Investigations of Twin Boundaries and Meso-Scale Phase Interfaces In NiTi

NiTi shape memory alloys exhibit complex microstructures that make the material capable of extraordinary mechanical behavior. Interfaces play a key role in determining the mechanical properties on several length scales. In this contribution, we  present recent Finite Element modeling results on (i) martensitic twin boundaries, and (ii) meso-scale phase interfaces in thin specimens during tensile loading. On the micro-scale, we consider stacks of thin martensite twins (type I, type II, or compound twins) under mechanical loading, and we analyze how elastic anisotropy results in considerable compatibility stresses. We discuss the potential effect of these stresses on microstructure evolution and on functional degradation in ultra fine-grained NiTi. On the macro-scale, we compare experimental and theoretical results on strains and phase distributions in the meso-scale phase boundaries between austenitic regions and fully martensitic bands in thin NiTi wires. Surface strains obtained by digital image correlation agree well with Finite Element modelling results based on a simple total transformation strain model. This allows a detailled analysis of bulk strains and phase volume fractions in the meso-scale interfaces, a characterization of their width and geometry as well as a discussion of how multi-axial stresses promote a stable, localized deformation in tensile specimens – or in geometrically more complex specimens and devices.