Orientation dependence of Au-195 and Cu-64 diffusion along symmetric [001] tilt grain boundaries in Cu

Under identical experimental conditions the grain boundary (gb) diffusion product P=s delta D-gb of Au-195 in Cu along a series of 12 different symmetric [001] tilt boundaries, exactly characterized by the Kossel-technique, with tilt angles near to the ideal Sigma = 5 Theta = 36.9 degrees (310) CSL-boundary (Delta Theta = 6 degrees) was measured as a function of temperature and tilt angle. In the temperature range of (0.48-0.78)T-m (Cu) the orientation dependence P(Theta) shows a characteristic cusp not exactly at but slightly below the ideal Sigma=5 CSL-gb for all investigated temperatures. The reason for this apparent result is discussed. The activation enthalpy Q(gb) of Au gb diffusion shows a strong orientation dependence at sufficiently low temperatures. With increasing temperature a negative deviation from a straight Arrhenius behaviour is observed and the orientation dependence of Q(gb) disappears. This result indicates to a temperature induced change in the gb structure.