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

Abstract

Grain boundary (gb) self-diffusion in pure Ni-rich Ni3Al was measured between 882 and 1374 K using the radiotracer 63Ni, a serial sectioning technique and sensitive liquid scintillation counting. The results of the gb diffusivity P = δDgb (δ : gb width, Dgb : gb diffusion coefficient) can be represented by the Arrhenius parameters P0 = 3.27 · 1013and Qgb = 168 kJmol. Additionally gb diffusion was investigated in boron-doped (0.24 at%) Ni-rich Ni3Al in the range from 882 to 1352 K yielding P0 = 1.24 · 10−12 m3/s and Qgb = 187 kJmol. The increase in the activation enthalpy Qgb 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 self-diffusion in pure Ni was remeasured yielding Qgb = 112 kJmol. 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 γgb were determined for arbitrary high angle gbs in pure and B-doped Ni3Al, resulting in 915 and 870 mJ/m2, respectively, at 1100 K.