Diffuse x-ray scattering studies of defect reactions in electron-irradiated dilute nickel alloys. II. Ni-Ge

Defect properties in dilute Ni-Ge alloys are investigated by Huang diffuse scattering and the results for this alloy (oversized impurity) are compared with those for Ni-Si (undersized impurity). Ni alloys containing 1 at.% and 0.05 at.% Ge were irradiated with electrons at 4.2K and were subsequently isochronally annealed. It was found that, prior to annealing, the Frenkel-pair resistivities and self-interstitial atom configurations were the same in the alloys as in pure Ni. The independence of the Frenkel-pair resistivity to Ge concentration indicates that the resistivities due to Frenkel pairs and Ge solute are linearly additive in Ni. After annealing through stage I to 85K, the average defect cluster size grew to 1.8, 3.0 and 3.0 interstitial atoms for the 1 at.% Ge, 0.05 at.% Ge and pure Ni specimens, respectively. These results demonstrate that self-interstitial atoms are not immobilised by single Ge atoms in Ni, but rather complexes involving several Ge atoms and/or two interstitial atoms are the stable defects at the end of annealing stage I. There is a strong suppression of interstitial cluster growth during stage II only for the 1 at.% Ge alloy. Although these results are qualitatively very similar to those for Ni-Si, there are three characteristic differences: (i) interstitial-atom clusters of equal size are mobile in Ni-Ge at lower temperatures than in Ni-Si; (ii) there appear to be different mechanisms for the transport of solute by interstitial-atom motion in the Ni-Si and Ni-Ge alloys; (iii) in annealing stage III there is a strong interaction of small vacancy agglomerates with Ge atoms but not with Si atoms.