Determination of the Solute Diffusion Coefficient by the Droplet Migration Method
- PDF / 419,311 Bytes
- 8 Pages / 594 x 792 pts Page_size
- 96 Downloads / 141 Views
Determination of the Solute Diffusion Coefficient by the Droplet Migration Method SHAN LIU, JING TENG, and JEONGYUN CHOI Further analysis of droplet migration in a temperature gradient field indicates that different terms can be used to evaluate the solute diffusion coefficient in liquid (DL) and that there exists a characteristic curve that can describe the motion of all the droplets for a given composition and temperature gradient. Critical experiments are subsequently conducted in succinonitrile (SCN)-salol and SCNcamphor transparent alloys in order to observe dynamic migration processes of a number of droplets. The derived diffusion coefficients from different terms are the same within experimental error. For SCN-salol alloys, DL 5 (0.69 6 0.05) 3 103 mm2/s, and for SCN-camphor alloys, DL 5 (0.24 6 0.02) 3 103 mm2/s. I. INTRODUCTION
LIQUID inclusions in a solid matrix can move in a temperature gradient field. The movement operates through the melting of the solid side in contact with the liquid inclusion with a slightly higher temperature and resolidification of the side with a slightly lower temperature. For simplicity of the subsequent discussion, we refer to the melting side as the leading edge and the resolidification side as the trailing edge. The driving force for the melting/resolidification is the composition gradient Gc, which is determined by the imposed temperature gradient GL through the relationship Gc 5 GL /m, where m is the liquidus slope. Since melting and regrowth occur simultaneously under the diffusive solute flux, this technique has been used to evaluate the solute diffusion coefficient in the melt, melting and growth kinetics, etc.[1–8] Migration of liquid pockets also plays a significant role in the side arm climbing/coarsening in a solidification process, microsegregation, and planar front initialization.[9–12] Liquid inclusion movement in a temperature gradient field can be described by (here neglecting solute diffusion in solid) V¼
DL GL mCL ð1 kÞ
[1]
where V is the droplet migration velocity, CL is the liquid composition at the solidifying edge (i.e., the trailing edge), k is the equilibrium solute distribution coefficient, and DL is the solute diffusion coefficient in liquid. Since m and k can be obtained from a phase diagram and GL is set externally, the diffusion coefficient can be calculated if V and CL can
SHAN LIU, Associate Scientist, is with the Materials and Engineering Physics Program, Ames Laboratory, Ames, IA 50011. Contact e-mail: [email protected] JING TENG and JEONGYUN CHOI, Graduate Students, are with the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011. This article is based on a presentation made in the symposium entitled ‘‘Solidification Modeling and Microstructure Formation: In Honor of Professor John Hunt,’’ which occurred March 13–15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee. METALLURGICAL AND AND MATERIALS MAT
Data Loading...
