Grain boundary self-diffusion in pure and boron-doped Ni3Al was measured using 63Ni, serial sectioning and sensitive liquid scintillation counting (figure 27). For comparison, grain boundary self-diffusion in pure Ni was re-investigated in type B-kinetics. It was shown that the temperature dependence of the grain-boundary diffusion parameter P = δDgb of 63Ni in pure Ni and in both Ni3Al materials was of Arrhenius type. The absolute values of P followed the sequence, P(Ni) > P(Ni3Al) > P(Ni3Al+B). Ordering of the lattice and the preservation of order up to the grain-boundary planes, as predicted in Ni3Al, therefore had a marked decelerating effect upon grain-boundary diffusion. The Ni diffusivity in the doped alloy was some 2 to 3 times lower than in pure Ni3Al. This was attributed to the strong segregation of B in Ni3Al grain-boundaries, which could lead to an increase in the vacancy formation enthalpy and to a blocking of energetically favourable diffusion paths in the grain-boundaries. By applying the semi-empirical relationship of Borisov et al. grain-boundary energies γgb were determined for arbitrary high-angle grain-boundaries in pure and B-doped Ni3Al; giving 915 and 870mJ/m2, respectively, at 1100K. The effect of stoichiometry upon grain boundary self-diffusion in Ni3Al was investigated for compositions between 73 and 78at%Ni. A V-shaped concentration dependence of P, with a minimum near to 75at%Ni, was observed. This behaviour was related to the change in composition and state of order of grain boundaries in non-stoichiometric Ni3Al.

Temperature and Concentration Dependence of Grain Boundary Diffusion of 63Ni in Pure and Boron-Doped Ni3Al. Frank, S., Rüsing, J., Herzig, C.: Defect and Diffusion Forum, 1997, 143-147, 1441-6

Table 27

Grain-boundary diffusivity of 63Ni in Ni3Al

continued

Table 27 (continued)

Grain-boundary diffusivity of 63Ni in Ni3Al