The effect of Cu addition on the hot deformation behavior of NiTi shape memory alloys

The influence of Cu addition on the hot deformation behavior of NiTi shape memory alloys was investigated using hot compression test. A series of alloys with different Cu contents of Ti50.4Ni49.6−xCux (x = 0, 3, 5, 7 at.%) were deformed under compression to a true strain of 0.7 at the temperature range of 700-1000 °C with 100 °C intervals and constant strain rate of 0.1 s−1. The stress-strain curves showed that the addition of Cu to NiTi alloy made the flow curves shift upward. This was confirmed by the calculated critical stress, σc, obtained from inflections in θ-σ plots, which is attributed to the formation of high strength Cu containing precipitates and solid solution hardening caused by substitution of Cu for Ni. It was also found that the maximum softening rate, ɛ*, obtained from θ-ɛ plots, was increased by increasing Cu content. The microstrutural analysis showed the evidence of dynamic recovery by revealing the elongated and serrated grain boundaries at 800 °C. However, a few dynamic recrystallized grains were found at this temperature. On the other hand, grain growth was also seen at 1000 °C.