Annu. Rev. Mater. Sci. 1981.11:1-32
INTRODUCTION
+8655Two theoretical approaches have been applied to the heterogeneous nucleation of martensitic transformations. One can be regarded as nucleation on common defects by improbable physics; the other amounts to nucleation on improbable defects by common physics. Of most interest in the first approach is the concept of a local mechanical instability of the crystal lattice!. However, the physics necessary to describe finite inhomogeneous deformations does not currently exist, and so it has not yet been possible to formulate a quantitative theory of martensitic nucleation along these lines.Before trying to invent a new physics, it is instructive to ask whether classical physics can indeed account for martensitic nucleation. We have found that the answer is "yes," provided that special defects of suffi ciently high energy exist; this thtm brings us to the "improbable defect" approach to martensitic nucleation. A crucial point in its favor is the extremely heterogeneous nature of martensitic transformations. Small particle experiments indicate that the number of deft;cts triggering martensitic nucleation at the Ms temperature in ferrous alloys is of the order of 106 per cm3, or one per 100 p.m grain of the parent phase (1, 2).This small number demonstrates that martensitic nucleation is in fact improbable, and the sparseness of the nucleation sites is one of the important observations that a viable theory of martensitic nucleation must ultimately.explain.We restrict our attention here! to martensitic transformations as re cently classified within the broader context of displacive-diffusionless phase transformations; a martensitic transformation involves lattice distortive shear displacements sufficiently large that the kinetics and 1 0084-6600/81/0801-0001 $0 1.00 Annu. Rev. Mater. Sci. 1981.11:1-32 transformation responsible for the hardening of steels can be regarded as the prototype martensitic transformation. Appropriate to such a transfor mation, a dislocation-dissociation mechanism of martensitic nucleation based on classical physics is reviewed, and the extent of its agreement with experiment assessed. As the dislocation-dissociation mechanism is founded on a discrete-dislocation description of interphase-boundary structure, we begin with these interfacial-structure concepts.