On the role of phase transformation events in crack formation and crack propagation during fatigue of NiTi-based shape memory alloys

Today, it is established that service lives of shape memory alloys (SMAs) are limited by functional or structural fatigue. Functional fatigue refers to changes in strains/stresses and transformation temperatures during cyclic application of shape memory effects, caused by the accumulation of microstructural defects during transformation events. In contrast, structural fatigue is based on the formation and growth of cracks, e.g. during dynamic loading. Like in conventional engineering materials, fatigue cracks emanate from small stress raisers like surface scratches, drawing marks, or inclusions. It is common to distinguish between these two functional and structural degradation phenomena in shape memory technology, which is entirely correct from a user’s or engineer’s point of view. However, from the perspective of the material, things look different. In fact, both types of fatigue are governed by identical microstructural processes, and they directly depend on martensitic and reverse transformations. The present work compiles examples where high resolution electron microscopy was used to document how fatigue cracks nucleate and grow during (1) stress-free thermal cycling, (2) thermomechanical cycling and (3) pure mechanical cycling. The results demonstrate that even a single stress-free transformation cycle can generate micro cracks in chemically complex SMAs, and that the presence of external stresses can boost crack nucleation events and crack growth rates. The results are discussed on the basis of lattice incompatibility stresses and cyclic transformation / stress-redistribution processes at crack tips and other stress raisers.