A quantitative model for the intergranular precipitation of M7X3 and M23,X6 in Ni-16Cr-9Fe-C-B

Abstract

A thermodynamic model has been constructed to explain the details of grain boundary chemistry in Ni-16Cr-9Fe doped with carbon and boron. In this model, Gibbs free energy of the precipitates is calculated using an ideal solution model for both the metallic and non-metallic sub-lattices in M23X6 and M7X3, and the activities of the five species in the matrix are calculated using a Kohler equation formalism involving only binary terms. The parameters required by the model are based solely on data on the constituent binary and ternary subsystems. The model yields correct results for the compositions of intergranular, Ni + B-rich M23X6 (M&80at.%Ni, X [approximate] 100at.% B) as well as Cr + C-rich M7C-rich M7C3 at 873 K and 973 K. The result that M23X6, is rich in Ni and B only occurs when the chemical activities in the matrix are those for a grain boundary in equilibrium with Cr7, C3, which implies that Ni + B-rich M23X6 develops as a consequence of intergranular Cr7,C3 and the resultant depletion of Cr at the grain boundary. The model does not successfully explain the precipitation of B rich M23X6 and M2,X3 at 1173 or 1273 K.