The Origin of Functional Fatigue in Shape Memory Alloys

There are a large number of elasto-caloric shape memory alloys (SMAs) with considerable future potential. However, the functionality of SMAs degrades with cycling and this is common to all shape memory alloys with no exception. The degradation of functionality means that the transformation strains decrease and the transformation domains reduce with cycling. The degradation results in changes in hysteresis and transformation stress as well. If the loops shakedown to an elastic state in the limit, the forward and reverse transformation no longer occurs and elasto-caloric effect would disappear. The presentation will explain the underlying causes of the cyclic-induced degradation in shape memory alloys from theoretical considerations. In particular, we will focus on generation of dislocations from crystallographic misfit at interfaces using advanced theory, including first principle calculations, anisotropic elasticity, and theory of martensitic transformations. This understanding at the lattice level will allow refined phenomenological models to be developed that can be utilized in engineering applications.