The effect of heat and mechanical treatments on the NiTi SMA tensile behaviour

Equiatomic NiTi alloys exhibit shape memory or superelastic behaviour. This remarkable property allows them to "remember" their original shape. The key lies in a reversible change in their crystal structure (phase transition) triggered by stress or temperature, without any permanent rearrangement of the crystals (recrystallisation). Unlike traditional metals, the grains can deform but maintain their overall arrangement. Initially, in shape memory alloys with pseudoelastic behaviour, the stable phase is the austenite, characterised by a cubic lattice. Under tension, austenite behaves elastically at first. However, a unique phenomenon occurs upon reaching a critical stress level: instead of yielding or plastically deforming, austenite transforms into a new phase called martensite with a monoclinic lattice. This study examined the tensile behavior of an equiatomic NiTi shape memory alloy characterised by a pseudoelastic effect after various thermomechanical treatments. The resulting stress-strain curves were employed to assess the influence of these treatments on parameters within two constitutive models from the literature: Di Cocco and Natali's microstructure evolution model and Bellini et al.'s model.