- Sam R. Coriell, Metallurgy Division
- Bruce T. Murray, ACMD
- Geoffrey B. McFadden, ACMD
- Alex A. Chernov, USRA and Russian Academy of Sciences
- During crystal growth or solidification of a binary alloy from a liquid
phase, temperature and/or 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 three years, 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.
- A recent phase of this research is to investigate more completely the
interaction of fluid flow with a stepped crystal-melt interface. The above
figure shows schematically a perturbed step bunch and the solute concentration
field above the solid/liquid interface for growth from a supersaturated solution.
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 above figure is a stability map given in terms of the spatial wavenumbers at which the system is neutrally stable as a function of growth velocity for an orientation slope of 0.01 and for shear rates of -0.1, 0.0, 0.0001, 0.001, 0.01, 0.1 and 0.5 for a linear Couette profile. The solid curves are numerical solutions of the complete linear stability equations while the dashed curves are from the analytic approximation which neglects the perturbed flow field. The two solutions are in excellent agreement except at small wavenumbers. The current objective is to quantify flow-interface interactions for a range of processing conditions for solution growth; the extension to more complex physical models and nonlinear interface morphologies will be part of the ongoing research.

Here is a list of recent publications related to this work:

- A. A. Chernov, S.R. Coriell, and B.T. Murray,
Morphological Stability of a Vicinal Face Induced by Step Flow,
*Journal of Crystal Growth*141, pp. 405-413, 1993. - S.R. Coriell, B.T. Murray, and A. A. Chernov,
Kinetic Self-Stabilization of a Stepped Interface: Binary Alloy
Solidification,
*Journal of Crystal Growth*141, pp. 219-233, 1994. - A. A. Chernov, S.R. Coriell, and B.T. Murray,
Kinetic Self-Stabilization of a Stepped Interface: Growth into a
Supercooled Melt,
*Journal of Crystal Growth*149, pp. 120-130, 1995. - S.R. Coriell, B.T. Murray, A. A. Chernov, and G.B. McFadden,
Effects of Shear Flow and Anisotropic Kinetics on the Morphological Stability
of a Binary Alloy,
*Met. Matls. Trans*27A, pp. 687-694, 1996.