Masatake Yamaguchi
Japan Atomic Energy Agency
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Featured researches published by Masatake Yamaguchi.
Journal of Physics: Condensed Matter | 2013
Tomohito Tsuru; Y Udagawa; Masatake Yamaguchi; Mitsuhiro Itakura; Hideo Kaburaki; Y Kaji
There is a pressing need to improve the ductility of magnesium alloys so that they can be applied as lightweight structural materials. In this study, a mechanism for enhancing the ductility of magnesium alloys has been pursued using the atomistic method. The generalized stacking fault (GSF) energies for basal and prismatic planes in magnesium were calculated by using density functional theory, and the effect of the GSF energy on the dislocation core structures was examined using a semidiscrete variational Peierls-Nabarro model. Yttrium was found to have an anomalous influence on the solution softening owing to a reduction in the GSF energy gradient.
Modelling and Simulation in Materials Science and Engineering | 2014
Tomoaki Suzudo; Masatake Yamaguchi; Akira Hasegawa
We report a series of ab initio studies based upon density functional theory for the behavior of rhenium and osmium atoms in body-centered-cubic tungsten crystal. Contrary to the fast one-dimensional migration of self-interstitial atoms, interstitials of these solute elements in tungsten have three-dimensional motion because they form a mixed dumbbell having a low rotation energy barrier. The migration of these solute elements strongly influences the effects of radiation upon the materials, and our results suggest that the low rotation energy barrier leading to three-dimensional migration is a property that is key to the explanation of the radiation effects experimentally observed in tungsten-rhenium and tungsten-osmium alloys.
Philosophical Magazine | 2012
Masatake Yamaguchi; Jun Kameda; Ken-ichi Ebihara; Mitsuhiro Itakura; Hideo Kaburaki
Atomistic mechanisms of hydrogen-induced cracking along a bcc Fe Σ3(111) symmetrical tilt grain boundary (GB) have been studied by first-principles calculations. The mobile and immobile effects of hydrogen on the GB decohesion are analyzed by calculating the dependence of hydrogen segregation energy on the coverage relevant to the repulsive interaction among segregated hydrogen atoms at the GB and on its fracture surfaces, together with generalizing McLeans formula. It was found that the segregation of combined mobile and immobile hydrogen atoms from the bulk and/or GB on the fracture surfaces causes much stronger reduction (70–80%) in the GB cohesive energy. It can occur even at a very low bulk hydrogen content of about 10−9 atomic fraction during slow cracking. This is in contrast to only 10–20% decohesion induced by immobile hydrogen at much higher hydrogen content during fast cracking. The mobile effect of hydrogen, giving rise to a profound reduction in the GB cohesive energy, is a key factor controlling the mechanism of hydrogen-induced GB cracking.
Philosophical Magazine | 2015
Hidetoshi Somekawa; Masatake Yamaguchi; Yoshiaki Osawa; Alok Singh; Mitsuhiro Itakura; Tomohito Tsuru; Toshiji Mukai
Magnesium alloys are very promising structural material, especially because of their low density, but their poor deformability at room temperature is making their commercial application impractical. Some alloying elements have been shown to improve the ductility dramatically, and a search for the yet better alloying elements is now a pressing task. In this study, we investigated the effects of several alloying elements on the mechanical and deformation behaviour of magnesium alloys using both first-principles calculations as well as experiments. The first-principles calculations indicate that the influential key factor for the activation of non-basal slips is the electronegativity and atomic radius. The experimental result proves that the alloys with a similar electronegativity as magnesium and a little larger atomic radius than that of magnesium, such as Ca, Sr and some rare earth elements, show superior ductility due to activation of non-basal dislocation slips. These results propose a promising design principle for the alloys with the improved deformability at room temperature ranges.
