Abstract: Superalloys feature multi-elements and complex elemental ranges, which makes the proper composition selection difficult. In fact, more strict composition standards generally apply in practical productions. The objective of this paper is to understand and eventually to renew the composition standard via example of the most common grade Inconel 718. We have recently shown that the alloy chemistry originates from a nearest-neighbor cluster [center-shell] plus a few next-neighbor glue atoms, or expressed in cluster formula [center-shell] (glue atoms). By grouping the elements into $\overline{Ni}$= (Ni, Co, Cu, Fe), $\overline{Cr}$= (Cr, Mn, Si, Mo), and $\overline{Nb}$= (Nb, Al, Ti), it is found that the reported alloys fall within a narrow composition zone $\overline{Ni}$_11.0-13.0-$\overline{Cr}$=_3.5-4.5-$\overline{Nb}$=₁ confined by cluster formulas of 16 and 18 atoms. This composition zone is also expressed in terms of 288-atom supercluster formulas, $\overline{Ni}$=_198-208-$\overline{Cr}$=_63-72-$\overline{Nb}$=_16-18, which leads to coordinated elemental variations in wt.%: 69.0 ≤ Ni+ Co + Cu + Fe ≤ 72.7; 19.8 ≤ Cr + Mn + 1.7 Si + 0.6 Mo ≤ 22.8; 8.7 ≤ Nb +3.2 Al + 1.9 Ti ≤ 9.8. Within this composition zone, $\overline{Ni}$₂₀₆-$\overline{Cr}$_65.5-$\overline{Nb}$_16.5 is further pinpointed and validated by our own experiments to possess the optimal match of strength and plasticity both at room and at 923 K.

Key words: Inconel 718 superalloy, Cluster-plus-glue-atom model, Cluster formula, Composition standard