The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales