Effect of Cu Alloying and Heat Treatment Parameters on NiTi Alloy Phase Stability and Constitutive Behavior

NiTiCu alloys with Cu-content up to 20 at.% were successfully made into fine wire with cold-drawn area reduction of 70%. These materials were heat treated and tested at different conditions to study phase transformation and constitutive behavior. It is found that: (1) At Cu-content less than 7.5 at.%, increasing Cu stabilizes the B2 austenite, suppresses R-phase, and slightly decreases B19’ transformation temperatures. At higher level, Cu suppresses B19’-phase, but promotes B19 martensitic phase. (2) Increasing Cu decreases thermal temperature hysteresis and lattice strains during phase transformation. (3) The effect of Cu on stress hysteresis depends on test conditions. A continuous increase in Cu does not guarantee a continuously decreased stress hysteresis. (4) Heat treatment affects Ti₂(Ni,Cu)₃ precipitation. The highest phase transformation temperatures were obtained after annealing at ~ 600 °C. (5) The \({\left\{102\right\}}_{\text{B}19}\) ${\left\{102\right\}}_{\text{B}19}$ and \({\left\{120\right\}}_{\text{B}19}\) ${\left\{120\right\}}_{\text{B}19}$ martensite texture comes from the \({\left\{111\right\}}_{\text{B}2}\) ${\left\{111\right\}}_{\text{B}2}$ austenite texture after phase transformation through {111} and/or {011} martensite twins. These texture patterns explain the relatively short transformation strains and suggest potential for large improvement through processing-induced texture optimization. (6) The combined effects of Cu addition and cold work strengthening largely increase thermal cycling stabilities of NiTiCu alloys and make them promising candidates for actuator applications.