Processing-microstructure-functional properties correlation for NiTi-based pseudoelastic shape memory alloy

Abstract

Ni-rich NiTi-based shape memory alloys in the pseudoelastic state are promising candidates for various applications such as in cardiovascular stents, sensors, actuators, etc. However, achieving the desired pseudoelasticity for such applications mandates the alloy to be subjected to a critically optimized thermomechanical processing schedule. In this study, cold-rolled Ni-rich NiTi specimens are subjected to aging treatment at three different temperatures that are of industrial concern for shape setting. Systematic microstructural investigation reveals the presence of equiaxed grains with martensite structure in the processed alloys. Distinctly different phase transformation characteristics are noted for the cold-rolled and aged NiTi alloys as well. This is reflected in attaining no pseudoelasticity at all for the cold-rolled alloy, while strain recovery is regained for the aged alloys. Interestingly, subtle variation in aging temperature is noted to significantly affect the mechanical performance of NiTi alloy. Nanoindentation-based investigation reveals that highest aging temperature is beneficial in achieving maximum localized hardness for the NiTi alloy that is particularly related to the occurrence of precipitates. In contrary, best indentation depth recovery is noted for the NiTi alloy aged at an intermediate temperature. This particular aging condition also revealed highest pseudoelastic strain recovery for the studied NiTi alloy, at the global scale under monotonic or cyclic incremental tensile loading. Overall, this systematic investigation plays a pivotal role in identifying the optimized thermomechanical treatment consisting of cold rolling and aging at 525 °C to achieve the best combination of mechanical properties including pseudoelasticity for NiTi alloy that is of industrial significance.