Self-diffusion in pure Ni-rich samples was measured at temperatures of between 882 and 1374K by using 63Ni radiotracer, serial sectioning and liquid scintillation counting techniques. It was found that the product of grain-boundary diffusivity and grain boundary width was described by:

D (m3/s) = 3.27 x 1013exp[-168(kJ/mol)/RT]

Grain boundary diffusion was also investigated in B-doped (0.24at%) Ni-rich samples at temperatures ranging from 882 to 1352K. These results could be described by:

D (m3/s) = 1.24 x 1012exp[-187(kJ/mol)/RT]

The increase in activation enthalpy, and the decrease in the pre-exponential product of grain-boundary diffusivity and grain boundary width, as a result of B-doping was attributed to the segregation of B in the Ni3Al grain boundaries. This was expected to lead to an increase in the vacancy formation enthalpy and to a blocking of energetically favorable diffusion paths in the boundaries. For the purposes of comparison, the grain boundary self-diffusivity in pure Ni was measured and was found to be governed by an activation enthalpy of 112kJ/mol. It was concluded that ordering of the lattice, and the preservation of ordering right up to the grain boundary planes, had a marked suppressing effect upon grain boundary diffusion. In fact, the effect was stronger than that on bulk diffusion. By using the semi-empirical Borisov relationship, the large-angle grain boundary energies in pure and B-doped Ni3Al were estimated to be 915 and 870mJ/m2, respectively, at 1100K.

S.Frank, J.Rusing: Intermetallics, 1996, 4[8], 601-11

Table 167

Bulk Diffusivity of 63Ni in B-Doped Ni3Al