Tailoring the thermal and stress induced martensitic transformation of NiTi shape memory alloy through He⁺ irradiation: The role of defect engineering

In the present study, various kinds of defects were introduced through He⁺ irradiation in NiTi shape memory alloy to tailor the thermal and stress induced martensitic transformation, further achieving the excellent martensitic transformation cycling stability and the larger transformation strain over the wider temperature windows as well as the higher elastocaloric effect. The results revealed that NiTi shape memory alloys were characterized by single B2↔B19ˊ martensitic transformation, regardless of He⁺ irradiation doses. The martensitic transformation temperatures of NiTi shape memory alloy were reduced owing to the blocking effect of irradiation defects on martensitic transformation. Notably, the thermal induced martensitic transformation cycling stability of NiTi shape memory alloy was improved due to He⁺ irradiation. In addition, irradiation induced defects can act as the pinning effect on the stress induced martensitic transformation, causing the discontinuous movement of martensite/austenite interface. In proportion, with He⁺ irradiation doses increasing, the critical stress dependent on the temperature became larger and larger, as well as transformation strain and transformation temperature window was reduced and shorten, respectively. Nevertheless, vacancy-type defects induced by He⁺ irradiation led to the lattice distortions, further contributing to the unit cell volume difference between martensite phase and austenite phase as well as elevating the transformational entropy change of 68 J/kg·K, ultimately achieving the giant elastocaloric response with an adiabatic temperature change of 6.5 K.