Temperature and concentration dependence of grain boundary diffusion of Ni-63 in pure and boron-doped Ni3Al

Grain boundary (gb) self-diffusion in pure and boron-doped Ni3Al was measured using the radiotracer Ni-63 a Serial sectioning technique and sensitive liquid scintillation counting. For comparison grain boundary self-diffusion in pure Ni was reinvestigated in type B-kinetics. It is shown that the temperature dependence of the grain boundary diffusion parameter P = delta D-gb of Ni-63 in pure Ni and in both Ni3Al materials is of the Arrhenius type. The absolute values of P follow the sequence P(Ni) > P(Ni3Al) > P(Ni3Al + B). Ordering of the lattice and the preservation of order up to the gb planes, as predicted in Ni3Al, therefore has a pronounced decelerating influence on gb diffusion. Ni-diffusivity in the doped alloy is about 2-3 times lower than in pure Ni3Al, attributed to the strong segregation of B in Ni3Al gbs, which may lead to an increase in the vacancy formation enthalpy and to a blocking of energetically favourable diffusion paths in the gbs. Applying the semi-empirical relation of Borisov et al. gb energies gamma(gb) were determined for arbitrary high angle gbs in pure and B-doped Ni3Al, resulting in 915 and 870 mJ/m(2), respectively, at 1100 K. The effect of stoichiometry on grain boundary self-diffusion in Ni3AI was investigated for compositions between 73 and 78 at% Ni. A V-shaped concentration dependence of P with a minimum near 75 at% Ni was observed. This behaviour is related to the change in composition and state of order of grain boundaries in non-stoichiometric Ni3Al.