Martensite twin formation and mechanical properties of B2 austenite NiTi shape memory alloy undergoing severe plastic deformation and subsequent annealing

B2 austenite NiTi shape memory alloy (SMA) with the chemical composition of Ni-49.3 at.%Ti underwent severe plastic deformation and subsequent annealing at 450 °C for various times, including 4, 8 and 12 h. As a result, nanostructured NiTi SMA can be obtained by crystallization of amorphous NiTi SMA. In particular, 1¯1¯1 type I martensite twin and {111} type I martensite twin are found to appear in nanostructured NiTi samples where B2 austenite is dominant in the matrix. Compared to the solution-treated NiTi SMA, nanostructured NiTi SMA possesses an extremely high yield stress. The effect of grain refinement plays a pronounced role in enhancing yield stress of NiTi sample. In addition, the critical stress for inducing martensite transformation decreases with the increase of grain size. This means that stress-induced martensite transformation of NiTi SMA is related to grain size and it is gradually inhibited with decreasing nanocrystalline grain size.