Two-way shape memory effect under multi-cycles in [001]-oriented Ni49Fe18Ga27Co6 single crystal

The effects of prior thermo-mechanical treatment on two-way shape memory effect (TWSME) and its stability under thermal cycles in [001]-oriented Ni49Fe18Ga27Co6 single crystals were investigated. In this study, TWSME in quenched, stress-free and stress-assisted aged crystals was induced by 100 isothermal loading/unloading cycles (epsilon = 6%, sigma(max) = 140 MPa). Maximum reversible TWSME strain up to 5.5% was observed in stress-assisted aged single crystals in which a full one-way transformation strain epsilon(tr) is achieved under minimum stress levels and the favourably oriented internal stress fields are formed by the selection of certain precipitate variants during stress-assisted ageing. In quenched and stress-free aged crystals TWSME strains are less than 2.2%. The mechanisms of cycling degradation of TWSME (decrease of TWSME strain and change in martensitic start temperature M-s) have also been discussed. In quenched single crystals, 100 thermal cycles result in irreparable degradation of TWSME. In contrast, age hardening in stress-free and stress-assisted aged crystals results in unchanged M, temperature during thermal cycles and the restoration of the TWSME strain due to additional training (10 isothermal loading/unloading cycles).