HISTORY OF THE CONTROVERSY OVER THE ROLES OF SHEAR AND DIFFUSION IN PLATE FORMATION ABOUT MD AND A COMPARISON OF THE ATOMIC MECHANISMS OF THESE PROCESSES

The evolution of the controversy over the roles played by shear and by diffusion in the formation of plate-shaped transformation products above M(d) is traced along its principal pathways. Growth by shear is taken to occur through the glide of dislocations, whereas diffusional growth is considered to occur by means of the biased random walk of individual atoms. Both mechanisms are based upon growth ledges, but during shear, risers, and in some cases terraces, are mobile in non-fcc/hcp transformations; however, during diffusional growth, only the risers (and more likely, only kinks on the risers) are mobile when there is a stacking sequence difference across the terrace of a growth ledge. The aura of the powerful and elegant phenomenological theory of martensite crystallography (PTMC) appears to have persuaded many researchers that even above M(d), plates usually form by shear because one or more of the multiple requirements for the applicability of the PTMC appears to be fulfilled. As the preceding article by Wayman in these conference proceedings emphasizes, however, all of these requirements must be fulfilled in a self-consistent manner if a plate is to qualify as the product of a shear transformation mechanism. Successful application of the invariant line (IL) component of the PTMC to precipitate plates and needles or rods by Dahmen and co-workers provides a phenomenological reason why the crystallographies and surface reliefs of ledgewise shear and ledgewise diffusional growth sometimes appear to be identical. While the IL-containing broad faces of a martensite plate minimize the transformation strain energy, the same situation in plates formed by ledgewise diffusional growth tentatively appears to engender a minimum in the kinetics of growth ledge generation and/or the lateral growth kinetics of such ledges.