Martensitic transformation of quaternary Ti 50.5− XNi 49.5Zr X/2 Hf X/2 ( X=0–20 at.%) shape memory alloys

Abstract

Martensitic transformation of Ti 50.5− X Ni 49.5Zr X/2 Hf X/2 quaternary alloys ( X=0–20 at.%) is studied by different thermo-mechanical treatments. These alloys have one-stage B2↔B19′ transformation and exhibit ≥80% shape memory recovery. Their DSC forward transformation peak M* can be raised from 50°C to 323°C with transformation hysteresis being slightly larger than that of Ti 50.5− X Ni 49.5Zr X alloys. In the early 10 cycles, thermal cycling can depress the M* temperature more significantly in Ti 35.5Ni 49.5Zr 7.5Hf 7.5 than in Ti 35.5Ni 49.5Zr 15 due to the former alloy having higher hardness in the matrix. Martensite stabilization can be induced by cold rolling at room temperature. The strengthening effects of cold rolling and thermal cycling on Ms temperature are found to follow Ms= T o− KΔ σ y, in which K values are related to the as-annealed hardness of these alloys. Ti 30.5Ni 49.5Zr 10Hf 10 alloy, aged in martensite phase can cause the phenomenon of thermal-induced martensite stabilization.