Journal of Materials Science, Vol.46, No.23, 7401-7409, 2011
Surface form memory by indentation and planarization of NiTi: displacements and mechanical energy density during constrained recovery
Indentation-induced two-way shape memory leads to pronounced temperature dependence of the depth of spherical indents made in martensitic NiTi shape-memory alloys. They are shallower when austenitic, and depth varies during both M -> A and A -> M transformations. If the impression is planarized, by metallographic grinding at T < M(f), a protrusion rises at the site when warmed past A(f). If cooled again this "exdent'' retreats, restoring optical flatness. The cycle is repeatable, and exdent heights can exceed 15% of prior indent depth. Since it maps between macroscopically distinguishable topographies, or forms, at orders greater length scale than the surface roughness, we call the effect "surface form memory''-SFM. Notable regarding potential applications is that, when loaded in compression by planar contact with a strong base metal, exdents exert sufficient pressure to indent the latter, suggesting that subsurface transformational mechanisms operate at volumetric work-energy densities >10(7) J/m(3), fully similar to 10% of the M -> A enthalpy.