SHAPE: Shape Memory Alloys via Halide-Activated Pack Eqilibration

Abstract

Achieving stringent chemistry standards is necessary for additive manufacturing of Nitinol shape memory alloys. This work describes an elevated temperature gas-solid reaction processing technique that can be used to precisely regulate the chemistry and phase transformation behaviors of Nitinol components. The technique, Shape memory alloys via Halide-Activated Pack Equilibration (SHAPE), employs established principles of chemical vapor transport to equilibrate a substrate against reactive pack mixtures designed to regulate the chemical potentials of nickel and titanium in accordance with Gibbs' phase rule as means to precisely control substrate phase and elemental composition. The results suggest that SHAPE may find crosscutting potential especially when paired with additive manufacturing or fusion welding of Nitinol to improve product quality. Notwithstanding future applications, SHAPE is limited by solid-state diffusion that, in turn, limits the practical thickness of suitable components to about 1 mm. Opportunities for continued development have been identified for application to other compositions, and to further refine microstructure control.