Internal states of temperature hysteresis in shape memory alloys

We propose a thermodynamic theory that permits us to connect the size of the temperature hysteresis loop to the interfacial energy between martensite and austenite. The free energy is a function of four variables, viz. deformation of austenite, deformation of martensite, phase fraction and temperature. A diagonal line inside the hysteresis loop in the deformation-temperature diagram is identified as the line of unstable phase equilibrium. Upon reaching this line the phase mixture loses its (meta-)stability, and the transformation from one phase to the other should occur. The theoretical results are supported by experiments on NiTi and CuAlNi specimens.