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.