Stability of nanoscale co-precipitates in a superalloy: A combined first-principles and atom probe tomography study

Inconel 718 is a nickel-iron based superalloy widely used in the aerospace industry. Its high temperature strength is attributed to the thermal stability of dense nanoscale precipitates γ^' [Ni₃Al] and γ^'' [Ni₃Nb]. There is experimental evidence that γ^' and γ^'' often form co-precipitates γ^'∕γ^'' or sandwichlike structure γ^'∕γ^''∕γ^' or γ^''∕γ^'∕γ^''. But how they stabilize under heat treatment or in service is still not well-understood. We have investigated the interfacial structure and chemistry of fine co-precipitates Ni₃(Al,Ti,Nb)∕Ni₃Nb(γ^'∕γ^'') in Inconel 718, using both first-principles density functional theory calculation and the three-dimensional atom probe technique. Our calculations confirm that Al atoms in the γ^' phase segregate to the γ^'∕γ^'' interface. The enrichment of Al helps to impede the assimilation of Nb from γ^' to γ^'' and reject Al from γ^'' to γ^', and therefore keeps such secondary precipitates at fine size. In the absence of Ti in the γ^' phase, our calculations predict an enhanced driving force for Al to accumulate at the interface. We have also characterized the microstructure of the γ^'∕γ^'' interface for an Inconel 718 sample taken from a commercial compressor blade serviced in an airplane engine for over 100.2em0ex0000.3em0exh at a temperature up to 6000.2em0ex^∘C using three-dimensional atom probe analysis. Interestingly, we find that Al enrichment sustains long-term service, suggesting that the coarsening of secondary precipitates is interface-controlled. The success of first-principles density functional theory computation in reproducing the experimental observation encourages extensive application of this powerful tool in the study of precipitates in superalloys.