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 degrees C, which results from boundary strengthening of hierarchical lath colonies. Hierarchical lath colonies are controlled by dislocations. Compression induces b =12 [111] screw dislocations on [111](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 recog-nized. 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.