KINETICS OF FCC-]BCC HETEROGENEOUS MARTENSITIC NUCLEATION .1. THE CRITICAL DRIVING-FORCE FOR ATHERMAL NUCLEATION

Employing available experimental data for athermal f.c.c. --> b.c.c. martensitic transformation in binary, ternary and multicomponent Fe-base alloys, a model is developed and tested for the critical driving force at the M(s) temperature. Incorporating the theory of solid solution hardening, we describe the composition dependence of the athermal frictional work for martensitic interface motion governing the kinetics of barrierless heterogeneous nucleation. The available data suggests that the composition dependence of the athermal frictional work is of the same form as that for slip deformation. We have evaluated the athermal strengths of 14 alloying elements Al, C, Co, Cr, Cu, Mn, Mo, N, Nb, Ni, Si, Ti, V and W from the experimental data. Except for Al, Ni and Co, the athermal strengths of the common substitutional alloying elements are similar in magnitude, while the interstitial solutes C and N exert a stronger influence. Previously proposed superposition laws are used to account for the presence of multiple solutes having different athermal strengths. With an improved assessment of the magnetic parameters of alloy systems, the model predicts M(s) temperatures within +40K for M(s)>300 K where thermal contributions to the frictional work can be neglected.