Shape memory characteristics of the P/M-processed Ti–Ni–Cu alloys

Abstract

Ti–Ni and Ti–Ni–Cu shape memory alloys (SMAs) were fabricated by the powder metallurgy (P/M) process using mechanical alloyed (MAed) powder. The phase transformation behavior shape memory characteristics and thermo-mechanical properties of the P/M alloys were investigated. The stress–strain curves of Ti–45.2 mol%Ni–5 mol%Cu and Ti–35.2 mol%Ni–15 mol%Cu alloys were stabilized by the thermo-mechanical training. The two-way shape memory effect appeared after the thermo-mechanical training. Only the Ti–30.2 mol%Ni–20 mol%Cu alloy showed clearly a two-step transformation. The transformation behavior of the P/M alloy was different than that of the wrought alloy because the Cu content of the P/M alloy was reduced by composition segregation. The narrowest hysteresis measured in Ti–35.2 mol%Ni–15 mol%Cu was 9 K, which is as narrow as that of the wrought alloy. The Ti–30.2 mol%Ni–20 mol%Cu alloy had the highest stress for slip deformation due to solid–solution hardening by Cu addition. The plastic strain of the alloy thermal-cycled at 175 MPa was 0.06%. The Ti–35.2 mol%Ni–15 mol%Cu alloy had the lowest value of d σ/d T and the alloy showed superelasticity in the austenite phase. Thus, it was found that the Ti–Ni and Ti–Ni–Cu SMAs with superior shape memory characteristics could be fabricated by the P/M process.