Surface form memory by indentation and planarization of NiTi: displacements and mechanical energy density during constrained recovery

摘要

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 107 J/m3, fully ~10% of the M → A enthalpy.