The enthalpy change of the hcp→fcc martensitic transformation in Fe–Mn alloys: composition dependence and effects of thermal cycling

Calorimetric measurements have been performed on Fe–Mn alloys with 16.7≤wt.% Mn≤24.7, in order to determine the composition dependence of the molar enthalpy difference between the fcc (γ) and the hcp phase (ε) in the states that are involved in the ε→γ martensitic transformation in this system. Two calorimeters, viz., a thermal analysis differential scanning and a Perkin–Elmer calorimeter were used to determine the heat effect associated with the ε→γ transformation, in alloys subjected to several cycles of transformation (γ→ε) and retransformation (ε→γ). The amount of transformed material for each alloy was determined by dilatometry, and the volume difference between the ε and γ structures was taken from a recent assessment of the lattice parameters. It is found that the amount of material transformed on cooling is strongly affected by the increase in the Mn content, and is at least partly due to the stabilizing effect upon the γ phase of the magnetic ordering reaction, and to the microstructural changes which are induced by the thermal cycling through the martensitic transformation. A key result of the work is that the enthalpy difference decreases monotonically with increasing Mn content. This decrease should be relevant in the thermodynamic modeling of the ε and γ phases.