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 (ε = 6%, σmax = 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 εtr 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 Ms) 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 Ms temperature during thermal cycles and the restoration of the TWSME strain due to additional training (10 isothermal loading/unloading cycles).