Papers by Author: Hisashi Hasegawa

Abstract: Usual static recrystallization treatment and a method to provide intense plastic deformation, ARB namely Accumulative Roll-Bonding, have been applied to two beta type titanium alloys, i.e. Ti-29Nb-13Ta-4.6Zr and Ti-15V-3Cr-3Sn-3Al. Microstructural change as well as work-hardening behavior was examined as a function of plastic strain. Both the work-hardening rate and the hardness at the initial as-hot-rolled state were smaller in the Ti-Nb-Ta-Zr alloy than in the Ti-V-Cr-Sn-Al alloy. Recrystallized grains of 14μm in size were obtained by the usual static recrystallization treatment, which was significantly smaller than that of the starting as-hot-rolled plate of 38μm. No significant change other than flattening and elongating of the original grains was found in the optical microscopic scale. It was revealed, however, from a TEM observation combined with selected area diffraction technique that geometric dynamic recrystallization occurred in the Ti-Nb-Ta-Zr alloy deformed at room temperature by a true strain of 5, resulting in an ultra-fine-grained microstructure where the grain size was roughly estimated to be about 100nm.

Abstract: The deformation characteristics of a beta-type Ti-29%Nb-13%Ta-5%Zr alloy, developed for biomedical application, and their relation with the microstructure are investigated. The cold-rolled specimen is subjected to a tensile test at high temperatures ranging from 700 to 800°C under a constant cross-head speed ranging from 1×10-4 to 1×10-2s-1. The elongations tested at different temperatures are compared with that of Ti-15%V-3%Cr-3%Sn-3%Al, a typical beta titanium alloy. The deformation mechanism is characterized from the parameter of the strain rate sensitivity. The microstructures before and after the tensile test are observed with optical microscope and the correspondent grain sizes are measured. The grain growth during the deformation is also described.