The grain boundary solute diffusion of Ni and Se in Ag polycrystals was investigated systematically, for the first time, by using the radio-tracer serial-sectioning technique with 63Ni and 75Se isotopes (tables 1 to 5). The diffusion of Ni was measured at temperatures ranging from 589 to 989K, and the diffusion of Se was measured at temperatures ranging from 371 to 877K. The measurements were performed in Harrison's 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 both Ni and Se was very small, but the phase diagrams were quite different. That is, Se formed an intermetallic compound whereas Ni did not; indicating the predominance of Se-Ag and Ni-Ni bonds, respectively. 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 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.

Nickel and Selenium Grain Boundary Solute Diffusion and Segregation in Silver T.Surholt, C.Minkwitz, C.Herzig: Acta Materialia, 1998, 46[6], 1849-59

Table 1

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

Table 2

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

Table 3

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

Table 4

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

Table 5

Arrhenius Parameters for Grain-Boundary Diffusion in Ag