Facile design and fabrication of two-way shape memory TiNi springs with narrow hysteresis

In this article, two-way shape memory TiNi springs with narrow hysteresis are fabricated using two-step successive constrained annealing treatments. The as-fabricated springs inherently exhibit two-way shape memory effect (TW-SME) after constrained annealing treatments. The introduction of stable, preferential internal stresses generated by coherent Ti3Ni4 phases into the TiNi matrix is indispensable to the TW-SME behavior, which is found to be closely associated with applied strains and the sequence of phase transformations. A simple structural criterion is proposed to ensure the narrow hysteresis. A chart concerning operating temperature points is also plotted as a reference for tracing annealing treatments. The advantages of the springs fabricated by the proposed design consist in its dimensional controllability and longterm cyclability, which are of great importance in the practical application of actuators.