The kinetics of isothermal martensitic transformation in an Fe-23.2wt.%Ni-2.8wt.%Mn alloy

In this paper the results of a detailed study of the isothermal martensitic transformation kinetics in an Fe-23.2wt.%Ni-2.8wt.%Mn alloy are reported. The transformation kinetics were determined as a function of test temperature, superimposed elastic stress field and prior plastic deformation of the austenite. Extensive quantitative metallographic measurements show that the overall transformation kinetics agree very well with the model that incorporates the vital contribution of autocatalysis. On the assumption that the rate-controlling step in martensitic nucleation is the thermally activated motion of dislocations, the activation energies for nucleation were calculated under an applied elastic stress or in work-hardened austenite. These show good agreement with the experimentally determined activation energies. The present results support the Olson-Cohen suggestion that the critical step in martensite nucleation is the thermally activated motion of the partial dislocations bounding the fault embryo.