CLASSICAL AND NONCLASSICAL MECHANISMS OF MARTENSITIC TRANSFORMATIONS

Abstract
Theoretical approaches to the nucleation of martensitic transformations have considered two types of nucleation paths : (a) the classical path, involving a nucleus of fixed structure and increasing size, and (b) nonclassical paths, involving a nucleus of varying structure. A unified approach is outlined which allows for the theoretical treatment of homogeneous and heterogeneous nucleation of coherent and semi coherent martensitic transformations by both types of nucleation paths, and identifies the circumstances favoring each. Nonclassical paths would be favored in the special case of homogeneous coherent nucleation near a mechanical instability ; however, quantitative treatment requires further knowledge of the energetics of homogeneous lattice deformations and the strain-gradient energies which control diffuse interfaces. Heterogeneous nucleation, which governs martensitic transformations in bulk materials, favors the classical path and involves relatively sharp interfaces. Classical heterogeneous nucleation is adequately treated by linear-elastic continuum and dislocation descriptions. Defect-nucleus interaction leads to "barrierless" nucleation with kinetics controlled by interfacial motion. Details requiring further attention are the microscopic mechanisms of martensitic interfacial motion in discrete crystals, and the precise nature of the nucleating defects, both those initially present and autocatalytically generated.