THE ELASTIC STRAIN-ENERGY OF GROWTH LEDGES ON COHERENT AND PARTIALLY COHERENT PRECIPITATES

The formation rate of growth ledges on a faceted precipitate strongly affects the growth kinetics and the shape of the precipitate. An Eshel by-type. model is used to compare the strain energy associated with th e nucleation of a ledge on different facet planes of a body-centered c ubic (bcc) precipitate in face-centered cubic (fcc) matrix. Ledge nucl eation is only likely at facet areas where the interaction energy betw een the ledge and: the precipitate is negative. The strain energy for ledge formation is not symmetric on any of the facet planes, but it is symmetric about the center of the precipitate. For coherent precipita tes comparable to those observed in the Ni-Cr system, ledges form with the lowest strain energy on the broad facet of the precipitate implyi ng that precipitate thickening should occur faster than lengthening an d widening. A procedure for modifying the Eshelby model is suggested i n order to allow Strain-energy calculations of partially coherent prec ipitates. The strain energy for ledge formation on at least one type o f partially coherent lath is lowest for a ledge located on the facet p erpendicular to the crystallographic invariant line (IL). This situati on favors precipitate lengthening in the invariant line direction.