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Harjo, S.; Kubota, Satoru*; Gong, W.*; Kawasaki, Takuro; Gao, S.*
Acta Materialia, 196, p.584 - 594, 2020/09
Times Cited Count：0
WB-STEM observation of dislocation loop behavior in reactor pressure vessel steel during post-irradiation annealingDu, Y.*; Yoshida, Kenta*; Shimada, Yusuke*; Toyama, Takeshi*; Inoue, Koji*; Arakawa, Kazuto*; Suzudo, Tomoaki; Milan, K. J.*; Gerard, R.*; Onuki, Somei*; et al.
Materialia, 12, p.100778_1 - 100778_10, 2020/08
In order to ensure the integrity of the reactor pressure vessel in the long term, it is necessary to understand the effects of irradiation on the materials. In this study, irradiation-induced dislocation loops were observed in neutron-irradiated reactor pressure vessel specimens during annealing using our newly developed WB-STEM. It was confirmed that the proportion of 
loops increased with increasing annealing temperature. We also succeeded in observing the phenomenon that two 
loops collide into a loop. Moreover, a phenomenon in which dislocation loops decorate dislocations was also observed, and the mechanism was successfully explained by molecular dynamics simulation.
Yamashita, Takayuki; Morooka, Satoshi; Harjo, S.; Kawasaki, Takuro; Koga, Norimitsu*; Umezawa, Osamu*
Scripta Materialia, 177, p.6 - 10, 2020/03
Times Cited Count：1 Percentile：100(Nanoscience & Nanotechnology)
113
deformation twinning in metastable 
-type Ti alloy focusing on stress-induced 
martensite phaseCho, K.*; Morioka, Ryota*; Harjo, S.; Kawasaki, Takuro; Yasuda, Hiroyuki*
Scripta Materialia, 177, p.106 - 111, 2020/03
Times Cited Count：2 Percentile：23.06(Nanoscience & Nanotechnology)Wang, Y.*; Tomota, Yo*; Omura, Takahito*; Morooka, Satoshi; Gong, W.*; Harjo, S.
Acta Materialia, 184, p.30 - 40, 2020/02
Times Cited Count：2 Percentile：23.06(Materials Science, Multidisciplinary)Harjo, S.; Kawasaki, Takuro; Grazzi, F.*; Shinohara, Takenao; Tanaka, Manako*
Materialia, 7, p.100377_1 - 100377_9, 2019/09
Ahadi, A.*; Khaledialidusti, R.*; Kawasaki, Takuro; Harjo, S.; Barnoush, A.*; Tsuchiya, Koichi*
Acta Materialia, 173, p.281 - 291, 2019/07
Times Cited Count：3 Percentile：43.71(Materials Science, Multidisciplinary)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：9 Percentile：8.05(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.
Bendo, A.*; Matsuda, Kenji*; Lee, S.*; Nishimura, Katsuhiko*; Toda, Hiroyuki*; Shimizu, Kazuyuki*; Tsuru, Tomohito; Yamaguchi, Masatake
Materialia, 3, p.50 - 56, 2018/11
Wang, B.*; He, H.*; Naeem, M.*; Lan, S.*; Harjo, S.; Kawasaki, Takuro; Nie, Y.*; Wang, X.-L.*; 7 of others*
Scripta Materialia, 155, p.54 - 57, 2018/10
Times Cited Count：18 Percentile：7.16(Nanoscience & Nanotechnology)Tsuru, Tomohito; Somekawa, Hidetoshi*; Chrzan, D. C.*
Acta Materialia, 151, p.78 - 86, 2018/06
Times Cited Count：17 Percentile：6.25(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 (
) and IVB (
) 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.
Somekawa, Hidetoshi*; Tsuru, Tomohito; Singh, A.*; Miura, Seiji*; Schuh, C. A.*
Acta Materialia, 139, p.21 - 29, 2017/10
Times Cited Count：8 Percentile：35.28(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.
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：12 Percentile：25.04(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 
-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.
neutron diffractionTomota, Yo*; Gong, W.*; Harjo, S.; Shinozaki, Tomoya*
Scripta Materialia, 133, p.79 - 82, 2017/05
Times Cited Count：13 Percentile：25.04(Nanoscience & Nanotechnology)Somekawa, Hidetoshi*; Tsuru, Tomohito
Scripta Materialia, 130, p.114 - 118, 2017/03
Times Cited Count：9 Percentile：31.39(Nanoscience & Nanotechnology)The impact of alloying elements on crack propagation and atomistic phenomenon at {10
2}-type twin boundaries in magnesium was investigated via both experiments and calculations. The alloying elements clearly affected crack propagation behavior. Cracks were difficult to propagate along matrix-deformation twinning interfaces in alloys that had high fracture toughness. In such magnesium alloys, the solute atoms, e.g., silver, manganese and zinc atoms, create adhesive interactions between magnesium atoms. Closed-shell and covalent-like bonding of these kinds of solute atoms would influence strong adhesion, which impedes the nucleation of a new surface at the twin boundary.
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：39 Percentile：4.2(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
Si
Mn
) 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 NiSi
(Fe,Cr)Mn.
Hase, Takayuki*; Otagaki, Tatsuya*; Yamaguchi, Masatake; Ikeo, Naoko*; Mukai, Toshiji*
Acta Materialia, 104, p.283 - 294, 2016/02
Times Cited Count：21 Percentile：13.08(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.
Suzudo, Tomoaki; Nagai, Yasuyoshi*; Schwen, D.*; Caro, A.*
Acta Materialia, 89, p.116 - 122, 2015/05
Times Cited Count：6 Percentile：53.88(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.
-phase and high-pressure 
-phase of pure group IV transition metalsAdachi, Nozomu*; Todaka, Yoshikazu*; Suzuki, Hiroshi; Umemoto, Minoru*
Scripta Materialia, 98, p.1 - 4, 2015/03
Times Cited Count：9 Percentile：41.41(Nanoscience & Nanotechnology)The deformation texture of the high-pressure -phase of pure group IV metals was investigated using neutron diffraction analysis. The 0001 direction gradually aligned to the shear direction with an increase in the shear strain introduced by the high-pressure torsion process. The deformation texture suggested that the main slip system of the -phase was {11-20}0001. We experimentally observed the orientation relationships of the mechanically stabilized -phase with respect to the reverse-transformed -phase, and determined that the relationship was similar to that proposed by Silcock and Rabinkin.
Gong, W.; Tomota, Yo*; Harjo, S.; Su, Y.; Aizawa, Kazuya
Acta Materialia, 85, p.243 - 249, 2015/02
Times Cited Count：72 Percentile：1.45(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.
