Phase stability and mechanical properties of Ti-Ni shape memory alloys containing platinum group metals

In order to develop Ti-Ni base shape memory alloys (SMAs), the effects of ternary additions on phase constitution and mechanical properties were investigated for TiNi alloys containing some platinum group metals: Ir, Rh and Pt. All the alloys fabricated were made by Ar arc melting method using high purity elemental materials followed by hot-forging at 1173-1673K in Ar and furnace cooling. Then X-ray diffraction analysis and tensile tests were carried out at room temperature (RT). It was found that, when the amount of ternary is lower than 10mol%, all the additions reduce martensitic transformation temperature (M-s) of TiNi and B2 phase becomes stable. Besides monoclinic, L1(0) and B19 phases appear for TiNi containing 40-50mol%Ir, 30-50mol%Rh and 20-50mol%Pt, respectively. The tensile ductility at RT decreases with increasing the amount of ternary additions, and the ductility becomes very limited when monoclinic (TiIr), L1(0) (TiRh) and B19 (TiPt) phases appear in the Ti-Ni-Ir, Ti-Ni-Rh and Ti-Ni-Pt systems, respectively. Strength strongly depends on M-s and crystal structure of the apparent phase. Small work hardening is recognized in all the alloy systems.