Diffusion-coefficient measurements in liquid metallic alloys
- PDF / 423,573 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 72 Downloads / 236 Views
I. INTRODUCTION
THE solute-diffusion coefficient in a liquid metal is a key parameter for quantitative prediction of many kinetic processes. In directional solidification it is directly related to the length scales of solidification microstructures, which, in turn, control the properties of the solidified product. Several methods have been used to determine this technologically significant parameter, that include solute-enriched thin-layer diffusion,[1,2] droplet movement in a temperature-gradient field,[3] thermal transport,[4] and the solute boundary-layer measurements in a quenched directionally solidified alloy.[5–12] The last method has been extensively used to evaluate the diffusion coefficient in metallic alloys. One of the alloy systems in which several measurements have been made is the Al-Cu system,[2–4,7–12] in which the reported values of the diffusion coefficient vary from (3.0 to 5.5) 103 mm2/s. This uncertainty in the value of the diffusion coefficient has, indeed, prevented the precise validation of theoretical models of solidification microstructures. A small part of the variation could be attributed to the different compositions used in these studies (from a very dilute to a eutectic composition). A major reason for this scatter in the data, however, is due to the influence of fluid flow on the composition profile in the liquid. Verhoeven[13] discussed the effect of possible convection patterns in the melt on the diffusion-coefficient measurements and categorized them into threshold and thresholdless convection, which depends upon whether the driving force for convection (i.e., density gradient in the melt) is parallel or perpendicular to the gravity vector. In a vertically upward growth, the fluid flow will be controlled by the density gradient in the melt at the interface. If this density gradient is positive (i.e., the rejected solute is lighter), the fluidJ.-H. LEE, Assistant Professor, is with the Department of Metallurgy and Materials Science, Changwon University, Changwon, South Korea 641-773. SHAN LIU, Scientist, is with Ames Laboratory, United States Department of Energy, Ames, IA 50011. H. MIYAHARA, Associate Professor, is with the Materials Science and Engineering Department, Kyushu University, Fukuoka, Japan. R. TRIVEDI, Senior Scientist, Ames Laboratory, is Professor, Materials Science and Engineering, Iowa State University, Ames, IA 50011. Contact e-mail: [email protected] Manuscript submitted June 9, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B
flow effect can be significant. In contrast, when the axial density gradient is negative (i.e., heavier solute is rejected), fluid flow can still occur if any horizontal thermal gradient is present. In Bridgman growth, the sample is heated radially by a furnace, and a difference in thermal conductivity between the ampoule and the sample is present, which will give rise to a horizontal temperature gradient[14–19] so that some fluid flow will always be present. A detailed numerical analysis has been carried out by Trivedi et al.[18]
Data Loading...
