preserves slim-hysteresis superelasticity at ultra-low temperatures. The new elastocaloric effect emerges at a temperature of about 90 K, exhibits an opposite sign than the conventional elastocaloric effect, and intensifies gradually with a decrease of temperature to 18 K. At 18 K, a large adiabatic temperature change of +3.4 K is measured upon rapid release of tensile stress. The measured adiabatic temperature change are larger and extend over a wider temperature span than the existing electrocaloric, piezocaloric, and barocaloric cryo-refrigeration materials.

論文

Microstructure evolution in a hydrogen charged and aged Al-Zn-Mg alloy

Bendo, A.*; 松田 健二*; Lee, S.*; 西村 克彦*; 戸田 裕之*; 清水 一行*; 都留 智仁; 山口 正剛

Materialia, 3, p.50 - 56, 2018/11

Microstructure evolution of alloy with composition Al-3.4Zn-1.9 Mg (at. %) was tracked using high resolution imaging and electron diffraction during ageing at 120 C after being initially natural aged for 4 days. Hydrogen was charged in microstructure by cutting samples in Electrical Discharge Machine immediately after being quenched from solid solution temperature. Transmission Electron Microscope observations revealed a higher presence of high aspect ratio GPII-zones in microstructure of hydrogen charged and aged samples. Correlation was made with previously reported hydrogen effect into vacancy formation.

論文

Deformation of CoCrFeNi high entropy alloy at large strain

Wang, B.*; He, H.*; Naeem, M.*; Lan, S.*; Harjo, S.; 川崎 卓郎; Nie, Y.*; Wang, X.-L.*; 他7名*

Scripta Materialia, 155, p.54 - 57, 2018/10

 被引用回数:35 パーセンタイル:95.89(Nanoscience & Nanotechnology)

The deformation behavior of an equi-atomic face-centered-cubic CoCrFeNi high entropy alloy was investigated by in-situ neutron diffraction under tensile loading up to 40% applied strain. A three-stage deformation behavior was fully captured by lattice strain and texture evolution. In spite of the chemical complexity, the deformation in CoCrFeNi is dominated by dislocation activities. Analysis of diffraction and microscopy data shows that the deformation progresses from dislocation slip to severe entanglement, where a sharp increase in dislocation density was observed. The neutron diffraction data, corroborated by transmission electron microscopy analysis, provided microscopic insights of the previously reported three-stage hardening behavior.

論文

Interfacial segregation and fracture in Mg-based binary alloys; Experimental and first-principles perspective

都留 智仁; 染川 英俊*; Chrzan, D. C.*

Acta Materialia, 151, p.78 - 86, 2018/06

 被引用回数:38 パーセンタイル:96.44(Materials Science, Multidisciplinary)

溶質元素による金属の破壊現象を理解するため、Mg合金に対して実験と破壊力学理論と第一原理計算の連携による検討を行った。Mg二元系合金の破壊を極めて脆性的であると仮定し、粒界や双晶境界の破壊をエネルギーベースの基準で評価することによって第一原理計算を有効に適用することを可能にした。また、化学結合が破壊に寄与する影響を詳細に検討し、IIIBやIVB族元素が破壊抵抗を向上する効果を有することを体系的に明らかにした。これまでに知られていない元素であるZrが破壊靱性を向上させるという実験結果とよく一致しており、計算科学を用いた元素戦略による材料設計が有効であることを示した。

論文

Effect of crystal orientation on incipient plasticity during nanoindentation of magnesium

染川 英俊*; 都留 智仁; Singh, A.*; 三浦 誠司*; Schuh, C. A.*

Acta Materialia, 139, p.21 - 29, 2017/10

 被引用回数:18 パーセンタイル:78.88(Materials Science, Multidisciplinary)

ナノインデンテーションにおけるポップイン挙動は転位の活動によって生じるため、結晶方位に強く影響する。我々はマグネシウムに対するナノインデンテーションに対する実験と分子動力学計算によって方位の影響を詳細に検討した。実験によるインデンテーションから、底面の押込みにおけるポップイン荷重とそれに付随した変位は柱面への押込みより大きくなることがわかり、原子シミュレーションによる荷重-変位関係からも同様の結果が得られた。これらの方位による違いは、押込みによって生成される転位のタイプによって特徴づけられ、柱面のポップインは押込み軸に垂直な底面すべりによって生じる一方、底面では押込み方向の変位を生成しない底面すべりだけでなく錐面上に転位が生成されるためであることがわかった。

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