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 ${\gamma }^{\prime }$ $\left[{\mathrm{Ni}}_3\mathrm{Al}\right]$ and ${\gamma }^{″}$ $\left[{\mathrm{Ni}}_3\mathrm{Nb}\right]$. There is experimental evidence that ${\gamma }^{\prime }$ and ${\gamma }^{″}$ often form co-precipitates ${\gamma }^{\prime }∕{\gamma }^{″}$ or sandwichlike structure ${\gamma }^{\prime }∕{\gamma }^{″}∕{\gamma }^{\prime }$ or ${\gamma }^{″}∕{\gamma }^{\prime }∕{\gamma }^{″}$. 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 ${\mathrm{Ni}}_3(\mathrm{Al},\mathrm{Ti},\mathrm{Nb})∕{\mathrm{Ni}}_3\mathrm{Nb}({\gamma }^{\prime }∕{\gamma }^{″})$ 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 ${\gamma }^{\prime }$ phase segregate to the ${\gamma }^{\prime }∕{\gamma }^{″}$ interface. The enrichment of Al helps to impede the assimilation of Nb from ${\gamma }^{\prime }$ to ${\gamma }^{″}$ and reject Al from ${\gamma }^{″}$ to ${\gamma }^{\prime }$, and therefore keeps such secondary precipitates at fine size. In the absence of Ti in the ${\gamma }^{\prime }$ phase, our calculations predict an enhanced driving force for Al to accumulate at the interface. We have also characterized the microstructure of the ${\gamma }^{\prime }∕{\gamma }^{″}$ interface for an Inconel 718 sample taken from a commercial compressor blade serviced in an airplane engine for over $10000\mathrm{h}$ at a temperature up to $600°\mathrm{C}$ 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.

• Received 9 August 2007

DOI:https://doi.org/10.1103/PhysRevB.76.224102