Shape memory effect of TiNi-based springs trained by constraint annealing

This study investigated shape memory effect (SME) behaviors of TiNi-based shape memory alloys springs trained by constraint annealing treatments. The results indicated that SME behaviors were determined by constraint annealing sequences and the chemical composition of alloys. Springs made by Ni-rich TiNi binary alloys showed both single-way SME (SW-SME) and two-way SME (TW-SME) depending on the constraint annealing sequences, while those made by TiNiCu alloys only showed SW-SME. The mechanism and stability of TW-SME induced by constraint annealing were discussed. The occurrence of TW-SME was found to be associated with oriented internal coherent strain caused by preferential Ti₃Ni₄ precipitates. A special method, which could achieve a stable post-inherent TW-SME and size selectivity, was proposed to provide an effective approach for devising actuators in Ni-rich alloys.