Facile Design and Fabrication of Two-Way Shape Memory TiNi Springs with Narrow Hysteresis

摘要

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 Ti_3Ni_4 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 long-term cyclability, which are of great importance in the practical application of actuators.