Effects of ledge density on the morphology and growth kinetics of precipitates in a Ni-Cr Alloy

Aging a quenched, face-centered cubic, Ni-45wt%Cr alloy at intermediate temperatures produces Cr-rich body-centered cubic precipitates with a lath morphology and two prominent boundary facets. Both the faceted boundaries migrate by a ledge mechanism. The density of ledges in these boundaries were measured as a function of time and related to the precipitate's shape and growth kinetics. A tendency of ledges to coalesce on one of the facets contributes to differences in the migration behavior of the two boundaries. One boundary has a distinct crystallographic habit, the other does not, and while both boundaries migrate with parabolic kinetics, they have different rate constants. The rate constants are influenced by the formation rate of ledges on the respective boundaries. Growth models are used in conjunction with the experimental measurements to show that the boundaries and ledges move at rates controlled by solute diffusion in the matrix.