Experimental determination of the driving force of the fcc-hcp martensitic transformation and the stacking fault energy in high-Mn Fe-Mn-Cr steels

By the use of several experimental techniques the difference in the Gibbs free energy between fcc austenite and hcp martensite has been determined for a wide range of compositions of Fe-Mn-Cr alloys where the martensitic transition takes place. Martensitic transformation temperatures were determined by dilatometry measurements, while the lattice parameters of both phases and the volume change between them were determined by X-ray diffraction. Combining these results with dilatometry and differential scanning calorimetry measurements, the amount of martensite and the heat exchanged during transformation were obtained. These values allowed obtaining the enthalpy change associated to the fcc-hcp transformation and, following thermodynamic criteria, the Gibbs free energy change (ΔG), leading to absolute values in the range from 165 J/mol up to 240 J/mol for the analyzed compositions. These values are high enough to overcome the known resistances to the transformation, like the strain energy of the transformation and the surface energy. The measured parameters enable obtaining the stacking fault energy which is also discussed in the present framework.