The temperature dependence of grain boundary self-diffusion in polycrystals was investigated systematically by using a 64Cu radiotracer and the serial sectioning technique. Two different high-purity materials were used: 5N8-Cu and 5N-Cu. The difference in impurity content was checked by using time-of-flight secondary-ion mass spectrometry. The materials were studied at temperatures ranging from 1066 to 720K (5N8-Cu) or from 973 to 784K (5N-Cu). A significant dependence of the grain boundary self-diffusion upon material purity was observed (table 41). This resulted in a different Arrhenius expression for each material:

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

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

The differences in grain boundary diffusion coefficient and activation enthalpy were explained in terms of strong impurity-Cu atom bonding in the boundary. In the present investigation, S was the dominant impurity. By using a semi-empirical relationship, the grain boundary energy of large-angle boundaries, and its temperature dependence, was determined from the self-diffusion data. The resultant value of the grain boundary energy was 650mJ/m2 at 1000K, and was in good agreement with grain-boundary energy measurements and model calculations.

T.Surholt, C.Herzig: Acta Materialia, 1997, 45[9], 3817-23

Table 42

Diffusivity of 64Cu in Cu at 919K along [001] Tilt Grain Boundaries

near to Σ = 5, 36.9º (310)[001]