Hysteretic and anhysteretic tensile stress–strain behavior of Ni–Fe(Co)–Ga single crystal: Experiment and theory

Recoverable strains have been measured in a single-crystalline Ni–Fe(Co)–Ga shape memory alloy in the course of tensile tests performed at different temperatures. Two substantially different types of deformation process were observed above the martensitic transformation temperature, TM ≈ 200 K. An unusual anhysteretic stress–strain behavior characterized by an enormous (∼14%) nonlinear deformation was found above 320 K, while stress–strain loops showing a pronounced hysteresis were obtained below this temperature. A sudden reduction of hysteresis was explained using the theoretical stress–temperature diagram of ferroelastic phase transitions. It has been shown that this almost anhysteretic deformation can occur when the temperature of the deformed alloy exceeds the critical value corresponding to the endpoint of the phase transition line in the phase diagram. A theoretical analysis showed that above the critical temperature a very small residual hysteresis (∼1 MPa) may be caused by the reconfiguration of crystal defects during the deformation cycle.