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Journal Articles

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

Harjo, S.; Kubota, Satoru*; Gong, W.*; Kawasaki, Takuro; Gao, S.*

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

 Times Cited Count:0 Percentile:100(Materials Science, Multidisciplinary)

Journal Articles

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

Wang, Y.*; Tomota, Yo*; Omura, Takahito*; Morooka, Satoshi; Gong, W.*; Harjo, S.

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

 Times Cited Count:3 Percentile:15.38(Materials Science, Multidisciplinary)

Journal Articles

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

Ahadi, A.*; Khaledialidusti, R.*; Kawasaki, Takuro; Harjo, S.; Barnoush, A.*; Tsuchiya, Koichi*

Acta Materialia, 173, p.281 - 291, 2019/07

 Times Cited Count:5 Percentile:24.25(Materials Science, Multidisciplinary)

Journal Articles

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

Yang, P.-J.*; Li, Q.-J.*; Tsuru, Tomohito; Ogata, Shigenobu*; 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

 Times Cited Count:11 Percentile:6.69(Materials Science, Multidisciplinary)

Body-centred-cubic metallic materials, such as niobium (Nb) and other refractory metals, are prone to embrittlement due to low levels of oxygen solutes. The mechanisms responsible for the oxygen-induced rampant hardening and damage are unclear. Here we illustrate that screw dislocations moving through a random repulsive force field imposed by impurity oxygen interstitials readily form cross-kinks and emit excess vacancies in Nb. The vacancies bind strongly with oxygen and screw dislocation in a three-body fashion, rendering dislocation motion difficult and hence pronounced dislocation storage and hardening. This leads to unusually high strain hardening rates and fast breeding of nano-cavities that underlie damage and failure.

Journal Articles

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

Tsuru, Tomohito; Somekawa, Hidetoshi*; Chrzan, D. C.*

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

 Times Cited Count:19 Percentile:6.61(Materials Science, Multidisciplinary)

We investigated the effect of solute elements on interfacial segregation and fracture in Mg alloys by experiments and first-principles density functional theory calculations in conjunction with interfacial fracture mechanics. Based on the assumption of brittle fracture in Mg alloys, the interfacial separation caused by segregated solutes in Mg can be efficiently described by the energy-based criterion of fracture, which is in good agreement with the fracture toughness obtained by experimental tests of Mg-M binary alloys. The electronic interaction, that is, the change in the electronic state between the interface and surface, mainly influences the ideal work of separation regardless of the type of interface. We found that IIIB ($$d^{1}$$) and IVB ($$d^{2}$$) solutes, such as Zr, show distinctive hybridization between the p band of Mg and the d band of the solute, which characterizes the strong fracture toughness of Zr-doped Mg alloys in both the calculations and experiments.

Journal Articles

Effect of crystal orientation on incipient plasticity during nanoindentation of magnesium

Somekawa, Hidetoshi*; Tsuru, Tomohito; Singh, A.*; Miura, Seiji*; Schuh, C. A.*

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

 Times Cited Count:9 Percentile:34.35(Materials Science, Multidisciplinary)

The effect of crystal orientation on incipient plasticity during nanoindentation was investigated by experiments and molecular statics simulation. Pop-in behavior is a result of dislocation activity, and is therefore influenced by crystal orientation. Experimental results using single crystals indicated that indentations on the basal plane had higher pop-in loads and larger pop-in displacements than those on the prismatic plane, an effect also captured by molecular statics simulation. The difference can be traced to the types of activated dislocations, with not only basal but also pyramidal dislocations active for indentations on the basal plane, but only basal dislocations triggered at the first pop-in on the prismatic plane.

Journal Articles

Chemical misfit origin of solute strengthening in iron alloys

Wakeda, Masato*; Tsuru, Tomohito; Koyama, Masanori*; Ozaki, Taisuke*; Sawada, Hideaki*; Itakura, Mitsuhiro; Ogata, Shigenobu*

Acta Materialia, 131, p.445 - 456, 2017/06

 Times Cited Count:13 Percentile:25(Materials Science, Multidisciplinary)

Most of the solute species show a significant interaction with the dislocation core, while only several solute species among them, such as Si, P, and Cu, significantly lower the Peierls potential of the screw dislocation motion. A first-principles interaction energy with the "Easy-core" structure excellently correlates with the change in the $$gamma$$-surface caused by solute atoms (i.e., chemical misfit). We show the availability of the interaction energy to predict the effect of each species on macroscopic critical resolved shear stress (CRSS) of the dilute Fe alloy. The CRSS at low and high temperature for various alloys basically agree with experiment CRSS. These results provide a novel understanding of the interaction between a screw dislocation and solute species from the first-principles.

