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 gamma' [Ni3Al] and gamma '' [Ni3Nb]. There is experimental evidence that gamma' and gamma '' often form co-precipitates gamma'/gamma '' or sandwichlike structure gamma'/gamma ''/gamma' or gamma ''/gamma'/gamma ''. 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 Ni-3(Al, Ti, Nb)/Ni3Nb(gamma'/gamma '') 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 gamma' phase segregate to the gamma'/gamma '' interface. The enrichment of Al helps to impede the assimilation of Nb from gamma' to gamma '' and reject Al from gamma '' to gamma', and therefore keeps such secondary precipitates at fine size. In the absence of Ti in the gamma' phase, our calculations predict an enhanced driving force for Al to accumulate at the interface. We have also characterized the microstructure of the gamma'/gamma '' interface for an Inconel 718 sample taken from a commercial compressor blade serviced in an airplane engine for over 10 000 h at a temperature up to 600 degrees 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.