On the relative fraction of ε martensite in γ-Fe-Mn alloys

Fe-Mn quenched alloys with Mn content between 13 and 27 wt% have been studied using X-ray diffraction, Moessbauer spectroscopy, differential scanning calorimetry and dilatometry. The samples have been structurally characterized using X-ray diffraction and Moessbauer spectroscopy. The composition dependence of the relative fraction of ε phase was determined by dilatometry and Moessbauer spectroscopy. Using a differential scanning calorimeter, measurements of the absorbed heat accompanying the ε → γ martensitic transformation were also performed. The relative fractions of ε phase determined by dilatometry agree well with those reported in Schumann's classical work [H. Schumann, Arch. Eisenchuettenw. 38 (8) (1967) 647-656] for Mn contents up to about 22 wt% Mn, but for higher concentrations, a larger fraction was found. The discrepancy was explained in terms of the differences between the present heat treatments and those applied by Schumann. However, relative fractions of ε phase determined by Moessbauer spectroscopy resulted systematically larger than those obtained from dilatometry. On the other hand, independent calculations of the transformation heat were performed for the different compositions. They resulted from multiplying the ε fraction by the calculated ε → γ enthalpy change obtained from a recent assessment of the Gibbs functions of the ε and γ phases from literature. Absorbed heat values calculated using the Moessbauer ε fractions, reproduced well the experimental differential scanning calorimetry data, except when a high density of stacking faults are expected in the sample