Abstract
The results of a study of solubility and bulk diffusion of 110Ag and 63Ni in lead, lead-silver and lead-nickel solid solutions in the temperature range 220 to 88°C are reported. Owing to the low solubility of silver and nickel in lead, Pick's solution corresponding to the boundary condition of a constant concentration of solute at the surface has been used. Depth-profile concentration analysis of 110Ag and 63Ni suggests a fundamental difference between the diffusion mechanisms of silver and nickel. Since silver penetration profiles in pure lead give diffusion coefficients independent of the penetration depth and silver concentration, it is suggested that slight decrease of silver diffusivity in lead-silver solid solutions (which remain within experimental error) have no significance. This implies that the interstitial silver atoms do not associate significantly with each other to form Ag-Ag dimers. In contrast, different behaviors of 63Ni depth profile concentration in pure lead and saturated PbNi solid solutions agree with a strong Ni-Ni interaction leading to the formation of less mobile dimers near the surface in pure lead. The model proposed by Decker to describe the diminution of gold diffusivity in PbAu solid solutions yields H = 9500 cal/mol and S = 0.9 k for the binding enthalpy and entropy of Ni-Ni substitutional dimers.
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