Correlation effects in grain boundary diffusion

Abstract

The grain boundary self-diffusion coefficient in silver has been measured using the sectioning technique, for polycrystals, 〈100〉 16° tilt bicrystals, and 〈112〉 18° tilt bicrystals over the temperature ranges 350–555°C, 400–555°C, and 350–498°C, respectively. The temperature dependence of D'δ from the Suzuoka solution is given by D'δ = 8.55 × 10 −10 exp (−17800/ RT)cm 3/sec for the polycrystals and D'δ = 3.1 × 10 −11 expt (− 11800/ RT) cm 3/sec for the 〈100〉 tilt bicrystals. The activation energy for the 〈112〉 tilt boundaries is significantly larger than that of the polycrystal or 〈100〉 tilt boundaries. The simultaneous diffusion of 105Ag and 110Ag in the grain boundaries of the polycrystals from 350–555°C and 〈100〉 tilt bicrystals at 400 and 498°C allowed for measurements of the isotope effect for grain boundary self-diffusion. The isotope effect results prove that there is very little isotopic separation during grain boundary diffusion. Within the experimental accuracies, the isotope effects for the random boundaries of the polycrystals and the 〈100〉 tilt boundaries are identical. The average value of f Δ K for the polycrystals is 0.46. This is significantly lower than the low-temperature lattice, self-diffusion isotope effect in silver. A vacancy model has been developed from which the correlation factor for diffusion along dislocations can be calculated. The isotope-effect measurements can be interpreted in terms of the modelistic calculations and many-body saddle point interactions. The results are consistent with a three-dimensional interpretation of grain boundary diffusion and a vacancy-dislocation binding energy that decrease at 0.05 to O.10 eV per atomic plane from the dislocation core. The silver isotope effect for grain boundary diffusion in polycrystalline copper at 466°C is 0.18. This value is consistent with a highly correlated vacancy mechanism.