NMR study of aluminium diffusion in Au-Al alloys

The atomic diffusion of Cu in solid Au:xCu alloys (x=5.7, 8.2, 9.1, 10.4, 13.2, 17.3 at%) has been studied by means of solute (Cu-63) nuclear spin relaxation (NSR) experiments in the rotating frame between 290 K and 110 K. Surprisingly, two distinct diffusion-induced maxima in the relaxation rate could be observed. The high-temperature mechanism around 950 K corresponds to the translational diffusion of Cu in the solid solution phase, as is suggested by comparison with tracer diffusion data. The second relaxation mechanism occurs at fairly low temperatures (about 700K) with a maximum intensity around 8 at% Cu. The origin of this contribution to the diffusional NSR rate is not quite clear. We attribute it to a short-range ordering effect of Cu atoms in the Au matrix. The activation enthalpies of both types of atomic depend only slightly on the Cu-concentration.