Constrained-Recovery of Surface Form in An Indented and Planarized NiTi Shape Memory Alloy

Spherical and cylindrical indenters may be applied to NiTi shape memory alloys in an indentation-planarization technique that results in two-way shape-memory training. This allows repeatable, cyclic generation of flat-to-wavy or flat-to-bumpy surface transitions on changing temperature. Deep spherical (or cylindrical) indents, after planarization (flat grinding just sufficient to remove traces of the original indent) are converted to reversible surface protrusions. Cyclic protrusion amplitudes, can be 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 training effect. In this paper we show that these cyclic protrusions (which may take arbitrary in-plane form) can perform appreciable force-distance work when displacing against a base-metal substrate (constrained recovery). This comprises a novel form of indentation in which no point-contact occurs. This “non-Hertzian” indentation, which appears to be able to exert the full energy density of SMA actuation, may have application to assembly of micromachines, bond-release, microforging and micro joining, electrical switching, microconnectors, variable heat transfer devices, and others.