Critical driving force for martensitic transformation fcc(gamma)-> hcp(epsilon) 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 st ability parameters of three elements from Dinsdale, the Gibbs free energy a s a function of temperature of the fcc(gamma) and hcp(epsilon) phases in th e Fe-Mn-Si system is reevaluated. The relationship between the Neel tempera ture of the gamma phase and concentration of constituents in mole fraction, T-N(gamma) = 67x(Fe) + 5402 x(Mn) + x(Fe)x(Mn) [761 + 689(x(Fe) - x(Mn))] - 850x(Si), is fitted and verified by the experimental results. The critica l driving force for the martensitic transformation fcc(gamma) --> hcp(epsil on), Delta G(C)(gamma-->epsilon), defined as the free energy difference bet ween gamma and epsilon phases at M-S of various alloys can also be obtained with a known M-S. It is found that the driving force varies with the compo sition of alloys, e. g. Delta G(C)(gamma-->epsilon) = - 100.99 J/mol in Fe- 27.0Mn-6.0Si and Delta G(C)(epsilon)(gamma-->) = - 122.11 J/mol in Fe-26.91 Mn-3.37Si. The compositional dependence of critical driving force accorded with the expression formulated by Hsu of the critical driving force for fcc (gamma) --> hcp(epsilon) transformation in alloys with low stacking fault e nergy (SFE), i. e. Delta G(C)(gamma-epsilon) = A . gamma + B, where gamma i s the stacking fault energy (SFE) and A and B are constants related to mate rials.