The nature and role of incoherent interphase interfaces in diffusional solid-solid phase transformations

In this article, some views on the nature of incoherent interphase interfaces, and their role in the nucleation and growth processes governing the evolution of microstructure in solid-state diffusional transformations (reconstructive transformations), are explored. It is argued that essentially incoherent interfaces can be involved in the initiation and propagation of polymorphic transformations and massive transformations as well as in various precipitation phenomena in metallic and ceramic systems. Similar views have already been advanced earlier in connection with studies of massive transformations. Faceting along the interphase interface during nucleation and growth can derive from thermodynamic, kinetic, and crystallographic factors independent of the bicrystallography of the conjugate phases. This idiomorphic behavior can be relevant to both intergranular and intragranular phase formation. The concept of one-dimensional (1-D) commensuration of phases through plane edge-to-edge/row matching is an interesting extension of the classic ideas of coherency and bicrystallography and potentially important in characterizing the behavior of certain types of boundaries. However, the general importance of these geometrical relations in real and reciprocal space will depend on the depth of the energy wells in orientation space associated with these special boundaries. This article is based on a presentation made in the “Hume-Rothery Symposium on Structure and Diffusional Growth Mechanisms of Irrational Interphase Boundaries,” which occurred during the TMS Winter meeting, March 15–17, 2004, in Charlotte, NC, under the auspices of the TMS Alloy Phases Committee and the co-sponsorship of the TMS-ASM Phase Transformations Committee.