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Martensitic transformation temperatures are the most important indicators for designing shape memory alloy (SMA) components. Subjected to alternating temperatures in service, they undergo functional fatigue, which affects little the thermal hysteresis. We investigate the thermal cycling induced functional fatigue in Ti44Ni47Nb9 wide hysteresis SMA, decorated with multi-scale Nb-rich particles. We identify an accelerated functional fatigue behavior leading to the hysteresis widening from 58 to 71 K after 20 thermal cycles, in contrast to which hysteresis maintains nearly constant in Ti-Ni binary SMAs. Compared to Ti49Ni51 with similar transformation temperatures and latent heats, the hysteresis widening in Ti44Ni47Nb9 is attributed to accelerated decrease in starting temperature of martensite transformation (Ms). Microscopic characterizations reveal an inhomogeneous microstructure, consisting of (Ti,Nb)Ni B2 matrix and multi-scale Nb-rich particles and (Ti,Nb)2Ni particles. The presence of unique multi-scale Nb-rich particles has been demonstrated at the core of the underlying mechanisms of accelerated functional fatigue. Densely distributed Nb-rich nanoparticles narrow the mean matrix distance of transformation to 156.3 nm, which restricts the size and cross-over of the transformation induced dislocation leaves. Our work provides a new mechanism to widen the thermal hysteresis of SMAs, which is probably applicable to the other SMAs systems with dense nanoparticles.
Liu et al. (Fri,) studied this question.