Martensitic transformation of quaternary Ti50.5-XNi49.5ZrX/2HfX/2 (X=020 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 a lloys have one-stage B2<->B19' transformation and exhibit greater than or e qual to 80% shape memory recovery. Their DSC forward transformation peak M* can be raised from 50 degreesC to 323 degreesC with transformation hystere sis being slightly larger than that of Ti50.5-XNi49.5ZrX alloys. In the ear ly 10 cycles, thermal cycling can depress the M* temperature more significa ntly in Ti35.5Ni49.5Zr7.5Hf7.5 than in Ti35.5Ni49.5Zr15 due to the former a lloy having higher hardness in the matrix. Martensite stabilization can be induced by cold rolling at room temperature. The strengthening effects of c old rolling and thermal cycling on Ms temperature are found to follow Ms = T-o-K Delta (sigmay), in which K values are related to the as-annealed hard ness of these alloys. Ti30.5N49.5Zr10Hf10 alloy, aged in martensite phase c an cause the phenomenon of thermal-induced martensite stabilization. (C) 20 00 Elsevier Science Inc. All rights reserved.