Relationship between the Solid Phase Equilibrium and the Isothermal Martensite Transformation in Fe-Ni-Cr and Fe-Ni-Mn Alloys

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The change in free-energy attendant upon the athermal and the isothermal transformations of austenite into martensite in Fe-Ni-Cr and Fe-Ni-Mn alloys was investigated. In Fe-Ni-Cr alloys containing 17 weight % Cr, the driving force necessary to initiate the athermal martensite transformation increases as Ni content increases, whereas in the isothermal transformation, this force decreases. When the Ni content in the alloys exceeds about 7 weight %, the martensite is formed isothermally at a temperature above M_s point and the driving force at this point is about 490 cal/mole. Similar phenomena were also observed in Fe-23% Ni-Mn alloy. In this alloy, the martensite is formed isothermally at a temperature above M_s point when the Mn content exceeds about 3.5 weight % and the driving force at this critical point is about 430 cal/mole. Temperatures for maximum rates of the isothermal martensite transformation in Fe-Ni-Cr and Fe-Ni-Mn alloys lower with the increases of Ni and Mn content, respectively. The driving forces for the athermal martensite transformation in the Fe-Ni-Cr and the Fe-Ni-Mn alloys used in the present work are larger than that of other alloys in which the martensite is formed only athermally. From these results, it is suggested that the nucleus of new phase in isothermal martensite transformation cannot develop into martensite plate only by the stored free energy, but probably some thermal fluctuation will play an important role.