Martensitic transformation of quaternary Ti_(50.5-X)Ni_(49.5)Zr_(X/2)Hf_(X/2) (X = 0-20 at. percent) shape memory alloys

摘要

Martensitic transformation of Ti_(50.5-X)Ni_(49.5)Zr_(X/2) quaternary alloys (X= 0-20 at. percent) is studied by different thermo-mechanical treatments. These alloys have one-stage B2 reversible B 19' transformation and exhibit >= 80 percent shape memory recovery. Their DSC forward transformation peak M* can be raised from 50 deg C to 323 deg C with transformation hysteresis being slightly larger than that of Ti_(50.5-X)Ni_(49.5)Zr_X alloys. In the early 10 cycles, thermal cycling can depress the M* temperature more significantly in Ti_(35.5)Ni_(49.5)Zr_(7.5)Hf_(7.5) than in Ti_(35.5)Ni_(49.5)Zr_(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_0-K#DELTA##epsilon#_y, in which K values are related to the as-annealed hardness of these alloys. Ti_(30.5)Ni_(49.5)Zr_(10)Hf_(10) alloy, aged in martensite phase can cause the phenomenon of thermal-induced martensite stabilization.