Study of the mechanism of fast diffusion of cobalt in thorium by diffusion and internal friction measurements
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INTRODUCTION
S I N C E Roberts-Austen [1] discovered that gold diffuses rapidly in lead, there has been considerable interest in the mechanism responsible for fast diffusion in metalmetal systems, t2-Tj Fast diffusion is said to occur in those systems in which the diffusivity of the solute is at least an order of magnitude greater than that of the host metal, and the activation energy for diffusion of the solute is much less than that for self-diffusion of the solvent. The small enhancement of the self-diffusivity of lead by the addition of copper, silver, and gold I8,91 suggests that fast diffusion in these systems proceeds by an interstitial mechanism, but the relatively large enhancement of the self-diffusivity of lead by two other fast diffusing solutes, cadmium and mercury, I7,1~ indicates that the fast diffusion mechanism is more complex than a simple interstitial jump process. This is supported by the small isotope effects of cadmium, silver, and copper in l e a d . [l l,12,13l
Frank and Turnbull I~4~ introduced the dissociative mechanism for fast diffusion, which assumes that the solute occupies both substitutional and interstitial sites, with solute migration being dominated by an interstitial mechanism. Warburton and Tumbull I2~ proposed the hostsolute diplon and Miller t6] and Warburton tT] proposed the interstitial-vacancy pair to explain the phenomenon. Several attempts have been made to characterize the solute defects present in various fast diffusing systems but no conclusive evidence has been presented. X-ray and neutron diffraction experiments performed by Bolze e t al. 115'161on dilute solutions of gold in lead indicate that the gold atoms exist primarily as substitutional defects, and channeling experiments by Miller e t al. t171 indicate that gold atoms also occupy substitutional sites primarily in tin. Although no evidence was cited for the type of defect associated with the fast diffusion of gold in either metal, it was stated that the concentration of such defects
S.C. AXTELL, formerly Graduate Student, Ames Laboratory, Iowa State University, is Assistant Professor with the Metallurgy Engineering Department, South Dakota School of Mines, Rapid City, SD 57701. O.N. CARLSON, Professor Emeritus and Associate, is with the Ames Laboratory, Iowa State University, Ames, IA 50011. Manuscript submitted January 31, 1989. METALLURGICAL TRANSACTIONS A
must be below the detectable limits for their measurement technique. Carpenter and Cathey t~8j used M6ssbauer measurements to study the diffusion of iron in yttrium. Their results indicate that the migration rate of the majority of the iron atoms in yttrium is much slower than that obtained from bulk diffusion experiments, suggesting that at least two migration mechanisms are operative. However, they were unable to identify the defect associated with the fast diffusion process. Vogl e t a / . H9] investigated the diffusion of cobalt in zirconium using quasi-elastic neutron scattering. Their results showed that a small fraction of the cobalt atoms di
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