Effects of Precipitation on the Thermomechanical Response of Ni-Ti-Hf High Temperature Shape Memory Alloys

Mills, Michael J.

Ni-Ti-Hf alloys display remarkable thermo-mechanical properties and good potential for high temperature actuators. However, the properties depend sensitively on the aging time and temperature to form precipitates. In particular, DSC data shows that critical temperatures for transformation gradually increase with aging time from 3 to 300 hr at 550°C. Over this time, the precipitate volume fraction remains relatively constant, but precipitate size increases from the nanometer to submicron scale. The precipitates strengthen the alloy against plastic ratcheting, thereby increasing the operative actuator stress and work output above typical values for near-equiatomic binary alloys.

These experimental features are studied using phase field and finite element based approaches that incorporate discrete precipitates into a surrounding B2 crystal. Of interest is that precipitation can alter the thermo-mechanical response, both through the stress field generated by precipitates and by the change in matrix composition. These “mechanical” and “chemical” effects are modeled by explicitly incorporating experimental measurements of precipitate misfit strain and matrix composition. Atom probe tomography reveals that precipitate formation reduces the Ni and Hf composition of the matrix. The resulting chemical effect is abrupt, shifting the critical temperature upward by ~40°C, regardless of aging time. In contrast, the mechanical effect decreases critical temperatures at short aging time, due to constraint imposed by nanoscale precipitates. At longer aging times, the constraint diminishes and critical transformation temperature increases. When the chemical and mechanical effects are combined, the net effect produces a graduate increase in critical temperature with aging, similar to experiments. In principle, this dual “chemical + mechanical effect” opens up approaches to optimize HT-SMA reponse.

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