Effect of H-phase Precipitates on Ni-Ti-Hf High Temperature SMA Performance

Ni-Ti-Hf/Zr alloys are promising High Temperature Shape Memory Alloys (HTSMAs) due to their low cost and good shape memory behavior. However, the realization and stabilization of this behavior depends sensitively on the aging conditions to form precipitates, whose effects are still poorly understood. In this project, we study the mechanical effect of H-phase precipitates in Ni_50.3-Ti_29.7-Hf₂₀ using finite element (FE) simulations. These are informed by DSC results, ab initio calculations and HRTEM observations, which provide the matrix and precipitate properties as well as the relative scaling between the precipitates and martensitic plates. The FE simulations show that the internal stress due to the matrix-precipitate lattice misfit has a relatively minor influence on both the forward and reverse phase transformations during heating and cooling. Rather, the major influence stems from the relative scale of the H-phase precipitates vs. martensite plates, as a function of aging. The results also motivate a hypothesis that nano-scale precipitates might deform inelastically, in order to relieve the large constraint they impose on the transformation process. Overall, the combined experiment-simulation approach reveals the dual mechanical-chemical effect of nanoscale precipitates in these alloys.

This work is supported by the Department of Energy, Office of Basic Sciences, DE-SC0001258 (John Vetrano, Program manager).