A general mechanism of martensitic nucleation: Part III. Kinetics of martensitic nucleation

Abstract

The growth of martensitic fault embryos in the fault plane, the development of their interfacial structure, and the thickening of the embryos normal to the fault plane are examined as possible rate limiting steps in the total martensitic nucleation process. Growth of the embryos in the fault plane appears the most probable rate limiting step, capable of accounting for both the observed isothermal and athermal kinetic behavior depending on the parameters (such as activation volume) which control the motion of the transformational dislocations. The thermally activated nucleation of dislocation loops responsible for lattice invariant deformations is a possible rate limiting step for some isothermal transformations, though such deformations are not required for all martensitic transformations. Embryo thickening by the nucleation of discrete loops of transformation dislocations appears improbable in bulk material; instead, a plausible pole mechanism for embryo thickening is presented which incorporates existing “forest” dislocations intersected by embryos growing in the fault plane. Lattice softening phenomena may lower the critical chemical driving force for nucleation, but are not essential for martensitic nucleation by the proposed faulting mechanism.