Dendritic solidification of alloys in low gravity
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INTRODUCTION
DENDRITIC growth processes are critical to the most common alloy casting techniques. Dendrite arm spacings correlate with mechanical strength. Interdendritic macrosegregation causes nonhomogeneous casting properties; yet, controlled segregation is utilized for precipitation strengthening processes. [1] Buoyancy-driven convective flow in the liquid perturbs metal alloy solute concentration profiles (and to a lesser degree temperature profiles [21) that control microstructure and properties. Gravity, thus, has strong influence on the processes of dendritic growth. Single dendrites of pure succinonitrile have been studied [3'41 as a function of undercooling, tip growth rate, and orientation to gravity. Gravity-independent diffusionlimited growth occurs at undercooling of 1 to 9 ~ Morphology and growth rate are dependent, however, at undercoolings less than 1 ~ on dendritic orientation to gravity. The growth rate perturbations observed at small undercooling are consistent with enhanced transfer of latent heat by natural convection. Low-gravity experiments tSJ with A1-0.4Cu show that convection can influence dendritic morphology and crystallographic orientation in metallic alloys. Thermal solutal convection and solidification shrinkage produce interdendritic fluid flow that is directly related to casting macrosegregation [6] and porosity, tT's] Convective flow is practically confined to 30 pct or less solid fraction, since permeability of the dendrite mushy zone increases rapidly with liquid fraction, t9'~~ Gravity-dependent interdendritic fluid flow for metal alloys has been modeled [H] numerically and (with simplifying assumptions) anal9] lytically. Interdendritic flow perturbs concentration and temperature fields that determine local solid composition in the casting. A1-Cu dendritic solidification perpendicular to the gravity vector has been analyzed as a function of magP.A. CURRERI, Materials Scientist, Space Science Laboratory, and J.E. LEE, Metallurgist, Materials and Processes Laboratory, are with NASA/Marshall Space Flight Center, Huntsville, AL 35812. D.M. STEFANESCU is Professor, Department of Metallurgical Engineering, The University of Alabama, Tuscaloosa, AL. This paper is based on a presentation made in the symposium "Experimental Methods for Microgravity Materials Science Research" presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.
METALLURGICALTRANSACTIONS A
nitude of gravity, t]2] Macrosegregation under normal gravity causes Cu fraction across the casting to vary 45 pct. Since flow due to solidification shrinkage is perpendicular to the thermal isotherms, theory predicts negligible macrosegregation in zero gravity. Low-gravity dendritic directional solidification of off-eutectic Pb-Sb, t13] Bi/Mn-Bi, [14] AI-Cu, and Sn-Pb I5] has resulted in negligible macrosegregation, indicating diffusive solute transport. Dendritic solidification of off-eutecti
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