Philosophical Magazine | 2011
Yutaka Udagawa; Masatake Yamaguchi; Tomohito Tsuru; H. Abe; Naoto Sekimura
We have investigated the effects of Sn and Nb on dislocation properties in a Zr lattice to elucidate the role of these alloying elements in hydride nucleation processes. According to experimental observations, γ-hydride habit planes are close to the prismatic plane in pure Zr and close to the basal plane in Zircaloy. Dislocation loops are observed around hydride precipitates, implying they play a part in hydride formation. Our ab initio generalized stacking-fault energy calculations showed remarkable effects of Sn on unstable-stacking energy and stacking-fault energy: these parameters for basal slip were considerably reduced while those for prismatic slip were increased in the presence of Sn. These results suggest selective stabilization and enhancement of dislocation spreading in the basal plane, promoting possible elementary processes of hydride precipitation with basal habit plane, i.e. screw-dislocation spreading and edge-dislocation emission in the basal plane.
Philosophical Magazine | 2014
Masatake Yamaguchi; Jun Kameda
A significant loss of fracture toughness () is induced by intergranular (grain boundary; GB) segregation of metalloid solute in alloy steels. Yet, the mechanism has not been clarified from a multiscale point of view. From a thermodynamic approach aided by first-principles calculations, it is shown that segregated solute with higher energetic stability on fracture surfaces causes a larger linear reduction in the ideal work to intergranular fracture (); i.e. the energy difference between a GB and its two fracture surfaces. Remarkably, the combined analysis with first-principles calculations and fracture mechanics experiments found several orders of magnitude more energy loss in for a specific range in the within only a few tenths of . These results illustrate that the GB of steel has the threshold energy of atomic cohesion under which catastrophic failure occurs. Our research scheme would play a key role in identifying specific solute elements to toughen the GB in metallic systems used under aggressive environments.
Modelling and Simulation in Materials Science and Engineering | 2013
Tomoaki Suzudo; Masatake Yamaguchi; Tomohito Tsuru
He atoms introduced into materials may lead them to fracture intergranularly, and understanding such an effect is a key issue in the design of future fusion reactors. In the current study, we investigated the decrease of grain boundary (GB) strength caused by He segregation at several kinds of α-Fe GBs by exploiting first principles calculations and a set of empirical potentials. We found enough evidence to support the notion that the GB cohesive energy, a critical measure of GB strength, approximately scales with the He concentration at the GB surface, regardless of the GB type.
Journal of Materials Science | 2018
Artenis Bendo; Kenji Matsuda; Seungwon Lee; Katsuhiko Nishimura; Norio Nunomura; Hiroyuki Toda; Masatake Yamaguchi; Tomohito Tsuru; Kyosuke Hirayama; Kazuyuki Shimizu; Hongye Gao; Ken-ichi Ebihara; Mitsuhiro Itakura; Tomoo Yoshida; Satoshi Murakami
The microstructures of precipitates in Al–Zn–Mg alloys in peak-aged condition have been studied using scanning transmission electron microscope. The same thermo-mechanical treatment was applied in all alloys. Investigation of peak-aged samples revealed that the most commonly found phases were η′ and η1 with their respective habit planes on {111}Al and {100}Al. η′ phases under [110]Al were analyzed and compared with η′ structure models. Furthermore, a close inspection of η1 phase as the second most found precipitate revealed that it incorporates an anti-phase resembling boundary, not observed in other orientation relationships that precipitates create with Al matrix, in addition, differences in matrix-precipitate interfaces between η′/η2 and η1 phases were noticed. This paper addresses the first part to the analysis of η′ phase. Next part is extended to the analysis of the η1 phase.
Modelling and Simulation in Materials Science and Engineering | 2015
Mitsuhiro Itakura; Hideo Kaburaki; Masatake Yamaguchi; Tomohito Tsuru
The cross-slip process of a screw
Journal of Astm International | 2009
Yutaka Nishiyama; Masatake Yamaguchi; Kunio Onizawa; Akihiko Iwase; Hiroshi Matsuzawa