Anomalous diffusion of Fe in Liquid AI Measured by the pulsed laser technique
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INTRODUCTION
THE gram atomic volumes of transition metals in solid and liquid A1 and the composition dependence of those volumes have been examined by Tumbull.tl] The partial molar volumes of most transition metals in liquid aluminum are less than in their pure forms. For Fe in liquid A1, this effect is much more pronounced than for most other transition metals in AI, for Fe has a negative partial molar volume in liquid AI: the addition of Fe to a fixed amount of A1 reduces the overall volume. Turnbull noted that this behavior, as well as anomalous viscosity and thermal expansion behavior, could be accounted for if tightly bound clusters of A1 surround individual Fe atoms in the liquid solution. He estimated the number of A1 atoms bound to a single Fe atom to be seven from the viscosity datat2] and 12 from the thermal expansion data. o] If these clusters indeed exist in the liquid state, we should not be surprised to find a reduced diffusivity (compared to the diffusivities of other transition metals and of AI atoms in liquid AI), as expected from the Stokes-Einstein relation. The diffusion coefficient of Fe and other solutes in liquid A1 has been measured by the capillary method by Ejima et a/. [4'5] The value he found for Fe, 2.98 • 10-9 mZ/s at 976 K, was about 46 pct less than his calculated self-diffusion coefficient for A1 (calculated by the principle of corresponding states) and was also smaller than the diffusion coefficients of most other transition elements in liquid AI. Although liquid diffusivities are most often measured by the capillary method, experimental difficulties such as conN. ISONO, formerly with the Division of Applied Sciences, Harvard University, is Research Scientist with Copper Research Department, Kobe Steel, Ltd., Shimonoseki, 572 Japan. PATRICK MICHAEL SMITH, formerly with the Division of Applied Sciences, Harvard University, is staff physicist with Lawrence Livermore National Laboratory, Livermore, CA 94550. D. TURNBULL, Professor Emeritus, and M.J. AZIZ, Gordon McKay Professor of Materials Science, are with the Division of Applied Sciences, Harvard University, Cambridge, MA 02138. Manuscript submitted February 9, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
vection, high reactivity, and melting points too high for containment tubes can severely complicate the measurement of diffusivities in some systems. The pulsed-laser melting techniquet61 can be used to measure diffusion coefficients of a solute in a melt without either convection or interface diffusion, owing to the thin film geometry, and the nanosecond time scale of the experiment confines heat to the thin film on the substrate, permitting refractory or highly reactive materials to be investigated. With spatially tmiform laser irradiation on optically fiat samples, a onedimensional melting and solidification geometry can be created, permitting simple but accurate measurements of important quantities, such as crystal/melt interface positions and velocities, and solute and temperature depth profiles.r~o]Furthermore, beca
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