Size Effects in Shape Memory Alloys: Competition between structural and microstructural features in determining grain scale performance

Stress induced phase transformation in polycrystalline NiTi is studied as a competition between two factors: extrinsic structural factors and intrinsic microstructural factors. Structural factors arise from stress concentrations around features such as holes and cavities. Microstructural factors include the effect of grain orientations and interactions between grain neighbors, which determine the critical stress for transformation and constrain the martensite microstructures allowed to form in grains. In this work, we systematically study the relative influence of structural and microstructural factors through a combined experimental and modeling effort.

The empirical component furnishes local strain fields using DIC and ex-situ microstructural information using EBSD, during superelastic tensile loading of planar NiTi specimens, with a pair of holes of varying size. The modeling effort consists of a phenomenological, continuum based model with uniform material properties, and a micro-mechanical model, which furnishes local martensite microstructure at the crystallographic scale. Using these data, we study the features of phase transformation around the holes, influence of the grain orientations and spatial position relative to the holes on martensite plate selection.

When the holes are much larger than the grains, we see a predominance of structural effects with a subtle influence of the microstructure effects. However, when the holes and grains are of comparable size, we see the complete domination of microstructural effects. These findings are key to understanding the dominant mechanism in porous SMAs, with pores comparable to grain size and also in choosing a reliable modeling framework for miniature SMA components like micron-sized sensors and actuators.