Journal Articles

The Two-step nucleation of G-phase in ferrite

Matsukawa, Yoshitaka*; Takeuchi, Tomoaki; Kakubo, Yuta*; Suzudo, Tomoaki; Watanabe, Hideo*; Abe, Hiroaki*; Toyama, Takeshi*; Nagai, Yasuyoshi*

Acta Materialia, 116, p.104 - 113, 2016/09

 Times Cited Count:44 Percentile:4.08(Materials Science, Multidisciplinary)

Atom probe tomography (APT) and TEM were combined for identifying the stage at which solute clusters transform into compounds crystallographically distinct from the matrix, in the precipitation of the G-phase (Ni$$_{16}$$Si$$_{7}$$Mn$$_{6}$$) from ferrite solid solution subjected to isothermal annealing at 673 K. Based on a systematic analysis of solute clusters as a function of annealing time, the nucleation of the G-phase was found to occur via a two-step process. Moreover, the structural change was found to occur via another two-step process. There was a time lag between the end of cluster growth to become a critical size and the start of the structural change. During the incubation period solute enrichment occurred inside the clusters without further size growth, indicating that the nucleation of the G-phase occurs at the critical size with a critical composition. Judging from the results of APT, TEM and the simulation of electron diffraction patterns, the critical composition was estimated to be Ni$$_{16}$$Si$$_{3.5}$$(Fe,Cr)$$_{3.5}$$Mn$$_{6}$$.

Journal Articles

Effect of aluminum or zinc solute addition on enhancing impact fracture toughness in Mg-Ca alloys

Hase, Takayuki*; Otagaki, Tatsuya*; Yamaguchi, Masatake; Ikeo, Naoko*; Mukai, Toshiji*

Acta Materialia, 104, p.283 - 294, 2016/02

 Times Cited Count:22 Percentile:13.62(Materials Science, Multidisciplinary)

We measured the impact toughness of three alloys (Mg, Mg-0.3 at.% Ca-0.6 at.% Zn, and Mg-0.3 at.% Ca-0.6 at.% Al) by the impact three-point bending test. The plastic deformability and impact toughness were higher in the ternary alloys than in pure Mg. The generalized stacking fault energy and grain boundary cohesive energy were estimated by first-principles calculations for Mg, binary Mg-Ca, ternary Mg-Ca-Zn, and ternary Mg-Ca-Al alloys. The calculation results agreed with the trend in the experimental results. We suggest that addition of Ca along with Zn or Al reduced plastic anisotropy and strengthened the grain boundaries, leading to higher in impact toughness of Mg alloys.

Journal Articles

Hardening in thermally-aged Fe-Cr binary alloys; Statistical parameters of atomistic configuration

Suzudo, Tomoaki; Nagai, Yasuyoshi*; Schwen, D.*; Caro, A.*

Acta Materialia, 89, p.116 - 122, 2015/05

 Times Cited Count:8 Percentile:50.79(Materials Science, Multidisciplinary)

By exploiting Monte Carlo methodology and molecular dynamics, we computationally simulate the spinodal decomposition of iron-chromium binary alloys and analyze the relationship between the increase of yield stress induced by the phase separation phenomenon, and statistical parameters of the atomistic configuration. We successfully model the experimentally-discovered proportional relationship between the hardness and the variation parameter (or V), and also found that the adequacy of the parameter V as an empirical indicator of hardening is limited, because it does not properly capture short-range atomistic configurations that influence the hardening. We suggest that the short-range-order parameter has more potential to become universal descriptor of the phenomenon.

Journal Articles

Effect of prior martensite on bainite transformation in nanobainite steel

Gong, W.; Tomota, Yo*; Harjo, S.; Su, Y.; Aizawa, Kazuya

Acta Materialia, 85, p.243 - 249, 2015/02

 Times Cited Count:76 Percentile:1.24(Materials Science, Multidisciplinary)

Nanobainite transformation behavior was comparably studied using in situ neutron diffraction measurements, scanning electron microscopy and electron backscatter diffraction observations for two heat treatments: with and without partial quenching before isothermal holding at 523 or 573 K. Prior martensite transformation was found to accelerate the subsequent nanobainite transformation. Bainitic lathes formed adjacent to a pre-existing martensite plate exhibited an almost identical orientation. Dislocations introduced in austenite due to stress relaxation of transformation strains are believed to assist bainite transformation accompanying variant selection.

