Divacancy reorientation in Al-rich B2 NiAl compounds

High-purity NiAl single crystals with different composition and different c rystallographic orientation have been prepared by crucible-free inductive z one melting under Ar atmosphere. Internal friction measurements of undeform ed Al-rich single crystals show a relaxation maximum with an activation ent halpy of (2.44 +/- 0.05) eV and a very small pre-exponential factor of 3.3. 10(-16+/-1) s. The relaxation strength of this maximum increases quadratica lly with the deviation from stoichiometry and shows an orientation dependen ce which is typical for atomic defects with tetragonal symmetry. In combina tion with the knowledge that structural defects in Al-rich NiAl are vacanci es on the Ni-sublattice, these experimental results allow evidently to assi gn the relaxation maximum to the reorientation of divacancies on the Ni-sub lattice. From the orientation dependence of the relaxation strength it is p ossible to determine the anisotropy of the elastic dipole tensor of a singl e divacancy to 0.2 +/- 0.05. (C) 1999 Elsevier Science Ltd. All rights rese rved.