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 approximately 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 temperature 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 x 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.