Internal Friction Studies of Fast Diffusing Solutes in Thorium
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INTRODUCTION
THEfast diffusion of metallic solutes in metals has been of interest since Roberts-Austin reported in 1896 that gold diffuses rapidly in lead. Recent papers by Warburton and Turnbull, 1Miller,2 Anthony, 3 and Peterson 4 have shown that there are a number of systems in which the diffusivity of a metallic solute is several orders of magnitude greater than the self diffusivity of the host metal. This rapid diffusion behavior is suggestive of an interstitial diffusion mechanism similar to that generally ascribed to carbon, oxygen, nitrogen, and hydrogen in metals. According to Hfigg's empirical rule, 5 interstitial solid solutions are limited essentially to systems in which the ratio of the atomic radius of the solute to that of the host, Rs/RH, is less than 0.59. In most of the systems in which fast diffusion is reported, however, the Hiigg limiting value is exceeded, and sometimes reaches a value of 0.90. Dyson, e t al 6 have suggested that interstitial solid solubility can occur for metallic systems in which there is no ion-ion overlap between a solute located in the largest interstitial site and the host ion. The most likely hosts for fast diffusion will therefore be those with large atomic Wigner-Seitz radii and relatively small ion-core radii. In an earlier paper 7 the authors reported that iron, cobalt, and nickel exhibit fast diffusion in thorium with diffusivities at least six orders of magnitude greater than the self diffusivity of thorium. On the basis of the above size criteria these solutes could occupy interstitial sites in thorium inasmuch as the Rs/Rn values are all approximately 0.70. Turner and associates 8 observed internal friction peaks in lead containing the fast diffusing solutes, gold, silver, and copper, although these results were not confirmed by later work. 9 The present investigation was therefore undertaken to determine if thorium containing the fast diffusing solutes, cobalt, iron, and nickel exhibit internal friction peaks and, if so, to attempt to identify and characterize the type of defect associated with them. Our interest in the fast diffusion phenomenon derives primarily from work on the ultrapurification of thorium l~ and the rare earth metals H by electrotransport which is a diffusion-controlled process. W. N. WEINS, formerly Research Assistant, Ames Laboratory, is now Assistant Professor of Mechanical Engineering, University of Nebraska, Lincoln, NE 68588. O. N. CARLSON is Professor, Materials Science and Engineering, and Senior Metallurgist, Ames Laboratory-DOE, Iowa State University, Ames, IA 50011. Manuscript submitted July 27, 1981. METALLURGICAL TRANSACTIONS A
THEORY
Several mechanisms have been proposed to explain the phenomenon of fast diffusion. They all assume that fast diffusion occurs by a bulk mechanism rather than along surfaces or line defects, and that more than one mechanism may be operational. As Warburton and Turnbull '2 have stated, the present evidence supports the conclusion that interstitial-type defects are primarily responsible fo
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