Evolution of microstructure and properties in NiTi shape memory alloy thin films after proton irradiation

Ni-51.5at.%Ti shape memory alloy (SMA) thin films were irradiated by 120 keV protons with fluences ranging from 1.0 × 10¹⁵ p/cm² to 2.0 × 10¹⁶ p/cm². Moreover, the effects of irradiation conditions on the microstructure, martensitic transformation behavior, shape recovery characteristics and internal friction of the thin films were investigated systematically. The experimental results and analysis revealed that proton irradiation with a fluence of 2.0 × 10¹⁶ p/cm² induced the transformation of a martensitic phase to austenite phase at the surface of thin films. The austenite layer was spatially uniform and terminates at an abrupt interface with the underlying martensite layer. Besides, the shape recovery characteristics significantly deteriorated and the internal friction of martensite obviously raised with the irradiation fluence increasing. The variations of shape recovery characteristics and internal frictions are closely related to the evolution of microstructure and the generation of multiple defect during proton treatment. Irradiation induced an amount of crystal defect, like vacancies, dislocation, stacking fault and nano-amorphous and so on.