Experimental and theoretical aspects of the elastic properties of NiTi on several length scales

The most simple and fundamental characteristics of NiTi shape memory alloys, the elastic properties, are not well understood today: From an engineer's point of view, the slopes of stress-strain data during loading and unloading of austenite/martensite describe the macroscopic elastic behavior and hence need to be implemented in simulation codes. But thermo-mechanical testing and microstructural investigations reveal that these rather low macroscopic moduli are affected by localized transformation, and by additional inelastic deformation processes. In consequence, the “true” elastic moduli are expected to be larger. In this contribution, we discuss experimental data on the macrosopic elastic moduli below, in, and beyond the pseudo elastic plateau, which first decrease from the austenitic to the martensitic “end of plateau” modulus, but increase with further straining. We rationalize this behavior in the light of the dominant microstructural processes (e.g. further transformation beyond the pseudoelastic plateau, reorientation, detwinning, plasticity). We present complementary finite element results on the effect of localized transformation bands on macroscopic moduli. Nanoindentation experiments, where martensitic NiTi is subjected to a multiaxial stress-state, reveal that considerably higher elastic moduli govern the indentation response. Finally, we relate the rather low experimentally observed elastic moduli to novel atomistic first-principles simulations which yield the full set of elastic constants for NiTi martensite, and which demonstrate that the fundamental elastic stiffness of martensite is even larger than that of austenite. Our combined experimental and theoretical study provides a detailed picture of NiTi elasticity on the several length scales and points of view involved.