Hot deformation processing of shape memory alloys: A review of effects on plastic flow behaviour, deformation mechanisms, and functional characteristics

Hot working processes are notable processes for transforming as-cast or wrought metallic materials, including shape memory alloys (SMAs), into semi-finished or finished products for various industrial applications. It is expected that this processing aimed at improving the workability of the SMAs, should preserve their mechanical strength and functional characteristics – in particular, their shape memory characteristics. In conventional alloys, material properties have been tailored through careful control of the hot deformation process parameters. In the case of SMAs, analyses of information on hot deformation processing and the impact on flow behaviour, mechanical and shape memory features, has not been comprehensively covered in review literatures. This review attempts to fill this gap through a systematical analyses of hot deformation behaviour of NiTi, Cu-based and Fe-based SMAs. The observations indicated that plastic flow progressed by work hardening, softening and steady-state flow mechanisms, and dynamic recovery and dynamic recrystallization, dominated as deformation mechanisms. Their prevalence was largely shape memory alloy (SMA) type, composition, strain rate and deformation temperature, dependent. The strength, hardness and ductility of the SMAs generally improved after hot deformation processing; however, the transformation temperatures and shape memory properties, were found to decrease in most cases (albeit, the Cu based SMAs exhibited improved superelasticity).