Kinetics and Morphology of the Precipitation of Gold from Dilute Lead

The kinetics of precipitation of the Pb(,3)Au intermetallic from dilute supersaturated Pb(Au) alloys have been measured resistometrically. The activation energies, Q, for precipitation have been determined; it was found that the activation energy for precipitation from Pb(500 atomic ppm Au) alloys is within experimental error of that measured previously for more concentrated alloys, about 20 kcal/mol, while that for Pb(100 at. ppm Au) specimens, approximately 10 kcal/mol, is on the order of the activation energy for Au tracer diffusion in Pb. The morphology of the Pb(,3)Au precipitate particles has been studied using scanning electron microscopy. By combining our morphological observations with the precipitation kinetics results we have demonstrated that the Au diffusivity limiting precipitation from Pb(500 atomic ppm Au) specimens is roughly that which is predicted by the earlier findings of Rossolimo and Turnbull; that diffusivity, D(,p), has been associated with the motion of a Au defect which is significantly slower than the interstitial Au reflected by the tracer diffusivity, D(,Au). On the other hand, D(,Au) is found to limit both precipitation from Pb(100 ppm Au) alloys and the Ostwald ripening in the Pb(500 ppm Au) specimens. In the former case the growth morphology was suggestive of the proposed ribbon-growth model while in the latter it was found that the grain boundary particles, six times as voluminous as those in the matrix, ripen at the expense of those smaller matrix particles; this leads to a different expression for the Ostwald ripening kinetics than that presented by the usual Lifschitz-Slyozov-Wagner model. The precipitate particle morphologies observed were quite striking; whiskers and ribbons of large aspect ratio developed upon precipitation from the Pb(100 ppm Au) specimens and Ostwald ripening of the 500 ppm specimens. If not the only such observation, this is one of the few findings of the growth of whisker-like second-phase particles in the solid state. Finally, the effects of the Au and Ag noble metal impurities on the power-law creep strength of Pb were investigated. Results are discussed in terms of two models of the Au dissolution behavior in Pb. In one model, the exciton model, the Au is dissolved primarily as a bound Au interstitial -lattice vacancy pair at high temperatures; these pairs collapse to the Au substitutional defect at lower temperatures. The second model calls for the association of the interstitial -vacancy pairs, or atomic excitons, with Au interstitials to form small clusters at lower temperatures and higher Au concentrations. (Abstract shortened with permission of author.).