A Study on Ternary Ti-rich TiNiZr Shape Memory Alloys

The martensitic transformation in Ti50.5-XNi49.5ZrXand Ti51.5-XNi48.5ZrX 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 Ti41.5Ni48.5Zr10 alloy than in the Ti40.5Ni49.5Zr10 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 Ms temperatures of these alloys were found to follow the expression Ms = T0-KΔσy, 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 Ti26.5Ni48.5Zr15 alloy may be due to the pinning effect on the interfaces of martensite plates by the point defects.