Interdiffusion in liquid tin
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Basic and Applied Research: Section I
Interdiffusion in Liquid Tin John Cahoon, Yuning Jiao, Kedar Tandon, and Mahesh Chaturvedi
(Submitted August 15, 2005; in revised form March 16, 2006) The interdiffusion coefficients of Zn, Ag, Sb, Pb, and Bi in liquid Sn were determined using both shear cell and long capillary techniques. These elements were chosen to provide a range of valences and atomic radii, variables that are expected to affect the interdiffusion coefficients. The results indicate that Sb and Ag diffuse in liquid Sn at the same rate as does Sn itself. Bi and Pb appear to diffuse more slowly in liquid Sn than does Sn. Zn appears to diffuse more rapidly in liquid Sn than does Sn itself. These results indicate that the atomic radius is an important variable for interdiffusion in liquid Sn. However, the results for the interdiffusion of Zn, Pb, and Bi, were more scattered than those for Ag and Sb, suggesting that some convective mixing, due possibly to transverse temperature gradients, may be occurring even in capillaries with only 1.5 mm diameters.
Keywords
interdiffusion, liquid tin, long capillary, shear cell
1. Introduction Atomic transport in liquid metals is an important phenomenon influencing the structure and properties of cast materials including metals, polymers, and ceramics. The main transport mechanisms are convective mixing and atomic diffusion in the liquid. Neither of these liquid transport processes is completely understood, and as a result the simulation of atomic transport processes during solidification for the purposes of establishing predictive models cannot be adequately accomplished. Knowledge of the liquid diffusion process in metals and alloys is not nearly as well developed as that for solid diffusion, and as a result many theories have been advanced to explain the experimental data. The “hole” theory of liquid diffusion[1,2] is equivalent to the vacancy mechanism for solid diffusion and results in the standard Arrhenius equation for the determination of the diffusion coefficient. However, entropy considerations render the “hole” mechanism implausible.[3] “Fluctuation” theories propose that liquid diffusion occurs via the movement of atoms through small and variable distances as a result of local density fluctuations and that such transport occurs by the cooperative motion of five atom groups.[4,5] The fluctuation theory results in a T2 temperature dependence for the liquid diffusion coefficient. Several models for liquid diffusion that involve the viscosity of the liquid have been developed.[6-8] In these models, the activation energy for liquid diffusion is that for viscous flow. Other models for liquid diffusion assume the liquid atoms to behave as solid rigid spheres and that difJohn Cahoon, Yuning Jiao, Kedar Tandon (deceased), and Mahesh Chaturvedi, University of Manitoba, Mechanical and Manufacturing Engineering, 15 Gillson St., Winnipeg, Manitoba R3T5V6. Contact e-mail: [email protected].
fusion in liquids is more similar to diffusion in gases than in solids.[9-
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