The intergranular segregation of boron in Ni3Al: Equilibrium segregation and segregation kinetics

The grain boundary B content of high-purity Ni-24 at.% Al alloys containing 0.048, 0.144, 0.240 and 0.480 at.% B (100, 300, 500, 1000 ppm mass) has been determined for samples aged from 1323 to 873 K for sufficient times to attain equilibrium. The B content was derived from Auger electron spectra of the intergranular fracture facets. Many facets were exposed during fracture at ≈ 300 K, and additional facets were formed upon fracturing following hydrogen charging after heat treatment. For each alloy sample, about 25 facets were analyzed. The grain boundary B contents were in the range of 0.5–2.5 at.%. The grain boundary B content increased with decreasing temperature and with increasing bulk B content in the alloys. The energy of binding of a B atom to the grain boundary was calculated using McLean's segregation theory and assuming a unique binding energy for each alloy. The values were in the range of 0.15–0.45 eV/atom, and increased with increasing temperature and with decreasing bulk B content. These results have been rationalized in terms of a spectrum of binding energies for a given alloy. However, when the entropy of adsorption was taken into account, an enthalpy of adsorption of B to the grain boundary of 0.13 eV/atom was obtained, independent of temperatire and bulk B content. This is interpreted to mean that the spectrum of binding energies is quite restricted. The grain boundary B content of these alloys has also been measured as a function of annealing time at 773, 873, 973 and 1173 K. The diffusion coefficient of B in Ni₃Al at 773 K is about 5 × 10⁻²¹ m²/s, and the equilibrium grain boundary B content is attained at about 3000 s. The diffusion coefficient at 973 K is between 10⁻¹⁶ and 10⁻¹⁷ m²/s. The activation energy for diffusion of B in Ni₃Al is between 200,000 and 300,000 J/mol.