The temperature dependence of grain-boundary self-diffusion in polycrystals was investigated using 64Cu radiotracer and serial sectioning techniques. Two different high-purity Cu materials were used: 5N8 and 5N. The difference in impurity content was checked using time-of-flight secondary ion mass spectrometry. The two materials were studied at 720 to 1066K (5N9) or 784 to 973K (5N). A significant dependence of grain-boundary diffusion upon material purity was observed, resulting in differing Arrhenius behaviours of the diffusivity, δDGB, where δ was the grain boundary width and DGB was the diffusion coefficient:

5N8:     D(m3/s) = 3.89 x 10-16exp[-72.47(kJ/mol)/RT]

5N:     D(m3/s) = 1.16 x 10-15exp[-84.75(kJ/mol)/RT]

The differences in the diffusion coefficients and activation enthalpies were explained in terms of strong impurity-Cu atom bonds in the boundary. In the present investigation, sulphur was the predominant impurity. Using a semi-empirical relationship, the grain boudary energy, γGB, of high-angle boundaries and its temperature dependence were deduced from the self-diffusion data. The resultant value, γGB = 650mJ/m2 at 1000K, was in good agreement with other energy measurements and with model calculations for Cu.

Grain Boundary Self-Diffusion in Cu Polycrystals of Different Purity. T.Surholt, C.Herzig: Acta Materialia, 1997, 45[9], 3817-23