Fcc/Hcp martensitic transformation in the Fe-Mn system: Part II. Driving force and thermodynamics of the nucleation process

This article, which continues the series started by Cotes et al. (Metall. Mater. Trans. A, 1995, vol. 26A, p. 1957-69), presents a study of the energetics of the heterogeneous nucleation of hcp martensite in the fcc matrix of the Fe-Mn system. A major goal of the work is the exploration of the various qualitative and quantitative consequences of applying the Olson and Cohen approach (OCA) to the most reliable information on the Fe-Mn system. To this end, an extensive assessment of the quantities involved in the OCA is performed. The selected Gibbs energy and the lattice parameter descriptions for fcc and hcp are based on recent experimental studies. Explicit calculations are presented of the key quantities in the OCA, including those quantities that have not yet been measured. In particular, a probable range of values is established for the surface energy parameter sigma, which plays a crucial role in the treatment of the nucleation size. On these bases, values are derived for the probable number of atomic planes (n(C)) of the embryo, which, according to the OCA, is at the condition of spontaneous growth. Predictions of the Mn content upon n(C) are also reported. The present attempt to calculate in detail the energetics of the fcc --> hcp martensitic transformation also revealed the lack of thermo-physical data for Fe-Mn alloys. The need for experimental studies of various relevant quantities is emphasized.