Solute diffusion and segregation in grain boundaries of silver and copper

Grain boundary (GB) solute diffusion of Se and Ni in Ag polycrystals was investigated by the radiotracer serial sectioning technique using Se-75 and Ni-63 isotopes. Measurements in the temperature ranges 877 to 371 K for Se and 989 to 589 It for Ni were carried out in Harrison's type B-regime at high temperature and in type C-regime at low temperature. In B-regime, the product s delta D-GB was determined (s being the solute GB segregation factor, delta the GB width, D-GB the GB diffusion coefficient)! while in C-regime D-GB values were measured directly. The D-GB values of Se are comparatively close to GB self-diffusion, whereas the GB diffusion activation enthalpy H-GB(Se) is larger than H-GB(Ag) of GB self-diffusion. This GB diffusion behaviour is explained together with our previous solute GB diffusion measurements in type-C regime (Te/Ag, Se/Cu and Au/Cu) in terms of formation of "embryos" of two-dimensional 2D phases. Within this argumentation also the influence of spurious impurities on recent GB self-diffusion measurements in nominally high purity Cu polycrystals is explained. The properties (surface tension, compounds in the phase diagram, lattice diffusion) of the system Ni/Ag are quite different from those of Se/Ag. Comparing the D-GB values of Ni with GB self-diffusion in Ag, a drastically decrease of the Ni atom mobility and an increase of H-GB(Ni) of Ni in Ag GBs is observed. This is discussed in terms of repulsive Ni - vacancy interactions and reduced jump frequencies of the Ni atoms which are mainly located at low energy positions in the GB. Combining the obtained S delta D-GB and D-GB values and assuming delta = 0.5 nm, the GB segregation factors and enthalpies of Ni (H-s(Ni) = -39.7 kJ/mol) and Se (H-s(Se) = -21.4 kJ/mol) were evaluated.