The effect of fluid flow on eutectic growth
- PDF / 1,245,862 Bytes
- 15 Pages / 612 x 792 pts (letter) Page_size
- 2 Downloads / 192 Views
NTRODUCTION
THE theoretical model of eutectic growth under diffusive growth conditions is now well established for lamellar growth in three dimensions.[1] In contrast, most directional solidification experiments in bulk metallic systems are carried out under conditions where significant natural convection is present during the growth process, and the effect of fluid flow on eutectic microstructures has remained controversial. For natural convection in the Bridgman system, Drevet et al.[2] and Curreri et al.[3] showed that the eutectic spacing of a sample unidirectionally solidified under microgravity is not the same as that on the ground. Drevet et al.[2] ascribed this effect to the presence of a long-range solute-boundary layer in offeutectic alloys that is altered by the presence of fluid flow in the melt. They developed a theoretical model based on the boundary-layer concept. Although this model is applicable for laminar flow in hypereutectic Al-Cu alloys in which a lighter solute is rejected, the bulk composition in the boundary-layer model changes with solid fraction so that the spacing and microstructure will be a function of solid fraction, so that a time evolution of the eutectic pattern needs to be examined. In addition, the boundary-layer model is not applicable for large-diameter samples in which convective flows can be more complex, and it is not appropriate for hypoeutectic Al-Cu alloys in which the rejected solute is heavier and fluid flow causes significant radial variations in composition.[4] All the previous studies were carried out to characterize the effect of fluid flow on eutectic spacing only, and Curreri et al.[3] concluded that the eutectic spacing can increase, decrease, or remain constant in a low-gravity environment, depending on the alloy system. A more quantitative study is, J.H. LEE, Associate Professor, is with the Department of Metallurgy and Materials Science, Changwon National University, Kyungnam, South Korea 741-773. SHAN LIU, Division of Materials and Engineering Physics, and R. TRIVEDI, Senior Scientist, Department of Materials Science and Engineering, are with Ames Laboratory, Iowa State University, Ames, IA 50011. Contact e-mail: [email protected] Manuscript submitted April 6, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
thus, required that precisely establishes how different modes of fluid flow influence not only the spacing but also the microstructure. The aim of this article is to present the results of detailed systematic experimental studies on the effect of fluid flow on the eutectic microstructure in the Al-Cu system. Since the fluid flow gives rise to composition variations in the liquid in the radial and/or axial direction, we shall first characterize these composition variations and then establish how it influences the eutectic spacing, primary phase–to-eutectic transition, and lamellar-to-rod or rod-to-lamellar transition. Experiments are carried out by using two concentric cylindrical ampoules[5,6,7] in which diffusive growth is present in the inner-capillar
Data Loading...
