The grain boundary solute diffusion of Se in Ag polycrystals was investigated systematically, for the first time, by using the radio-tracer serial-sectioning technique with the 75Se isotope (tables 11 and 12). The diffusion of Se was measured at temperatures ranging from 371 to 877K. The measurements were performed in the Harrison type-B regime at high temperatures and in the type-C regime at low temperatures. In the type-B regime, the product of solute grain-boundary segregation factor, grain-boundary width, and grain-boundary diffusion coefficient, was determined. In the C-type regime, grain-boundary diffusivity values were measured directly. The solid solubility of Se was very small, and Se formed an intermetallic compound; indicating the predominance of Se-Ag bonds. The measured grain-boundary diffusivity values for Se were comparatively close to the values for grain-boundary self-diffusion, but the activation enthalpy was higher for Se grain-boundary diffusion than for Ag grain-boundary diffusion. This behavior was explained in terms of the formation of so-called embryos of 2-dimensional phases at high-energy grain-boundary sites. The results could be described by:

sδD (m3/s) = 3.0 x 10-14 exp[-63.5(kJ/mol)/RT]

D (m2/s) = 4.1 x 10-4 exp[-84.8(kJ/mol)/RT]

T.Surholt, C.Minkwitz, C.Herzig: Acta Materialia, 1998, 46[6], 1849-59

Table 12

Grain-Boundary Diffusion (Type-C Regime) of 75Se in Ag