Understanding indentation-induced two-way shape memory effect

Spherical indents in NiTi shape memory alloys can have reversible depth change: deeper depth in the martensitic phase at low temperature and shallower depth in the austenitic phase at high temperature. This is the indentation-induced two-way shape memory effect. After polishing the indents, two-way reversible surface protrusions can occur on the shape memory alloy surfaces upon heating and cooling. The height of the surface protrusion is about the same as the depth of the reversible indent. Further polishing reduces the height of the surface protrusion, which disappears completely when the polished depth is about the length of the contact radius. By comparing finite element analysis and experimental data, we show that the depth at which a protrusion disappears is close to the 10% strain boundary. This suggests that slip-plasticity is responsible for the observed indentation-induced two-way shape memory effect.