THERMODYNAMICS OF THE GAMMA/EPSILON MARTENSITIC-TRANSFORMATION IN FE-MN ALLOYS - MODELING OF THE DRIVING-FORCE, AND CALCULATION OF THE M(S) AND A(S) TEMPERATURES

By using recent measurements of the martensitic transformation temperatures in the Fe-Mn system we perform an evaluation of the Gibbs energy (G(m)) difference between fcc (gamma) and hcp (epsilon) as a function of temperature and composition. The approach involves the use of thermodynamic models that account for the magnetic contribution to G(m), and the evaluation of a set of optimum model parameters. Among the quantities evaluated by us there is one related to the entropy of antiferromagnetic ordering in fcc, and parameters representing the Gibbs energy barriers for the start of the transformation. The calculated driving forces are confronted with results from other authors. A detailed comparison is reported with the usual thermodynamic evaluation method that relies on the estimation of T-0 temperatures as the average of M(S) and A(S).