NiTi shape memory alloys are finding various small scale applications in medicine such as stents and in the engineering sector in micro-actuators form. However, a thorough understanding of the correlation between microstructure and mechanical properties at small length scale is still missing. In this study, we investigate the formation of martensite during spherical nanoindentation in austenitic NiTi shape memory alloys. We use a special NiTi alloy in which stress induced martensite is stable at room temperature. This specificity allows post-mortem analysis to characterize the stress induced martensite which formed during deformation [1]. Austenite grains with (111) surface were selected for ex-situ and in-situ spherical nanoindentation in a scanning electron microscope. The in-situ load-displacement curves show several pop-ins which occur concomitantly with the formation of traces around the contact between the indenter tip and the sample. These traces exhibit a threefold symmetry around the remnant indent. A detailed study of the indentation-induced surface relief by atomic force microscopy before and after shape recovery allows to identify the formation of six twinned martensite plates. The activation of the experimentally observed twinned martensite plates is rationalized by analytical calculations of resolved shear stress on the basis of the elastic equations for a half space loaded by a sphere [2]. Finally, the in-situ nanoindentation results in combination with the post-mortem microstructural characterization show that the most likely deformation mechanism responsible for pop-in events corresponds to sudden increases of the thicknesses of twinned martensite plates.
- G Laplanche, J Pfetzing-Micklich, G Eggeler: Orientation dependence of stress induced martensite formation during nanoindentation in NiTi Shape memory alloys, Acta Materialia 2014;68:19-31
- G Laplanche, J Pfetzing-Micklich, G Eggeler: Sudden stress-induced transformation events during nanoindentation of NiTi shape memory alloys, Acta Materialia 2014;78:144-160