Papers by Author: H. Kyogoku

Abstract: Ti-Sn-Cr shape memory alloys were fabricated by the powder metallurgy (PM) process. The mixed powders in composition of Ti-3at%Sn-7at%Cr were prepared using a V-blender. The mixed powders were filled into a graphite die, and then sintered at a temperature between 1023 K and 1223 K using a pulse current sintering equipment. The sintered alloy was solution-treated and then aged. The phase structure and the tensile properties of the alloy obtained were investigated. The relative density of the as-sintered alloy increased with elevating sintering temperature, and the maximum relative density was 99.5 % at a sintering temperature of 1223 K. Although the as-sintered alloy was high density, the microstructure of the alloy was inhomogeneous. Thus it was found that the solution treatment was needed to obtain the homogeneous microstructure of the alloy. The alloy which is mainly consisted of the β phase was obtained by performing solution treatment. The plateau region due to the slip deformation was observed in the stress-strain curve of the solution treated alloy, but that due to the stress induced martensite transformation could not be done. The plateau region due to the stress induced martensite transformation, however, was appeared in the stress-strain curve of the aged alloy. This means that the aged alloy has shape memory properties. It was also found from the result of bending test that the alloy fabricated by the PM process has obviously shape recovery properties.

Abstract: In this research, we investigated the fabrication conditions of Ti-Ni alloy powder by MA process and the shape memory characteristics of the sintered alloy by a pulse-current sintering equipment using mechanically alloyed powder (MA powder). The microstructure of the sintered alloy of the MA powder was more homogeneous than that of the alloy of the elemental powders. The application of the MA powder makes the width between transformation temperatures of the shape memory treated alloy of the MA powder became narrower, that is, it improves the temperature response of the compact. The tensile strength and elongation of the shape memory compact of the MA powder were approximately 780 MPa and 7.5 %, respectively. This is one of the superior tensile properties of SMA compact fabricated by powder metallurgy process. The superelastic behavior took place in the alloy of the MA powder. Thus, the MA process in short process time resulted in lower contamination of the MA powder and the application of the MA powder resulted in superior shape memory characteristics of the sintered alloy.

Abstract: Ti-Ni-Cu shape memory alloy by elemental powders was fabricated by using a pulse-current pressure sintering method, and it was subjected to solid-solution heat treatment after sintering to homogenize the microstructure of the alloy. The influence of the thermo-mechanical cyclic deformation on the shape memory and thermo-mechanical characteristics of Ti-Ni-Cu alloy was investigated. The thermo-mechanical cyclic deformation improved the deformation behavior of the sintered alloy and made the thermo-mechanical behavior of the alloy stable caused by the introduction of dislocations. As a result, the recovery stress of the alloy after the cyclic deformation fairly increased. The transformation temperatures of the alloy after the cyclic deformation became lower than those of the alloy before the cyclic deformation. Therefore, the superelastic-like behavior of the alloy after the cyclic deformation appeared in an isothermal tensile test at lower holding temperature.