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

Abstract 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 Ti 2 (Ni,Cu) 3 precipitation. The highest phase transformation temperatures were obtained after annealing at ~ 600 °C. (5) The $${\left\{102\right\}}_{\text{B}19}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mfenced> <mml:mn>102</mml:mn> </mml:mfenced> <mml:mrow> <mml:mtext>B</mml:mtext> <mml:mn>19</mml:mn> </mml:mrow> </mml:msub> </mml:math> and $${\left\{120\right\}}_{\text{B}19}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mfenced> <mml:mn>120</mml:mn> </mml:mfenced> <mml:mrow> <mml:mtext>B</mml:mtext> <mml:mn>19</mml:mn> </mml:mrow> </mml:msub> </mml:math> martensite texture comes from the $${\left\{111\right\}}_{\text{B}2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mfenced> <mml:mn>111</mml:mn> </mml:mfenced> <mml:mrow> <mml:mtext>B</mml:mtext> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> 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.