Thermodynamics of thermally induced martensitic transformations in Cu–Al–Ni shape memory alloys

The dependence of martensitic transformation characteristics on the type ({beta}{sup '}, {gamma}{sup '} or {beta}{sup '}+{gamma}{sup '}) of thermally induced martensite has been studied in Cu-Al-Ni shape memory alloys. The energy balance of the martensitic transformation (MT) has been analyzed for the three possible transformations by calorimetric measurements. When a single MT, {beta}{yields}{beta}{sup '} or {beta}{yields}{gamma}{sup '} is present, the dissipative term linked to the irreversible entropy S{sub i} has been evaluated. This term that accounts for the hysteresis is higher for the {beta}{yields}{gamma}{sup '} transformation than for the {beta}{yields}{beta}{sup '} one, according to the hysteresis behavior. On the other hand, during mixed {beta}{yields}{gamma}{sup '}+{beta}{sup '} transformation, a higher irreversible entropy and a negative total exchanged heat have been measured. In order to explain this behavior, a modification of the energy balance, which takes into account the presence of plastic deformation when martensite coexistence occurs, has been proposed.