Investigation of Interstitials in Electron-Irradiated Aluminum by Diffuse-X-Ray Scattering Experiments

The diffuse scattering of x rays ($\mathrm{Cu}{K}_{\alpha l}$) from single crystals of aluminum has been investigated after low-temperature electron irradiation (defect concentrations 3 × ${10}^{-4\mathrm{}}$ and 5 × ${10}^{-4\mathrm{}}$). The measurements were made near the (200), (400), (220), and (222) Bragg reflections in directions both parallel and perpendicular to the scattering vector. These measurements were repeated after a number of steps in an isochronal-recovery program. The data taken immediately after the irradiation showed all the characteristic features predicted by the theory for scattering from point defects with weak displacement fields (Huang scattering). The conclusions are the following: (i) The interstitial in Al has slightly tetragonal or cubic symmetry; the crowdion configuration can be excluded. (ii) By comparing the diffuse-scattering intensities with measurements of the lattice parameter the absolute defect concentration and the volume change $\Delta V$ per defect was determined. ($\Delta V=1.9\mathrm{}\pm 0.2$ atomic volumes per Frenkel defect.) From the absolute concentration and from electrical-resistivity change measurement on the same sample, the resistivity change was determined to be ${\rho }_F=3.9\mathrm{}\pm 0.6$ μΩ cm per atomic percent Frenkel defects. In the recovery measurements an increase of the scattering intensity per defect was observed at the end of stage I. This gives direct evidence that the interstitials agglomerate during their free migration in stage I. In stage II a further increase of the scattering intensity per interstitial is observed close to the Bragg peaks, suggesting an increase of the average interstitial cluster size throughout this stage. The changes in the intensity distribution around the reflections were in good agreement with the predictions of the scattering theory for defects with strong displacement fields (Stokes-Wilson approximation). From the data, the mean number of interstitials per cluster can be estimated. It is 2 at the end of stage I and increases to about 10 at the beginning of stage III. There is good indication that the larger interstitial clusters are dislocation loops.