Study of the diffusion of Au and Ag in Be using ion beams

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Abstract
The diffusion of Au and Ag in single−crystal α−Be has been studied using ion beam techniques. The time−dependent diffusion profiles were monitored by ion backscattering. Diffusion couples were created both by ion implantation and by vapor deposition, with consistent results. Diffusion data were obtained at two temperatures, 665 and 780 °C, for both c and a crystallographic directions. Because of the low solid solubilities of Au and Ag in Be, ≲0.1 at.% at the above temperatures, a surface layer of ∼ 1000 Å thickness was always present in which the solubility was greatly exceeded. During annealing this layer served as a source from which the solute diffused into the underlying Be. The surface layer was characterized by transmission electron microscopy in the case of Au implantation. No evidence of precipitation was found following the room−temperature implantation, but subsequent annealing above about 300 °C caused regions of AuBe5 to form in the layer. In the underlying bulk, the observed diffusion behavior indicates that the dilute solute is migrating within the α−Be lattice without formation of an intermetallic compound, and that its concentration is limited by the solid solubility. By solving the diffusion equation with appropriate boundary conditions, the diffusion coefficients and solid solubilities in α−Be are extracted. The diffusion rates for Au in Be are found to be very similar to those for Cu in Be in magnitude, in temperature dependence, and in anisotropy (Dc/Da ?0.5 in both cases). However, for Ag the diffusion coefficients are larger by about an order of magnitude and have the opposite anisotropy (Dc/Da?2). This variation among the I B solutes is shown to correlate with pseudopotential parameters. Diffusion measurements were also made for Au in polycrystalline Be with a mean grain size of 10 μm. This effective diffusion coefficient is found to be much larger than in the single−crystal material, a factor of ∼100 at 665 °C, and to have a weaker temperature dependence.