The stability of transition phases in Fe-Mn-Si based alloys

Various transition phases, featured by nH(n>2), are possible in the Fe-Mn-Si based alloys. Using the Chou-Model, their Gibbs free energies are calculated and their stabilities are ranked. The thermodynamical parameters of those structures were deduced from those of fcc and hcp structures which have been established based on the stacking fault energy (SFE) and the SF density. The results indicate that at the temperatures below thermal equilibrium (To) the stability of transition phases is in an order of 2H(hcp)>9H(9R)>4H>6H>fcc and in the reverse order when the temperature is above To, consistent with the experimental observations. In the case that an external stress field exists, the strain energy plays an important role on the phase stabilities and, therefore, their order would be modified.