Kinetics of growth of platelike precipitates

Abstract

In order to resolve existing difficulties in the interpretation of the lengthening and thickening kinetics of platelike precipitates, these kinetics have been measured on θ' and η plates in Al-4 % Cu alloy and Al-0.2% Au alloy respectively, in conjunction with extensive observations of the ledge and misfit dislocation structures at the broad faces of the plates. Thickening has been found, in agreement with a well established theory of morphology, to occur by a ledge mechanism, where the migration rates of the ledges were diffusion controlled. Surprisingly, the lengthening rates of coherent plates were found to be much greater than those of the same kinds of plates when semicoherent. In the latter condition, lengthening rates approximate those to be expected on the basis of volume diffusion control. This result is explained by the role of the broad face coherency strain field which accelerates the lengthening. Thus a previous suggestion that such accelerated kinetics are due to diffusional short circuit is shown to be invalid.