THE INVARIANT LINE AND PRECIPITATE MORPHOLOGY IN FCC-BCC SYSTEMS

Second-phase precipitates in many face-centered cubic-body-centered cubic (fcc-bcc) systems (e.g., Ni-Cr, Cu-Cr, Fe-Cu, and alpha-gamma stainless steels) have a lath-shaped morphology, the long axis of the lath being an invariant line of the transformation. The invariant line direction and major (habit) facet plane of the product phase can be predicted by 0-lattice (0-line) models. For N-W- and K-S-oriented precipitates, the habit plane is shown to be an unrotated plane of the transformation. This contains a single set of dislocations lying parallel to the invariant line, with their Burgers vector in the habit plane. Structural ledge models for the habit-plane interface also are considered. For the range of lattice parameter ratios of interest in this study, structural ledge and 0-line models can make almost identical predictions as to the optimum habit plane. A variety of elasticity calculations for the energy of fully constrained or fully relaxed precipitates is presented. These models are shown to have limited predictive capabilities. It is suggested that better atomic matching along or near to the invariant line direction might explain the preference for K-S-related precipitates in many systems.