Nickel and selenium grain boundary solute diffusion and segregation in silver

For the first time, the grain boundary (GB) solute diffusion of Ni and Se in Ag polycrystals was investigated systematically by the radiotracer serial sectioning technique using Ni-63 and Se-75 isotopes. Measurements in the temperature ranges 589-989 K for Ni and 371-877 K for Se were carried out in Harrison's type B regime at high temperatures and in type C regime at low temperatures. In the 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 the C-regime D-GB values were measured directly. The solid solubility of both solutes, Ni and Se, in Ag is very small. The equilibrium phase diagrams, however, are quite different: Se forms an intermetallic compound whereas Ni does not form any compound with Ag, which indicates dominating Se-Ag and Ni-Ni bonds, respectively. Comparing the solute D-GB values with C regime measurements of GB self-diffusion in Ag, a drastic decrease of the Ni diffusivity and an increased activation enthalpy of Ni diffusion, H-GB(Ni), in Ag GBs is observed. This behaviour is discussed in terms of repulsive Ni-vacancy interactions and decreased jump frequencies of the Ni atoms which are mainly located at low energy positions in the GB. The measured D-GB values of Se, on the other hand, are comparatively close to GB self-diffusion, whereas H-GB(Se) is again larger than H-GB(Ag) of GB self-diffusion. This behaviour is explained in terms of the formation of "embryos" of two-dimensional phases at high energy GB sites. Combining the obtained s delta D-GB and D-GB values and assuming delta = 0.5 nm, the GB segregation factors and segregation enthalpies of Ni (H-s(Ni) = -39.7 kJ/mol) and Se (H-s(Se) = -21.4 kJ/mol) in Ag were evaluated. (C) 1998 Acta Metallurgica Inc.