Papers by Author: Yoshinao Mishima

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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: Masataka Yoshino, Yoshinao Mishima, Yoshiaki Toda, Hideaki Kushima, Kota Sawada, Kazuhiro Kimura
Abstract: The precipitation behavior of MX carbonitride during a normalizing heat treatment with and without ausageing was investigated in a modified 9Cr-1Mo steel. The normalizing heat treatment was performed at 1150 oC for 1800 s. Ausageing was conducted at 765 and 500 oC for 1800 to 86400 s during the cooling from the heat treatment. The matrix of the steel was austenite single phase during normalizing and ausageing, except for that ausaged at 765 oC for 86400 s. The initial austenite grain size and hardness were not influenced by ausageing, except for the sample ausaged at 765 oC for 86400 s. Although Nb-rich MX (NbX) and cementite were observed, V-rich MX (VX) was not observed under any of the conditions investigated. The amount of NbX in the steel ausaged at 500 oC was at least twice as large as that under the other conditions, and the amount in the steel ausaged at 760 oC was slightly larger than that in the steel that did not undergo ausageing. The precipitation of NbX took place during ausageing in the austenite matrix. On the other hand, it is well known that VX precipitates during tempering. An equilibrium mole fraction of VX in the austenite matrix calculated by Thermo-Calc. was larger than that of NbX at the ausageing temperatures. It is proposed that VX is an equilibrium phase at the ausageing temperature; however, VX nucleation takes much longer in the austenite matrix. It is postulated that the precipitation of VX is more strongly influenced by the interfacial energy rather than supersaturation. It is concluded that the precipitation of MX carbonitride, especially NbX, can be controlled by ausageing during cooling after a normalizing heat treatment.
Authors: Seiji Miura, J.H. Kim, Kenji Ohkubo, Yoshisato Kimura, N. Sekido, Yoshinao Mishima, Tetsuo Mohri
Authors: Yoshisato Kimura, Kiichi Sakai, Shinya Teramoto, Yoshinao Mishima
Abstract: Aiming for further improvement of mechanical properties of Co3AlC-based heat resistant alloys, microstructure control was conducted using optical floating zone (OFZ) melting. Unidirectional solidification was performed to align Co3AlC/a(Co) two-phase eutectic microstructure. Co3AlC single phase poly-crystal alloys were successfully fabricated for the first time by taking advantage of OFZ. Mechanical properties were evaluated for selected alloys by compression tests at ambient temperature, 1073 K and 1273 K. Excellent elevated temperature strength is achieved in Co3AlC single phase alloys and ductility is sufficiently improved in Co3AlC/a(Co) two-phase alloys.
Authors: Yoshisato Kimura, Mototsugu Ohsaki, Hiroki Fujita, Yoshinao Mishima
Abstract: Aiming for the design of simple thermal coatings including Ir as a diffusion barrier layer with B2 aluminides bond coat such as CoAl, diffusion behavior and phase equilibria at the Ir/CoAl interface have been investigated to understand thermal and chemical stability of the interface. Diffusion couples were prepared with various conditions of Ir, plate, powder and film deposited to examine the effects on diffusion behavior. Moreover, the interdiffusion coefficient of binary Ir-M in the Ir solid solution, DIr-M was determined using Boltzmann-Matano method, where M is selected form elements used in heat resistant alloys. The potential of Ir as a diffusion barrier has been evaluated particularly against diffusion of Al.
Authors: Keisuke Yamamoto, Yoshisato Kimura, Yoshinao Mishima
Abstract: Precipitation behavior of intermetallic phases in ferrite matrix is investigated by transmission electron microscopy (TEM) in Fe-10Cr-1.4W-4.5Co (at%) alloys with and without 0.3at%Si. It is intended to provide basic information for the alloy design of ferritic heat resistant alloys strengthened by intermetallic compounds. In the alloy containing Si, icosahedral quasicrytalline phase (I-phase) is found to precipitate during aging at 873K. It is confirmed that selected area diffraction (SAD) patterns of the precipitates exhibit two-, three- and five-fold symmetry and have diffraction spots in the positions related to the golden section. In the Si-free alloy, the R-phase precipitates instead of I-phase at 873K, and the Laves phase precipitates in both alloys during aging at higher temperature, 973K. The Laves phase formed at 973K transforms to the I-phase in the Si-added alloy but to the R-phase in the Si-free alloy during subsequent aging at 873K. The factors in controlling the phase stability of I-phase, R-phase and Laves phase precipitates in Fe-based alloys are discussed by the atomic size ratio and electron concentration factor (e/a).
Authors: Andrey Belyakov, Kaneaki Tsuzaki, Yoshisato Kimura, Yoshinao Mishima
Abstract: 15%Cr ferritic stainless steel was machined in rectangular samples and then processed by multiple forging to a total cumulative strain of 7.2 at an ambient temperature. The large strain deformation resulted in almost equiaxed submicrocrystalline structure with a mean grain/subgrain size of 230 nm and about 2.2×1014 m-2 dislocation density in grain/subgrain interiors. The annealing at a relatively low temperature of 500oC did not lead to any discontinuous recrystallizations. The grain/subgrain size and the interior dislocation density slightly changed to 240 nm and 2.1×1014 m-2, respectively, after annealing for 30 min, while the Vickers hardness decreased from 3140 MPa in the as-processed state to 2900 MPa. This annealing softening was attributed to remarkable release (by 50%) of internal stresses, which are associated with a non-equilibrium character of strain-induced grain/subgrain boundaries.
Authors: Takeshi Shinoda, Kinichi Masuda-Jindo, Shin Takeuchi, Yoshinao Mishima, T. Suzuki
Authors: Yoshisato Kimura, Chihiro Asami, Yaw Wang Chai, Yoshinao Mishima
Abstract: A new fabrication process was proposed for half-Heusler type TiNiSn thermoelectric alloys. Based on the result that the TiNiSn phase can be formed easily at the Sn(Liquid)/TiNi(Solid) interface, the liquid-solid reaction-sintering process was developed using TiNi and Sn powders. The TiNi compound powders were prepared by the atomization method using argon gas atmosphere. We have fabrictaed nearly single-phase TiNiSn alloys and evaluated their thermoelectrical properties; the presnt TiNiSn alloys have large electrical power factor of about 3.5 mWm-1K-2, and the maximum value of dimensionless figure of merit, ZT = 0.67, can be achieved at around 700 K even without tuning of the carrier concentration through alloying elements.
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