On the Impact of Thermal Treatments on the Shape Memory Effect of ‹001›-Oriented Fe–Ni–Co–Al–Ti Single Crystals

This study presents a comprehensive and systematic assessment of the impact of aging heat treatments on the functional actuation response in 〈001〉-oriented Fe–Ni–Co–Al–Ti single crystals. The results demonstrate that excellent functional properties can be obtained, however in a relatively small aging heat treatment window. Three aging process regimes have been identified and clearly illustrate the significance of aging time and temperature to adjust transformation strains and temperatures, stress bearing capability as well as thermal hysteresis widths. Cooling-heating experiments under constant stress have demonstrated a considerable variation in the transformation strain, i.e., from 0.6 to 7.7% following different aging heat treatments. Transmission electron microscopy (TEM) studies revealed the presence of finely dispersed γ′-precipitates, with an average size of ~ 3 to 6 nm, which determine the functional material response. The size of the precipitates as well as the resulting matrix hardness were significantly influenced by different aging parameters. The Fe–Ni–Co–Al–Ti SMA demonstrates favorable functional behavior dependent of the aging treatment conducted. For longer aging times and higher aging temperatures, in turn, the formation of β phase precipitates was observed, which further affect the overall alloy performance.