*Invited* On the Effect of Crystallographic Compatibility on Functional Fatigue in Niti-Based Shape Memory Alloys

The service life of shape memory components is often limited through a degradation of functional properties, which is referred to as functional fatigue. Both shape memory actuators and pseudoelastic components accumulate irreversible strain during thermomechanical and mechanical cycling, respectively. It has been observed that this behavior is related to the introduction of dislocations in the microstructure within each cycle. In the last decades, materials scientists and engineers have identified various strategies which aim at minimizing functional fatigue. These strategies have in common that they all represent measures to make the shape memory alloy (SMA) stronger against plastic deformation. Both nanocrystalline grain structures and precipitation hardening have been found to be very effective in reducing dislocation activity and thus, in improving cyclic stability. In the present work, we now introduce a different strategy. The accumulation of dislocations and associated irreversible effects are directly related to the crystallographic compatibility between martensite and austenite crystal structures. A better compatibility lowers the nucleation barrier (as indicated through a lower thermal / mechanical hysteresis width) and requires less accommodation. Thus, a more stable bahavior is obtained. Most importantly, the crystallographic compatibility between the high and the low temperature phase depends on alloy composition, and alloying copper and palladium reduces the misfit between austenite and martensite. The results of the present study can be helpful for the design of new SMAs with a better functional stability.