The role of interfacial structure in the evolution of precipitate morphology

Three aspects of precipitate growth by a ledge mechanism in a Ni-45wt%Cr alloy were investigated. The strain energy for ledge formation and ledge growth kinetics and the emission of structural defects were studied experimentally during the growth of bcc laths from an fcc matrix.; The elastic strain energy of a growth ledge as a function of the ledge location was estimated using an Eshelby-type model. Ledge nucleation is only likely at facet areas where the interaction energy between the ledge and the precipitate is negative. Ledges form with the lowest strain energy on the broad habit plane of coherent precipitates. On a partially coherent lath the strain energy is lowest for a ledge located on the facet perpendicular to the crystallographic invariant line. This situation favors precipitate lengthening in the invariant line direction.; Experimental measurement of growth kinetics of the precipitate was made to examine the mechanistic relationship between precipitate growth kinetics and its morphology. TEM was employed to measure overall precipitate growth kinetics as function of time, crystallographic orientations and ledge density. Results show the precipitates widen and thicken by a ledge mechanism following parabolic growth laws. Morphology of precipitates during aging is closely related to the ledge density.; Several types of defect emission from partially coherent interphase boundary in the alloy were observed using conventional and in situ hot stage TEM techniques. Prismatic dislocation loops expand and glide off from the precipitate. Perfect {dollar}{lcub}alphaover2{rcub}langle 110ranglesb{lcub}fcc{rcub}{dollar} dislocations glide away from the broad habit plane. Stacking faults emanated from the broad face of the laths were observed during precipitate growth. These defects result in steps in the interface and appear to compensate misfit in the broad face of the lath.