Thermodynamics of fcc–fct martensitic transformation in Mn–X(X=Cu,Fe) alloys

Highlights: • A universal method was proposed to obtain the thermodynamic parameters of fct martensite in Mn-based alloys. • The critical chemical driving force of fcc–fct martensitic transformation in Mn-based alloys was first calculated. • The entropy, enthalpy and heat capacity of two phase transitions were calculated and compared. • The orders of martensitic transformation and paramagnetic-antiferromagnetic transition were studied. Mn-based antiferromagnetic alloys, e.g. Mn–Fe and Mn–Cu, are widely used for their magnetic shape memory effect and reversible shape memory effect, which are closely related to their fcc–fct martensitic transformation. However, due to a lack of thermodynamic parameters of fct martensite phase, the thermodynamics of such a transformation has not yet been well established. In this work, a method was proposed to solve this question for the first time and could be applied to other systems. By using the sub-regular solution model and thermal equilibrium condition, the Gibbs free energy of fct phase is calculated for the first time and expressed as a function of temperature and alloy composition. Furthermore, the dependences of chemical driving force on temperature for a given composition, that of critical chemical driving force on composition, and that of other thermodynamic quantities, i.e. entropy, enthalpy and heat capacity, on temperature were investigated for the first time. Finally, the influences of the calculating method and the assumption on the calculation were discussed, and the order of fcc–fct martensitic transformation was assessed from the viewpoint of thermodynamics semi-quantificationally, as well as the order of paramagnetic–antiferromagnetic transition.