Thus, it is imperative to understand the nature of dimensional instability, responsible mechanisms, and their relative contributions to permanent shape change under cyclic thermo-mechanical loading. This study focuses on the heating-cooling response of Ti₅₀Ni₂₀Pd₃₀ HTSMA under various constant loads. The effect of increasing upper-cycle temperature on residual strains is investigated. It is found that the material exhibits large residual strain accumulation above 370°C which is attributed to martensite pinning and creep deformation. The higher the upper-cycle temperature and stress are, the greater the detected dimensional instability. The effect of heating/cooling rate on transformation temperatures and strains are also examined. Interestingly, the results demonstrate that the faster the heating/cooling rate, the larger the observed transformation strain.

The thermal-cycling experiments under zero stress following the load-biased heating-cooling tests demonstrate the recovery of large accumulated residual strains. Recovery of up to 4.3% residual strain by increasing upper-cycle temperature under zero stress shows that creep mechanisms may help relieve internal stresses built up during prior thermo-mechanical cycles, thereby unpinning oriented martensite variants, and recovering residual strains. In the light of these extensive experimental results, the competing mechanisms of creep, plasticity, and phase transformation will be discussed as a function of thermo-mechanical loading conditions.