Grain boundary self-diffusion of Ni-63 in pure and boron-doped Ni3Al

Grain boundary (gb) self-diffusion in pure Ni-rich Ni3Al was measured between 882 and 1374 K using the radiotracer Ni-63, a Serial sectioning technique and sensitive liquid scintillation counting. The results of the gb diffusivity P = delta D-gb (delta:gb width, D-gb:gb diffusion coefficient) can be represented by the Arrhenius parameters P-0 =-3.27 . 10(-13) m(3)/s and Q(gb) = 168 kJ/mol. Additionally gb diffusion was investigated in boron-doped (0.24 at%) Ni-rich Ni3Al in the range from 882 to 1352 K yielding P-0 = 1.24 . 10(-12) m(3)/s and Q(gb) = 187 kJ/mol. The increase in the activation enthalpy Q(gb) and the decrease of P upon boron-doping is explained by the 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. For comparison gb seif-diffusion in pure Ni was remeasured yielding Q(gb) = 112 kJ/mol. Ordering of the lattice and the preservation of ordering up to the gb planes, as predicted in Ni3Al, therefore has a pronounced decelerating influence on gb diffusion, stronger than on bulk diffusion. Applying the semi-empirical relation of Borisov et al. (Phys. Met. Metallogr., 17 (1964) 80) 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. (C) 1996 Elsevier Science Limited