SHAPE: Shape memory alloys via halide-activated pack equilibration

Fabrication of shape memory alloy components is challenging due to the precision with which elemental composition must be controlled to achieve desired shape memory behavior. This presentation describes a method to precisely regulate chemistry in a NiTi shape memory alloy via halide-activated pack equilibration (SHAPE) against a constant chemical potential reservoir. To demonstrate efficacy of the SHAPE process, an initially titanium-deficient specimen (pure nickel foam) has been equilibrated against an excess of an intimately mixed two-phase pack (NiTi + Ti₂Ni) in the presence of a vapor phase transport agent (iodine). The two-phase pack pins chemical potentials in this two-component system in accordance with Gibbs’ phase rule, thereby regulating the equilibrium state of the specimen. Ti-rich NiTi foams produced in this manner exhibit reproducible and well-defined phase transformation behaviors. The SHAPE process is thought to be advantageous to produce shape memory components with well-defined elemental compositions of varying size, shape, or initial degree of compositional deficiency owing to independence of the equilibrium state of the specimen from the details of process kinetics. Current limitations and prospects for the application of this method to enhance the manufacture of shape memory alloy technologies are briefly discussed. Based in part on the thesis submitted by author ASK for the MS Degree in Materials Science and Engineering at Alfred University in 2019.