The effect of nanoprecipitates on the superelastic properties of FeNiCoAlTa shape memory alloy single crystals

FeNiCoAlTa shape memory alloys were recently discovered to possess a combination of high transformation strain and high resistance to plastic deformation. However, the transformation strain observed from single crystals is much smaller than theoretically predicted, which could be related to γ′ (L12) precipitates. Therefore, we examined the effect of nanosized γ′ precipitates on various superelastic properties of Fe–28%Ni–17%Co–11.5%Al–2.5%Ta single crystals, and found that they have profound influence on the superelastic stress hysteresis, transformation temperatures, stress–temperature phase diagram, and the characteristics of the stress–strain response along the 〈1 0 0〉 orientations. The size and volume fractions of precipitates were determined quantitatively with 3-D atom probe tomography, and the composition of these precipitates was determined for the first time. Aging at 600 °C for 7 h and above resulted in little or no change in the volume fraction of the precipitates, but coarsening of the precipitates accompanied by modest changes in their chemical compositions was observed with increasing aging time, which resulted in an increase in the transformation temperatures. Furthermore, the change in the precipitate size affected tensile and compressive superelastic behavior differently. An increasing size of the precipitates, and thus decreasing number density, caused tension–compression asymmetry in the superelastic characteristics, such as recoverability, stress hysteresis, and intensified the critical stress vs. temperature slope. We explain this observation based on the inherent differences in the morphology and variant structures of the martensite formed during tension and compression superelastic experiments.