KINETICS OF FCC-]BCC HETEROGENEOUS MARTENSITIC NUCLEATION .2. THERMAL-ACTIVATION

A critical analysis of the experimental data for the activation energy for f.c.c --> b.c.c. martensitic nucleation in Fe Ni, Fe-Ni-C, Fe Ni-Cr and Fe-Ni-Mn alloys has been performed employing the theory of dislocation discrete-obstacle interaction. It is shown that the normalized activation energy Q (T)/mu(T) vs normalized driving force DELTAg(n)/DELTAg data for isothermal martensitic nucleation follows the general form proposed by Kocks, Argon and Ashby [in Thermodynamics and Kinetics of Slip, Progress in Materials Science, Vol. 19. Pergamon Press, Oxford (1975)] for the kinetics of conventional slip deformation in solid solution strengthened alloys. Curvature of the Q (DELTAg) function defines a temperature dependent activation volume for isothermal martensitic nucleation. The analysis is consistent with rate control by the interaction of solute atoms with the thermally-assisted motion of the martensitic interface. The contribution of a solute to the thermal component of interfacial friction is found to scale with its athermal contribution. The analysis incorporates kinetic data for the effects of applied elastic stress and plastic pre-strain, providing a quantitative estimate of the interfacial friction from dislocation forest hardening.