Grain-boundary and interphase-boundary diffusion was investigated in the Ag/Ni system by using the radiotracer serial-sectioning technique. The Ni segregation in Ag grain boundaries was strongly temperature-dependent, and led to the conclusion that Ni atoms were located mainly in the bulk planes adjacent to the grain boundary. From interphase boundary-diffusion measurements in (110)-oriented Ag/Ni interphase boundaries, it was deduced that the structure of this interphase boundary was incoherent. The Arrhenius relationships for 63Ni grain boundary diffusion in Ag polycrystals (figure 1) were:

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

D (m2/s) = 3.1 x 10-6 exp[-110.1(kJ/mol)/RT]

By comparing these results, the segregation factor was deduced to be:

s = 1.2 exp[40.9(kJ/mol)/RT]

The characteristic features of the results were this strong segregation, and the slow grain boundary diffusion of Ni in Ag as compared with Ag grain boundary self-diffusion. These features were attributed to the high surface tension of Ni, which was markedly higher than that of Ag, and to a repulsive vacancy-Ni solute interaction. It was concluded that Ni did not segregate in the grain boundary core, but in the adjacent planes of the bulk. Radiotracer Diffusion of Ni and Ag in Ag and Ni Grain Boundaries and Oriented Ag/Ni Interphase Boundaries. C.Minkwitz, C.Herzig, B.Straumal, W.Gust: Materials Science Forum, 1999, 294-296, 541-4

Figure 1

Grain-Boundary Diffusivity of Ni in Ag