Critical driving force for martensitic transformation fcc(γ)→hcp(ε) in Fe−Mn−Si shape memory alloys
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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,T N γ =67x Fe+540x Mn+x Fe x Mn[761+689(x Fe−x Mn)]−850x si, is fitted and verified by the experimental results. The critical driving force for the martensitic transformation fcc(γ)→hcp(ε), ΔG C γ→ε , defined as the free energy difference between γ and ε phases atM s of various alloys can also be obtained with a knownM s . It is found that the driving force varies with the composition of alloys, e. g. ΔG C γ→ε =−100.99 J/mol in Fe−27.0Mn−6.0Si and ΔG C γy→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. ΔG C γ→ε =A·γ+B, where γ is the stacking fault energy (SFE) andA andB are constants related to materials.
Keywordscritical driving force martensitic transformation Fe−Mn−Si alloy
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