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Influence of Annealing on the Stress‐Assisted Two‐Way Memory Effect in Cold‐Worked NiTi Wire

NiTi shape memory alloys (SMAs) have attracted great attention in the past decades due to their novel performances, 45.3wt% Ti. The as-received wire (35% cold-drawn) was annealed in argon atmosphere at 350, 450, 550, 600, 700, 800, such as shape memory effect (SME), pseudoelasticity, and high damping. These functional properties are related to the reversible martensitic transformation. As the research of SMA-related applications expands in various industries, such as actuators, SMA pipe couplings, and SMA composites, the necessity for understanding and accurately predicting the SMA thermomechanical response grows faster. One typical issue is the stress-assisted two-waymemory effect (SATWME) of NiTi alloys. The occurrence of SATWME has been characterized by the stress-assisted martensitic transformation and the reverse transformation evoked during thermal cycling under a constant stress. Themagnitude of SATWME is affected not only by pre-strain, applied stress, and temperature but also by prior thermomechanical treatments, such as cold-work and annealing. Wada and Liu have studied the shape recovery behavior of NiTi alloys under constant stresses and variable pre-strain conditions. Bigeon and Morin investigated the influence of constant stress on the SATWME of TiNi alloy, and its maximum value appeared at about 200MPa. Miller and Lagoudas also observed the occurrence of maximum SATWME in NiTi alloy at a similar constrained stress level provided that the alloy underwent certain cold-work and heat treatment. However, the mechanism of the development of SATWME has not been explored in depth or fully understood. In order to provide more convincing experimental data, this study investigated the effect of annealing on the SATWME in cold-workedNiTi wire. Such understanding is of significance to the application of these alloys. Some new results are presented in this paper.