High-purity single crystals with various compositions and crystallographic orientations were prepared. Internal friction measurements of undeformed Al-rich crystals revealed a relaxation maximum, with an activation enthalpy of 2.44eV and a pre-exponential factor of 3.3 x 10-16s. The relaxation strength of this maximum increased quadratically with deviation from stoichiometry, and exhibited an orientation-dependence which was typical of atomic defects with tetragonal symmetry. When combined with the information that the structural defects in Al-rich material were vacancies on the Ni sub-lattice, the experimental results permitted the relaxation maximum to be attributed to the reorientation of divacancies on the Ni sub-lattice. From the orientation-dependence of the relaxation strength, the anisotropy of the elastic dipole tensor of a single divacancy was deduced to be 0.2.

Divacancy Reorientation in Al-Rich B2 NiAl Compounds. D.Schaible, M.Hirscher, H.Kronmuller: Intermetallics, 1999, 7[3-4], 309-14