Shape memory behavior and tension-compression asymmetry of a FeNiCoAlTa single-crystalline shape memory alloy

A 〈1 0 0〉 textured polycrystalline FeNiCoAlTa shape memory alloy was recently shown to possess large superelastic strain and stress levels. In this study, the shape memory behavior of a Fe-28Ni-17Co-11.5Al-2.5Ta (at.%) single-crystalline material oriented along the 〈1 0 0〉 direction was studied, for the first time, by thermal cycling under constant stress levels in both tension and compression. When γ′ precipitates with an average size of 5 nm are introduced by an aging heat treatment, the single crystals show fully recoverable transformation strains up to 3.75% in tension and 2% in compression. The change in transformation temperatures for a unit change in applied stress level was higher in compression than in tension, in accord with the lower transformation strains in compression obtained both from theoretical calculations and experimental observations. However, in all specimens, the observed transformation strain levels were lower than theoretically predicted, possibly owing to significant volume fraction of non-transforming precipitates, incomplete martensite reorientation due to martensite variant interactions, and a slightly higher-than-expected martensite c/a ratio in the samples used in this study. The ramifications of relevant structural parameters and microstructural features on reaching theoretical transformation strain and high strength levels are also discussed.