The Effect of Rolling Process on the Actuation Fatigue Behavior of Ni₅₀Ti₂₅Hf₂₅ High Temperature Shape Memory Alloy

NiTiHf High Temperature Shape Memory Alloys (HTSMAs) with high Hf content are promising candidates for very high temperature actuation applications. However, high operating temperatures at which HTSMAs are used are detrimental to their actuation capabilities since they lose their actuation fatigue performances with the increase of plastic deformation via number of thermal cycles. Therefore, stable actuation behavior at high temperatures is vital to achieve long actuation fatigue lives. In this work, the stability of the shape memory characteristics was investigated by conducting thermo-mechanical deformation studies on ternary Ni₅₀Ti₂₅Hf₂₅ (at. %) HTSMA. Cold rolling and warm rolling operations at 500 and 600 °C with 5% thickness reduction were applied to improve the stability of the shape memory properties of the alloy. DSC studies were done to determine the transformation temperatures. Furthermore, thermo-mechanically treated samples were subjected to actuation fatigue experiments under 200 MPa. Actuation strain, transformation temperatures, thermal hysteresis and austenite strains were determined from the experiments. Although the thermal cycles were run at very high temperatures it was observed that actuation stability of the ternary Ni₅₀Ti₂₅Hf₂₅ (at. %) HTSMA was enhanced and plasticity due to phase transformation at high temperatures and under constant load was mitigated after rolling operations.