Nanoindentation of pseudoelastic NiTi shape memory alloys: Thermomechanical and microstructural aspects

Nanoindentation allows local characterization of the microstructural processes associated with pseudoelastic recovery in NiTi shape memory alloys. We discuss nanoindentation and tensile testing results, both carried out at different temperatures on a NiTi alloy with three different microstructures (containing no, small and large Ni-rich precipitates). Maximum shape recovery in indentation experiments and during tensile testing occurs in microstructures with fine precipitates. Thermomechanical constraints affect pseudoelasticity differently in micro- and macro-scale testing: Indentation at temperatures as close as possible to the transformation start temperatures is associated with better shape recovery than testing at higher temperatures, whereas maximum shape recovery is only observed at slightly higher temperatures in macroscopic testing. These results are rationalized by considering the effect of temperature on stress-induced martensitic transformation and dislocation plasticity.