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:

sδD (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.

C.Minkwitz, C.Herzig, B.Straumal, W.Gust: Materials Science Forum, 1999, 294-296, 541-4