Internal friction and elastic modulus measurements of AI-0.07 at.% Mg single crystal after fast neutron irradiation at 5–7 K are made as a function of temperature. The recovery stages are also studied by electrical resistivity measurements and compared with that of pure Al. Several internal friction peaks and elastic modulus changes due to dislocation pinning are observed. The peaks, observed only in the alloy, could be related to the reorientation of interstitial-solute atom complexes. Symmetry and annealing processes of interstitials and interstitial-solute atom complexes are discussed using ⟨111⟩ and ⟨100⟩ oriented specimens. The relaxation strengths and the activation energies for these peaks are also given.