Study of Cu diffusion in Be using ion backscattering

The diffusion of Cu in single-crystal Be has been studied as a function of temperature, crystal orientation, and method of Cu introduction. Concentration-versus-depth profiles were obtained from the energy spectra of backscattered 2-MeV ${\mathrm{He}}^{+}$ and 2-MeV protons. The Cu was introduced by ion implantation at 100 keV or by vapor deposition. Reliable values of the dilute diffusion coefficient $D$ were obtained from the implanted samples when the Cu had diffused more than a factor of 2 beyond the initial implanted range ($\simeq 900$ Å), whereas anomalous diffusion rates were found over shorter distances. The vapor-deposited Cu films were not used to determine $D$ values, primarily because of substantial surface oxidation. However, the qualitative behavior of the three-component system Be-Cu-O was interpreted in terms of intermetallic compound formation and the heats of formation of the respective metal oxides. For example, whenever Be and Cu are present together, gettering of oxygen by Be is observed, and a surface layer of BeO is formed from which Cu is excluded. Also, the solubility of Cu in hcp Be at 750°C was measured to be 5.5 ± 1.0 at.%, in good agreement with the published phase diagram. The coefficient $D$ was measured for Cu diffusion along both the $c$ and $a$ crystallographic axes of the hcp Be lattice, between 420 and 640°C. There is a significant anisotropy, with $\frac{D_c}{D_a}=0.5\mathrm{}\pm 0.1$ at 600°C. These results merge smoothly with those obtained by Dupouy, Mathie, and Adda over the range 700-1000°C, by the radioactive-tracer method. The combined data represent a variation in $D$ of a factor of $\simeq 5\times {10}^6$ and permit an accurate determination of the activation energies. These are 2.00 ± 0.10 eV for the $a$ axis and 2.05 ± 0.10 eV for the $c$ axis. The LeClaire theory of heterovalent impurity diffusion was used to compare the Cu results with Be self-diffusion data of Dupouy, Mathie, and Adda. This theory was found to be inconsistent with the observed overall difference in activation energies between Cu and self-diffusion, and also with the observed anisotropy for Cu diffusion. In order to determine the sensitivity of the diffusion rate to radiation damage, a sample was bombarded with a flux of $\simeq 3\times {10}^{13}$ ${\mathrm{Ne}}^{+}$/${\mathrm{cm}}^2$ sec during a 30-min anneal at 500°C. The resulting rate of atomic displacement, $\simeq 0.01$ dpa/sec for depths