Geoffrey B. McFadden and Bruce T. Murray, ACMD
Sam R. Coriell, Materials Science and Engineering Laboratory
Alex A. Chernov, Russian Academy of Sciences
During crystal growth or solidification of a binary alloy from a liquid phase, temperature and solute gradients are inherently present. In a gravitational field, these gradients can give rise to fluid flow in the melt. The interaction of fluid flow with the crystal-melt interface plays an important role in determining the properties of the solidified material. Convection in the melt and interface instability may both produce solute inhomogeneities. In the absence of fluid flow, the conditions for the onset of morphological instability are well established. However, the coupling between morphological instability and fluid flow can be complicated; interfacial instabilities depend on temperature and solute gradients which may be strongly influenced by the flow field. The flow field, in turn, may be influenced by the morphology of the interface.
There has been a very successful long-term collaboration between individuals in CAML and MSEL which has resulted in the development of predictive models for a variety of crystal growth techniques from the bulk liquid phase. These models consist of both analytic relations under restrictive simplified cases and relatively sophisticated numerical algorithms to treat the nonlinear behavior under more general conditions. The recent focus of this modeling research addresses the specific concerns associated with crystals that grow with faceted or stepped interfaces. This situation occurs for crystalline materials which have high anisotropy (preferred orientations) in either their surface energy or atomic attachment kinetics.
In the last year, the effect of anisotropic interface kinetics on interfacial stability has been investigated for three crystal growth configurations and for materials of current practical interest. The dependence of the interface kinetic coefficient on crystallographic orientation is based on the motion and density of steps. As a result of the modeling work, it has been determined that anisotropic kinetics can provide a significant enhancement of interface stability. The growth conditions under which this stability enhancement can be obtained has been quantified as an outcome of this research effort.
The next phase of this research is to investigate more completely the interaction of fluid flow with a stepped crystal-melt interface. Shear flows along the interface interact strongly with the step motion and cause decreased stability for a flow in the same direction as the step motion and enhanced stability for flows counter to step motion. The objective is to quantify flow-interface interactions for a wide range of processing conditions for melt and solution growth with extension to more complex physical models and nonlinear interface morphologies.