Surface form memory in NiTi shape memory alloys by laser shock indentation

An indentation-planarization method for NiTi shape memory alloys has been developed that produces a robust surface topographical memory effect that we call "surface form memory", or SFM. Surface form memory entails reversible transitions between one surface form (flat) and another (say, wavy) that occur on changing temperature. These transitions are cyclically stable and exhibit very high mechanical energy density. Our previous study has demonstrated SFM transitions in NiTi alloys derived from quasistatic (i.e., low strain rate) spherical indents, as well as other geometries. Here, we report on experiments using confined laser ablation to indent a similar martensitic NiTi substrate, but in the dynamical regime (very high strain rate). As in the quasistatic case, subsurface plastic strain gradients are created via martensite twinning reactions, and later by dislocation-mediated slip. The resulting defects and stress fields support the two-way shape memory effect underlying SFM. In the dynamical case however, relative cyclic two-way displacements are found to be significantly larger, when normalized to the initial indent depth, than is the case with quasistatic indentation. This confers certain processing and boundary condition advantages. Analysis of the shock dynamics is found to be consistent with the observed surface displacements.