Journal Articles

The Effect of hydrogen atoms on the screw dislocation mobility in bcc iron; A First-principles study

Itakura, Mitsuhiro; Kaburaki, Hideo; Yamaguchi, Masatake; Okita, Taira*

Acta Materialia, 61(18), p.6857 - 6867, 2013/10

 Times Cited Count:52 Percentile:4.67(Materials Science, Multidisciplinary)

The interaction between dislocations and impurity atoms in metals determines various properties of plastic deformation, such as the dependence of the yield stress on the impurity contents. Since the direct observation of atomistic structure of screw dislocation is almost impossible, several hypothetical assumptions have been employed to explain conveniently experimental observations. Recent advancement of computational hardware, as well as the development of elaborated techniques to reduce the size-effect in the first-principles calculation, enabled direct calculations of dislocation-impurity interaction. We have succeeded to evaluate the effect of hydrogen atoms on the dislocation mobility in iron.

Journal Articles

First-principles study on the mobility of screw dislocations in bcc iron

Itakura, Mitsuhiro; Kaburaki, Hideo; Yamaguchi, Masatake

Acta Materialia, 60(9), p.3698 - 3710, 2012/05

 Times Cited Count:55 Percentile:5(Materials Science, Multidisciplinary)

Irradiation hardening of nuclear materials are caused by lattice defects which hinder the motion of dislocations and thus suppress plastic deformations. To understand the irradiation hardening, the precise knowledge about the dislocation motion inside the material is indispensable. In bcc metals the mobility of dislocations is determined by the atomic structure of the dislocation and thus quantum mechanical calculation is required to estimate the mobility of dislocations. We have devised a new method to calculate the dislocation properties combining linear elasticity theory and quantum mechanical calculations, and also developed a new method to control the two-dimensional motion of dislocation in the simulation. These new methods allowed us to identify the reason why wrong dislocation motion is observed in the current molecular dynamics simulations, and to present a guideline to improve these simulations. This work opened a way for the quantitative simulations of irradiation hardening.

Journal Articles

Ab initio study on plane defects in zirconium-hydrogen solid solution and zirconium hydride

Udagawa, Yutaka; Yamaguchi, Masatake; Abe, Hiroaki*; Sekimura, Naoto*; Fuketa, Toyoshi

Acta Materialia, 58(11), p.3927 - 3938, 2010/06

 Times Cited Count:65 Percentile:4.6(Materials Science, Multidisciplinary)

Journal Articles

Nanoscale contact plasticity of crystalline metal; Experiment and analytical investigation via atomistic and discrete dislocation models

Tsuru, Tomohito; Shibutani, Yoji*; Kaji, Yoshiyuki

Acta Materialia, 58(8), p.3096 - 3102, 2010/05

 Times Cited Count:20 Percentile:25.92(Materials Science, Multidisciplinary)

Nanoscale incipient plastic deformation in crystalline metals occurs as the result of the collective motion of dislocations. It is known as "nanoplasticity" and recognized as the elementary process of the macroscopic deformation. In the present study, experimental tests are first conducted to educe the unique nature of the nanoscale deformation. Subsequently large-scale atomistic simulations are performed to predict the incipient plastic deformation and a new discrete dislocation model combined with the boundary element analysis is constructed to capture the collective motion of the dislocations. Our results suggest that the incipient plastic deformation requires much higher critical shear stress than the theoretical shear strength due to high compressive stress distribution beneath the indenter, and that the displacement burst is induced by surface rearrangement corresponding to hundreds of dislocation dipoles.

Journal Articles

A New method for the quantitative analysis of the scale and composition of nanosized oxide in 9Cr-ODS steel

Onuma, Masato*; Suzuki, Junichi; Otsuka, Satoshi; Kim, S.-W.; Kaito, Takeji; Inoue, Masaki; Kitazawa, Hideaki*

Acta Materialia, 57(18), p.5571 - 5581, 2009/10

 Times Cited Count:94 Percentile:2.46(Materials Science, Multidisciplinary)

Journal Articles

Effects of neutron-irradiation-induced intergranular phosphorus segregation and hardening on embrittlement in reactor pressure vessel steels

Nishiyama, Yutaka; Onizawa, Kunio; Suzuki, Masahide; Anderegg, J. W.*; Nagai, Yasuyoshi*; Toyama, Takeshi*; Hasegawa, Masayuki*; Kameda, Jun*

Acta Materialia, 56(16), p.4510 - 4521, 2008/09

 Times Cited Count:48 Percentile:9.12(Materials Science, Multidisciplinary)

The effects of intergranular P segregation and hardening on the ductile-to-brittle transition temperature (DBTT) in several neutron-irradiated reactor pressure vessel steels with different bulk contents of P and Cu have been investigated using a scanning Auger microbe, a local electrode atom probe and positron annihilation spectroscopy. Increasing the neutron fluence at 563 K promotes intergranular P segregation. The content of P more significantly affects irradiation hardening than that of Cu due to distinct formation of P-rich precipitates arising from the stabilization of vacancies. Analyzing the correlations between the P segregation, hardening, fraction of intergranular fracture and DBTT, it is found neutron irradiation mitigates an embrittling effect of segregated P, and therefore the hardening more strongly affects the DBTT shift than the P segregation.

