Critical driving force for martensitic transformation fcc(γ)→hcp(ε) in Fe−Mn−Si shape memory alloys

By the application of Chou's new geometry model and the available data from binary Fe−Mn, Fe−Si and Mn−Si systems, as well as SGTE DATA for lattice stability parameters of three elements from Dinsdale, the Gibbs free energy as a function of temperature of the fcc(γ) and hcp(ε) phases in the Fe−Mn−Si system is reevaluated. The relationship between the Neel temperature of the γ phase and concentration of constituents in mole fraction,TNγ=67xFe+540xMn+xFexMn[761+689(xFe−xMn)]−850xsi, is fitted and verified by the experimental results. The critical driving force for the martensitic transformation fcc(γ)→hcp(ε), ΔGCγ→ε, defined as the free energy difference between γ and ε phases atMs of various alloys can also be obtained with a knownMs. It is found that the driving force varies with the composition of alloys, e. g. ΔGCγ→ε=−100.99 J/mol in Fe−27.0Mn−6.0Si and ΔGCγy→ε=−122.11 J/mol in Fe−26.9Mn−3.37Si. The compositional dependence of critical driving force accorded with the expression formulated by Hsu of the critical driving force for fcc(γ)→hcp(ε) transformation in alloys with low stacking fault energy (SFE), i. e. ΔGCγ→ε=A·γ+B, where γ is the stacking fault energy (SFE) andA andB are constants related to materials.