As FPs are produced in isolation during electron irradiation, the properties of single point defects and their interactions with impurities and sinks can be systematically investigated. An example is shown in Figure 11(a), where the results of low — temperature isochronal annealing of Cu are shown following 1.4 MeV electron irradiation at 6K.2 Recovery is observed to occur in ‘stages.’ These stud­ies have revealed that interstitial atoms become mobile at very low temperatures, always below 100 K, in so-called Stage I, while vacancies become mobile at higher temperatures, Stage III. The various substages IA-IE seen in Figure 11(a) arise from the interaction between interstitial-vacancy pairs, which are produced in close proximity. Stage IE refers to the free migration of interstitials in the lattice, away from its own vacancy, and annihilation at distant vacancies; these interstitials are freely migrating as discussed earlier. For comparison, Stage I annealing of Cu following neutron irradiation is shown in Figure 11(b). Notice that the close pair substages are suppressed during neutron irradiation, illustrat­ing the dramatic difference in the defect production process for these types of irradiation. Similarly, annealing studies on electron-irradiated Al doped with Mg or Ga impurities are shown in Figure 12.22 For these, it is observed that Stage I recovery is suppressed as interstitials trap at impurities and do not recombine. The recovery at higher temperature, in Stage II, reveals distinct subannealing stages.

These annealing stages are generally attributed to either the interstitial dissociating from the impurity, or the interstitial-impurity complex migrating to a vacancy or a defect sink. Migrating interstitial-solute complexes lead to segregation. A compilation of the properties of point defects for many metals, and their interactions with impurities can be found in Ehrhart.2 This information has played a crucial role in develop­ing an understanding of radiation damage in more complex engineering alloys and under more complex irradiation conditions.