Microstructural evolution and phase transformation in proton-irradiated NiTi films

In the present work, the microstructural characteristics, martensitic transformation behavior, and mechanical performance of NiTi thin films irradiated with different proton fluences were investigated. Multiple-layer structures formed in the NiTi thin films owing to proton irradiation. As the proton fluences were 1.0 x 1015 p/cm2 and 5.0 x 1015 p/cm2, a two-layer structure containing a B2 phase and a B19' martensite phase was introduced. In addition, thickness of B2 phase layer becomes larger and larger for irradiated NiTi thin films at room temperature, as the proton irradiation fluence increases. With further increasing proton irradiation fluence to 2.0 x 1016 p/cm2, an amorphous layer forms at the outmost layer of NiTi thin film. Moreover, an R phase and GP zones were detected in the interior of the B2 layer. The twin type of B19' martensite also changed to release the stored elastic energy induced by proton irradiation. The martensite variant related (111) type I twins and the substructure of the 011 type II twins gradually evolved into (111) type I twins and (001) compound twins, respectively, with increasing proton irradiation fluence. The formation of a multiple-layer structure resulted in the presence of two endothermic and exothermic peaks corresponding to the B19' B2 martensitic transformation. The introduction of lattice defects was responsible for the reduction in the martensitic transformation temperature. The strength of the irradiated NiTi films was enhanced as a result of dislocation strengthening. However, the introduction of higher density of defects also leads to the deterioration of ductility.

Automated summary

The study found that proton irradiation had an influence on the microstructural characteristics, martensitic transformation behavior, and mechanical performance of NiTi thin films, introducing multiple-layer structures and new transformation behaviors. Additionally, the effects of irradiation on NiTi thin films were enhanced with increasing irradiation fluence.