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 shapememory 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 TMf, a protrusion rises at the site when warmed past Af. 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.