Martensitic transformation of quaternary Ti50.5−XNi49.5ZrX/2HfX/2 (X=0–20 at.%) shape memory alloys

Martensitic transformation of Ti50.5−XNi49.5ZrX/2HfX/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 Ti50.5−XNi49.5ZrX alloys. In the early 10 cycles, thermal cycling can depress the M* temperature more significantly in Ti35.5Ni49.5Zr7.5Hf7.5 than in Ti35.5Ni49.5Zr15 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=To−KΔσy, in which K values are related to the as-annealed hardness of these alloys. Ti30.5Ni49.5Zr10Hf10 alloy, aged in martensite phase can cause the phenomenon of thermal-induced martensite stabilization.