Investigators: Sam R. Coriell, James A. Warren and William J. Boettinger

Technical Description:

Numerical and analytic models are being developed to predict solute segregation during solidification as a function of processing conditions. These segregation patterns play an important role in determining the properties of the solidified crystals and castings. The effect of fluid flow on the breakdown of the crystal-melt interface from planar to nonplanar cellular shapes, as well as the growth of fully-dendritic shapes are being studied. The fluid flow research is relevant to materials processing in microgravity.

Technical Objectives:

• Perform solidification modeling for application to microgravity directional solidification research including: the initial and final transients of a finite length sample, the effect of diffusion in the product liquid phase on monotectic composite solidification, and the effect of fluid flow on the formation of macrosteps and associated solute nonuniformities in crystals with anisotropic growth kinetics.

• Perform numerical analysis of melt convection due to thermal and solute gradients for the growth of lead bromide-silver bromide crystals.

• Develop a unified numerical technique for the simulation of dendritic growth that includes tip kinetics, solute redistribution and coarsening.

Anticipated Outcome:

• Improved scientific understanding of completed and planned experiments being carried out at a number of research laboratories involved with microgravity research. These experiments probe the fundamental role of the absence of melt convection during solidification. These include experiments on directional solidification of bismuth-tin alloys at the University of Florida and the growth of monotectic composite alloys of aluminum-indium at the University of Alabama in Birmingham.

• Optimized commercial growth conditions to produce quality lead bromide-silver bromide crystals as a non-linear optical material.

• Supercomputer calculations of a single dendrite will permit construction of improved approximate models for equiaxed castings that will lead to improved mechanical properties.

Accomplishments for FY 1995:

• The concentration profiles obtained in the initial and final transients were simulated allowing a more accurate prediction of microsegregation. Diffusion in the product liquid phase of a monotectic composite has little effect on the composite spacing. For crystals growing by the motion of steps, a shear flow in the direction of the step motion destabilizes the interface and causes solute inhomogeneities.

• Growth conditions to avoid melt convection due to temperature and solute gradients were computed for lead bromide-silver bromide crystals.

• Simulations of alloy dendritic growth using the phase-field, diffuse interface, numerical technique showed the effect of recalescence (reheating due to latent heat release) on the microsegregation pattern.

Impacts and Technical Highlights:

• Knowledge gained from experiments and analysis of faceted Bi-Sn alloys will provide increased understanding of faceted materials in general, such as semiconductors.

• Numerical calculations of the onset of fluid flow during the growth of lead bromide-silver bromide crystals have provided guidance to the Westinghouse Science and Technology Center on the best processing conditions for this non-linear optical material.

• Many international scientific groups have begun to utilize the phase field methods developed at NIST for the simulations of dendritic growth (Nature 375 (1995) 103).

Back to Table of Contents

Last modified: Mon Jan 06 09:46:15 1997 Metallurgy Webmeister