Prediction of Sigma-Phase Formation in Nickel-Base Superalloys

Abstract

Experimental determinations of γ-matrix compositions in nickel-base superalloys have shown that small amounts of aluminium may be retained in solid solution after γ′ precipitation. Average electron-vacancy concentrations and critical electron-vacancy concentrations have been calculated for the residual γ-matrix compositions of 30 commercial nickel-base superalloy heats. The γ-matrix compositions were determined experimentally or by a computational procedure. Critical electron-vacancy concentrations were calculated by two methods in which aluminium is assumed to act as (a) a γ-stabilizer or (b) a sigma-promoter. Predictions of γ-phase stability based on these calculations are compared with experimental observations. Optimistic predictions of γ stability are obtained when aluminium is treated as a γ-stabilizer, while good correlation between observed and calculated stability is obtained by the alternative approach. The validity of this latter approach is discussed in terms of the known effects of aluminium on σ-phase stability and a third alternative approach is suggested.