Journal Articles

Microstructure formation and in situ phase identification from undercooled Co-61.8 at.% Si melts solidified on an electromagnetic levitator and an electrostatic levitator

Li, M.*; Nagashio, Kosuke*; Ishikawa, Takehiko*; Mizuno, Akitoshi*; Adachi, Masayoshi*; Watanabe, Masahito*; Yoda, Shinichi*; Kuribayashi, Kazuhiko*; Katayama, Yoshinori

Acta Materialia, 56(11), p.2514 - 2525, 2008/06

 Times Cited Count:21 Percentile:25.15(Materials Science, Multidisciplinary)

Co-61.8 at.% Si (CoSe-CoSi$$_{2}$$) eutectic alloys were solidified on an electromagnetic levitator (EML) and an electrostatic levitator (ESL) at different undercooling levels. The results indicated that there is only a single recalescence event at low undercooling with the CoSi intermetallic compound as primary phase, which is independent of processing facilities, on either an EML or an ESL. The microstructure, however, is strongly dependent on the processing facility. On high undercooling, double recalescence takes place regardless of levitation condition. In situ X-ray diffraction of alloys solidified on the EML demonstrates that the CoSi$$_{2}$$ compound becomes the primary phase upon the first recalescence, and the CoSi intermetallic phase crystallizes during the second recalescence. In addition to phase identification, real-time diffraction patterns can also provide additional evidence of the fragmentation of the primary phase.

Journal Articles

Molecular dynamics study on the formation of stacking fault tetrahedra and unfaulting of Frank loops in FCC metals

Kadoyoshi, Tomoko; Kaburaki, Hideo; Shimizu, Futoshi; Kimizuka, Hajime*; Jitsukawa, Shiro; Li, J.*

Acta Materialia, 55(9), p.3073 - 3080, 2007/05

 Times Cited Count:39 Percentile:12.33(Materials Science, Multidisciplinary)

Critical conditions have been determined for intrinsic transformation of a vacancy Frank loop into a stacking fault tetrahedron in a face centered cubic metal by the molecular dynamics method. We found that a stacking fault tetrahedron can be formed from the scalene hexagonal vacancy Frank loops of wide range of sizes due to the dissociation of dislocations. We have also found atomistically the dynamical process in which vacancy and interstitial faulted Frank loops transform into perfect loops by the application of the external shear stress or by raising the temperature. We have determined numerically the critical shear stress and temperature for the initiation of unfaulting. The simulation results clearly unveiled the important role of temperature in the unfaulting mechanism of an interstitial Frank loop.

Journal Articles

Yield point of metallic glass

Shimizu, Futoshi; Ogata, Shigenobu*; Li, J.*

Acta Materialia, 54(16), p.4293 - 4298, 2006/09

 Times Cited Count:174 Percentile:0.9(Materials Science, Multidisciplinary)

Shear bands form in most bulk metallic glasses (BMGs) within a narrowrange of uniaxial strain $$epsilon_{rm y} simeq 2%$$. We proposethis critical condition corresponds to embryonic shear band (ESB)propagation, not its nucleation. To propagate an embryonic shearband, the far-field shear stress $$tau_inftyapprox Eepsilon_{rm y}/2$$must exceed the quasi steady-state glue traction $$tau_{rm glue}$$of shear-alienated glass until the glass-transition temperature $$T_{rm g}$$ is approached internally due to frictional heating, atwhich point ESB matures as a runaway shear crack. The incubationlengthscale $$l_{rm inc}$$ necessary for this maturation is estimatedto be $$sim 10^2$$ nm for Zr-based BMGs, below which size sample-scaleshear localization does not happen. In shear-alienated glass, the lastresistance against localized shearing comes from extremely fastdownhill dissipative dynamics (DDD) of timescale comparable to atomicvibrations, allowing molecular dynamics (MD) simulations to capturethis recovery process which governs $$tau_{rm glue}$$. We model4 metallic glasses: a binary Lennard-Jones system, two binaryembedded-atom (EAM) potential systems, and a quinternary EAMsystem. Despite vast differences in the structure and interatomicinteractions, the four MD calculations give $$epsilon_{rm y}$$predictions of $$2.4%, 2.1%, 2.6%$$ and $$2.9%,$$ respectively.

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