Microstructural Evolution of Plastic Deformation of NiTi Shape Memory Alloy at Low Temperature

Nickel-titanium shape memory alloy(NiTi SMA) which possesses crystal structure of B2 austenite at room temperature was subjected to plastic deformation at low temperature(-150 ℃) by means of local canning compression.The microstructural evolution of NiTi SMA at the different deformation degree was investigated by transmission electron microscopy(TEM) and high resolution transmission electron microscopy(HRTEM).At the deformation degree by 15%,a high density of dislocations occurs in the deformed NiTi sample.At the deformation degree by 25%,the deformed NiTi sample exhibits the martensite morphology due to the pinning of dislocations at the grain boundaries.At the deformation degree by 50%,a small amount of nanocrystalline phase arises in the deformed NiTi sample.At the deformation degree by 80%,severe plastic deformation(SPD) leads to the occurrence of a great deal of amorphous and nanocrystalline phase.