Atomic motion of copper in solid Au:xCu alloys studied by nuclear magnetic resonance

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 (63Cu) nuclear spin relaxation (NSR) experiments in the rotating frame (T1p) between ambient temperature and 1100 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 ( approximately=700 K) with a maximum intensity around 8 at.% Cu. The origin of this contribution to the diffusional NSR rate is not quite clear. The authors attribute it to a short-range ordering effect of Cu atoms in the Au matrix. The activation enthalpies of both types of atomic motion depend only slightly on the Cu concentration.