INTERSTITIAL ATOM CLUSTERING AT CO-57 IMPURITY ATOMS IN ALUMINUM AFTER ELECTRON AND NEUTRON-IRRADIATION

From Moessbauer measurements on Al**5**7Co samples irradiated above stage I with 2. 8 MeV electrons it is found that the temperature dependence of the Debye-Waller factor of **5**7Co impurities having trapped interstitials becomes weaker with increasing number of interstitials trapped at a **5**7Co atom. This behavior is explained by a linear superposition of two phenomena: The dynamics of **5**7Co impurities with only one trapped interstitial (mixed dumbbell) is determined by jumps in an octahedral cage. When two or more interstitials are trapped, the cage jumps are suppressed and the mixed dumbbells can only perform low frequency vibrations. Annealing in stage I leads to substantial fraction of trapped single interstitials, thus indicating free migration of single interstitials. In stage II the fraction of trapped single interstitials is reduced while the fraction of trapped interstitial clusters grows.