The grain boundary solute diffusion of Ni in Ag polycrystals was investigated systematically, for the first time, by using the radio-tracer serial-sectioning technique

with the 63Ni isotope (tables 9 and 10). The diffusion of Ni was measured at temperatures ranging from 589 to 989K. 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 Ni was very small, and it did not form an intermetallic compound; indicating the predominance of Ni-Ni bonds. Upon comparing the grain-boundary diffusivities with C-regime measurements, a sharp decrease in the Ni diffusivity and an increased activation enthalpy for Ni diffusion in the grain boundaries was observed. This behavior was explained in terms of repulsive Ni-vacancy interactions, and decreased jump frequencies of the Ni atoms; which were located mainly at low-energy positions in the grain boundaries. The grain boundary diffusivity of 63Ni could be described by:

sδD (m3/s) = 1.9 x 10-15 exp[-69.2(kJ/mol)/RT]

D (cm2/s) = 3.1 x 10-6 exp[-110.1(kJ/mol)/RT]

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

Table 10

Grain-Boundary Diffusion (Type-C Regime) of 63Ni in Ag