A Study on Ternary Ti-rich TiNiZr Shape Memory Alloys

Abstract

The martensitic transformation in Ti 50.5- X Ni 49.5Zr X and Ti 51.5- X Ni 48.5Zr X alloys ( X = 0–25 at.%) was studied by using thermomechanical treatments. These alloys have a B2↔B19′ transformation sequence, and their transformation peak temperature M* can be raised to 50–450°C by different additions of Zr. Although a great many second-phase particles exist around (Ti,Zr)Ni grain boundaries, these alloys still exhibit ≥80% shape-memory recovery. Thermal cycling can depress the M* temperature more significantly in the Ti 41.5Ni 48.5Zr 10 alloy than in the Ti 40.5Ni 49.5Zr 10 alloy in the first ten cycles, owing to the former’s having greater hardness and more second-phase particles. Martensite stabilization can be induced by cold rolling at room temperature for Ti-rich ternary TiNiZr alloys. The strengthening effects of cold rolling and thermal cycling on M s temperatures of these alloys were found to follow the expression M s = T 0- KΔσ y, in which K values are related to the as-annealed hardness of these alloys. For the study of 400°C aging effects, the martensite stabilization appearing in the Ti 26.5Ni 48.5Zr 15 alloy may be due to the pinning effect on the interfaces of martensite plates by the point defects.