Influence of phase composition and microstructure on the high temperature creep properties of a model single crystal nickel-base superalloy: An atom probe/AEM study

Abstract

Atom probe field ion microscopy (APFIM) and analytical electron microscopy (AEM) have been used to characterize the microstructure of as-quenched and creep-tested model single crystal nickel-base superalloys having the nominal bulk composition Ni- x at.% Mo-2.0% Ta-13.7% Al ( x = 7.8–9.2%). The results showed that Ta partitioned preferentially to the γ′ phase whereas Mo partitioned to the γ matrix. Atom probe composition profiles across γ—γ′ interfaces did not reveal any evidence of solute segregation and statistical analyses of the data did not indicate ultrafine solute clusters in the γ matrix. Acicular δ phase precipitates were detected in number densities approximately one order of magnitude higher in the 9.2% Mo alloy as compared to the 8.6 and 8.8% Mo alloy. There was some indication that the DO 22-ordered γ″ phase is stable at 982°C, the temperature at which the creep tests were performed. The absence of interfacial solute segregation and clustering in the γ matrix suggested that the high creep life of this model alloy system is influenced by the nature of the dislocation network at the γ—γ′ interface with a possible strengthening contribution from the DO 22-ordered phase.