Transformation twinning and deformation twinning of NiTi shape memory alloy

Abstract

Transformation twinning and deformation twinning of NiTi shape memory alloy are investigated on the basis of a nominal chemical composition of Ni50Ti50 and Ni50.9Ti49.1 (atomic percent), respectively. In the case of Ni50Ti50 alloy, {111¯} type Ⅰ twin and 〈011〉 type Ⅱ twin can be observed in the NiTi matrix, but it seems that {111¯} type Ⅰ twin is dominant. In the case of as-rolled Ni50Ti50 alloy subjected to heating for 2h at 850°C and subsequent quenching into liquid nitrogen, in particular, the primary twin and the secondary twin can be observed. The occurrence of the secondary twin plays a critical role in guaranteeing the compatibility of martensitic transformation. In the Ni50.9Ti49.1 samples aged at 600°C, the occurrence of martensitic twins is attributed to the local inhomogeneous composition in the microstructures and the dislocation networks at the interface between the incoherent Ni4Ti3 precipitates and the B2 matrix. The (001) martensitic compound twins appear in the Ni50.9Ti49.1 sample subjected to local canning compression at room temperature and subsequent annealing at 600°C. The phenomenon is attributed to the fact that the lattice distortion along with the dislocation defects frequently occurs in the coarse-grained NiTi sample derived from crystallization of the amorphous phase and thus contributes to the occurrence of the (001) martensitic compound twins. A high density of dislocations is distributed in the B2 austenite matrix and {114} B2 austenitic compound twin can be observed in the Ni50.9Ti49.1 sample subjected to local canning compression at 300°C, which indicates that dislocation slip and deformation twinning play an important role in local canning compression of Ni50.9Ti49.1 alloy at 300°C.