Papers by Author: Tetsuo Mohri

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Authors: Kazufumi Sato, Satoshi Takizawa, Tetsuo Mohri
Abstract: We investigate the melting transition of the solids interacting through a simple pairwise potential using conventional and Wang-Landau Monte Carlo simulation. In the simulations, the atomic displacement is discretized for describing the atomic vibration and each atom is confined within its Voronoi polyhedron. The melting point can be uniquely determined by Wang-Landau approach while the temperature hysteresis inevitably appears in the conventional method. The obtained results show typical feature of first-order transition which is the discontinuous change in the internal energy. We discuss the relation between the limit of superheated state and intrinsic instability of the system through the comparison with two results.
Authors: Seiji Miura, Tatsuichi Tanahashi, Yoshinao Mishima, Tetsuo Mohri
Abstract: In order to spheroidize -Nb5Si3 strengthening phase embedded in Nb matrix for attaining a good room temperature toughness of Nb-Si alloy, the authors have proposed a microstructure control technique by combining eutectic and eutectoid reactions. Nb3Si intermetallic compound formed during solidification is a key phase for the microstructure control, but its stability is very sensitive to the alloying elements. Nb3Si disappears by adding as small as 3 at% of W and Mo, while these elements are very effective for the solid solution strengthening of Nb phase. For a further alloy development, establishment of an alloy design concept based on the control of phase stability of Nb3Si is needed. Similarly to ferrous alloys such as stainless steels where Cr and Ni are added to control the stability of bcc phase and fcc phase, two alloying elements (one is a stabilizing element and the other is a destabilizing element for Nb3Si phase) are added to a Nb-Si binary master alloy and their microstructure is investigated using SEM. The stabilizing element Ta is found to enlarge the composition area where Nb3Si exists even with the destabilizing element Mo, and it is confirmed that the phase stability concept is useful for designing Nb-Si based alloys.
Authors: Tetsuo Mohri
Abstract: Cluster Variation Method (CVM) has been widely recognized as one of the most reliable theoretical tools to study phase equilibria in metallic alloy systems. The conventional CVM, however, does not allow atomic local displacements and, therefore, calculated results often encounter various inconveniences such as the overestimation of transition temperatures. Continuous Displacement Cluster Variation Method (CDCVM) was proposed to circumvent such deficiencies of the conventional CVM. Preliminary studies on an order-disorder phase diagram based on CDCVM indicate that the transition temperature is shifted downward reproducing experimental tendencies. In the present study, lattice thermal vibration effects are also incorporated through Morse potential. It is concluded that the local lattice distortion effects are quite effective to reduce the transition temperature.
Authors: Seiji Miura, Suguru Yamamoto, Kenji Ohkubo, Tetsuo Mohri
Authors: Tetsuo Mohri, Munekazu Ohno, Ying Chen
Abstract: First principles calculations have been applied in various fields in Materials Science. The authors have been attempting to reproduce a binary phase diagram by combining FLAPW electronic structure total energy calculations with Cluster Variation Method of statistical mechanics. Such a first principles calculation for static equilibrium has been quite successful for a series of Febased alloy systems. Recently, main attention is directed towards the extension of the first principles calculation to phase transformation dynamics by incorporating Phase Field Method. A series of preliminary calculations on disorder-L10 ordering in Fe-Pd and –Pt are satisfactory and the evolution process of Anti-Phase Boundaries was reproduced. In the present report, first-principles calculations of phase equilibria and phase transformation are briefly reviewed. Particular focuses are placed on coarse graining operation which authors developed and and time scaling as a remaining problem.
Authors: Tetsuo Mohri
Abstract: The detailed behavior of the free energy of Cluster Variation Method in the vicinity of spinodal ordering transition is examined. The generalized phase diagram proposed in the previous study is modified and spinodal ordering transition is reinterpreted as a limiting case of the ideal glass transition.
Authors: Seiji Miura, Hiroyuki Shimamura, Kenji Ohkubo, Tetsuo Mohri
Abstract: Investigation on the crystallographic orientation relationships among D022-Al3Ti, A15-Mo3Al and high temperature bcc phase consisting of an Al-Mo-Ti ternary alloy with an equi-axed two-phase structure was conducted by FESEM/EBSD analysis. The grains of intermetallic phases have certain crystallographic orientations each other, while the crystallographic orientation distributions become random after a high temperature deformation. This strongly suggests the grain boundary sliding and grain rotating govern the high temperature deformation, which is consistent with the fact that the strain-rate sensitivity m is 0.3 or higher during a steady-state compressive deformation.
Authors: Seiji Miura, Hélio Goldenstein, Kenji Ohkubo, Hisashi Sato, Yoshimi Watanabe, Tetsuo Mohri
Abstract: Ni3Al-based alloys possess good oxidation resistance, moderate room and high temperature strength and ductility. Introduction of Cr-carbide particles through a solidification route is attempted to provide higher hardness and wear resistance. The mechanical and physical properties are measured at room temperature for several alloys with various carbon concentrations up to 2.0 wt.%. Hardness and wear resistance as well as compressive strength increase with increasing carbon concentration, while bend ductility decreases. Adhesion between carbides and matrix phase is good, and cracks propagate mainly through carbides. The crystallographic orientation relationships between constituent phases are also attempted using electron back-scattering diffraction (EBSD) technique. The thermal conductivity is found to be less sensitive to the alloy composition.
Authors: Seiji Miura, J.H. Kim, Kenji Ohkubo, Yoshisato Kimura, N. Sekido, Yoshinao Mishima, Tetsuo Mohri
Authors: Munekazu Ohno, Ying Chen, Tetsuo Mohri
Abstract: Multi-scale simulation of ordering process from electronic, atomistic scales to microstructural scale was carried out by hybridizing Phase Field Method (PFM) and Cluster Variation Method (CVM). The hybrid model was applied to disorder-L10 ordering process in Fe-Pd system. Furthermore, computation of relaxation constants in the PFM was attempted based on Path Probability Method (PPM) which is the time evolution version of the CVM, within a linearized analysis of order-order relaxation process.
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