Influence of annealing on NiTi shape memory alloy subjected to severe plastic deformation

The amorphous phase with the retained nanocrystalline phase, the deformation bands and the amorphous bands coexist due to inhomogeneous plastic deformation in nickel–titanium shape memory alloy (NiTi SMA) subjected to severe plastic deformation (SPD) based on local canning compression. Transmission electron microscopy, X-ray diffraction and differential scanning calorimetry are used to investigate microstructural evolution and phase transformation of NiTi sample subjected to SPD in the case of annealing for 2 h at 300 °C, 450 °C and 600 °C, respectively. Annealing at 300 °C and 450 °C leads to nanocrystallization of amorphous NiTi sample, while annealing at 600 °C results in the coarse-grained NiTi sample, where the (001) martensite compound twins are found. The precipitation phases such as Ni4Ti3 and Ni3Ti are suppressed in NiTi sample formed as a result of crystallization of the amorphous structure and thus occur more easily in the deformation bands in the case of annealing. Martensitic phase transformation is influenced by the grain size and is suppressed with the decrease in the nanocrystalline grain size.