Surface Form Memory in NiTi: Energy Density of Constrained Recovery During Indent Replication

Spherical indentation of NiTi shape memory alloys (SMA) to depths greater than about 3% of the indenter radius results in two-way shape-memory training in a deformation zone beneath the indent. If deep spherical or cylindrical indents are subsequently machined away just sufficiently to remove traces of the original indent (in the martensitic condition), a thermally induced and cyclically reversible flat-to-protruded surface topography is enabled. We term the phenomenon surface form memory. The amplitude of cyclic protrusions, or ‘exdents’, is related to the existence of a subsurface deformation zone in which indentation has resulted in plastic strains beyond that which can be accomplished by martensite detwinning reactions. Dislocation generation in this zone is thought to underlie the observed two-way shape-memory (TWSME) training effect. In this article, we show that these cyclic exdents can perform appreciable mechanical work when displacing under load against a base-metal substrate (constrained recovery). This “non-Hertzian” indentation, which appears to be able to exert the full energy density of SMA actuation, may have use for assembly of micromachines, bond-release, microforging, microjoining, electrical switching, microconnectors, and variable heat transfer devices, among many other potential applications.