Precipitation in star sapphire II. Elastic accommodation of the precipitate

Publisher Website
Google Scholar
Abstract
The growth of rutile needle precipitates in star sapphire is accompanied by an interfacial dislocation structure in the rutile-sapphire interface, and a set of pyramidal dislocations which climb into the sapphire matrix. In addition, the rutile is internally twinned. The dislocation array in the interface provides almost perfect accommodation for the lattice mismatch between precipitate and matrix. These interface dislocations are vacancy-type loops and are formed by the rejection of interstitials into the matrix, due to a combination of elastic and chemical effects accompanying precipitation. The rejected interstitials cause the formation of the set of climbing dislocations. The twinning provides improved lattice matching between rutile and sapphire, but the twin orientation is such that twins cannot have formed by shear to accommodate growth stresses; rather they are growth twins formed either during the early stages of growth or perhaps during nucleaten of the precipitates.