Hierarchical lath colonies induced by dislocation rearrangement improve thermal cyclic stability of NiTi shape memory alloy

High thermal cyclic stability is achieved in Ti-50.8at.%Ni alloy by fabricating hierarchical lath colonies along {112}B2 planes using isothermal compression and aging. Transformation temperature shift between the 1st and 20th thermal cycle is 0.03∼0.27°C, which results from boundary strengthening of hierarchical lath colonies. Hierarchical lath colonies are controlled by dislocations. Compression induces b=12[1¯11] screw dislocations on [1¯11](121¯) slip system with anti-phase boundaries (APBs) located on (121¯)B2 plane, while aging promotes dislocation motion and rearrangement, and adjustment of atomic site occupation in local structure near APBs. When distance of APBs is large, one-direction lath-colony boundaries and one-direction lath colony form. When distance of APBs reduces, clover-shaped lath-colony boundaries form, and hierarchical lath colonies are recognized. If area of APB reduces, lath-colony boundaries and lath colony vanish. These findings show the potential for improving properties of NiTi alloy by dislocation engineering.