Abstract
The change of free-energy attending the athermal and isothermal martensite transformations in Fe-Ni-Cr and Fe-Ni-Mn alloys was investigated. In Fe-Ni-Cr alloy containing 17 weight%Cr, the driving force necessary for initiating athermal martensite transformation increases as Ni content increases, whereas in the case of the isothermal transformation, this force decreases. When the Ni content in this alloy exceeds about 7 weight%, the martensite is formed isothermally at a temperature above Ms and the driving force at this critical point is about 450 cal/mol. Also the similar phenomena were observed in Fe-23%Ni-Mn alloy. In this alloy, the martensite is formed isothermally at a temperature above Ms when the Mn content exceeds about 3.5 weight% and the driving force at this critical point is about 430 cal/mol. Temperatures for maximum rate of the isothermal martensite formation in the Fe-Ni-Cr and Fe-Ni-Mn alloys are lowered with the increase of Ni and Mn content respectively. The driving forces for athermal martensite transformation in the Fe-Ni-Cr and 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 nuclei of isothermal martensite can not develop into the martensite plates only by the stored free energy, and probably thermal fluctuations will play the important role.
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