Orientation dependence of stress-induced phase transformation and dislocation plasticity in NiTi shape memory alloys on the micro scale

NiTi shape memory alloys can be used as micro actuators and small scale pseudoelastic components. Therefore there is a need to characterize their mechanical properties on the micro scale. In several previous studies, such tests (nanoindentation, pillar compression) were performed for different NiTi alloys. However, no consistent results concerning the coupling between plastic deformation and martensitic transformation were obtained. Moreover it is unclear whether the material's response to loading on the micro scale reflects its large scale mechanical anisotropy. In this study, we investigate a binary, solution annealed precipitate free NiTi alloy and compress small pillars in <0 0 1>–, <1 0 1>– and <1 1 1>–directions. Mechanical results are analyzed in the light of SEM and post-mortem TEM investigations. We identify deformation mechanisms and show that there is deformation anisotropy. We show that micro pillar testing yields results which are in good qualitative agreement with previous work from macroscopic investigations.

► NiTi micro pillars can show pseudoelastic behavior. ► Micro mechanical behavior is strongly dependent on crystallographic orientation. ► Deformation mechanisms differentiate with crystallographic orientation. ► Martensitic transformation is accompanied by dislocation formation. ► Remnant deformation is due to dislocation plasticity.