Influence of annealing on incomplete detwinning and deformation twinning in equiatomic NiTi shape memory alloy undergoing severe plastic deformation

Abstract

Equiatomic NiTi shape memory alloy (SMA) undergoing solution treatment, which contains {1¯11} type Ⅰ twin and 〈011〉 type Ⅱ twin, suffers from severe plastic deformation (SPD) using local canning compression, where nanocrystalline grains, amorphous phase, deformation twin and detwinning can be observed. SPD-processed NiTi alloy is subjected to subsequent annealing for 2 h at 300, 450 and 600 °C, respectively. 〈011〉 type Ⅱ twin still arises in NiTi sample annealed at 300 °C and it is attributed to incomplete detwinning of 〈011〉 type Ⅱ twin in the solution-treated NiTi sample. (001) compound twin appears in NiTi sample undergoing annealing at 450 °C and it results from the retained deformation twin due to plastic deformation of solution-treated NiTi sample. However, both (001) compound twin and 〈011〉 type Ⅱ twin occur in NiTi specimen annealed at 600 °C and they are generated in the new grains as a result of crystallization. The existence of the twins substantially influences phase transformation of NiTi SMA. Following annealing at 300 and 450 °C, NiTi SMA exhibits one-stage phase transformation during heating and cooling where the corresponding martensitic transformations of the involved NiTi samples are related closely to the retained twins or the retained coarse grains resulting from SPD. NiTi sample annealed at 600 °C exhibits two-stage transformation of B2-R-B19' on cooling, whereas it presents one-stage transformation from B19' to B2 phase on heating, where the corresponding martensitic transformation of the involved NiTi sample is completely dependent on the occurrence of the twins in the new crystallized grains.