Soft solution state and hard aging state of Inconel 718 reached via optimized precipitation elements using cluster formula approach

Abstract

The superior high-temperature performance of Inconel 718 is originated mainly from the precipitation strengthening via Nb, Al, and Ti alloying. Inappropriate contents of the precipitation elements may lead to over-strengthened solution state and insufficiently strengthened aging state. We have previously identified the alloy composition formula [(Nb,Al,Ti)1-(Ni,Fe,Cr,Mo)12](Ni,Fe,Cr,Mo)5 using the cluster formula approach, where the 18-atom structural unit is composed of a nearest-neighbor cluster, centered by one precipitation elements, plus five glue atoms situated at the next neighbors. On the basis of this formula, the present work designs a series of alloys using (Nb,Al,Ti) = 0.93, 1, and 1.06, with 4.9 and 5.5 wt % Nb, 0.68 and 0.5 wt % Al, and 1.10 wt% Ti corresponding to the upper-to-middle ranges in the ASTM standard. In particular, when the total number of atoms in the formula is equal to one, (Nb0.6Al0.2Ti0.2), the alloy is soft at the solution state (σUTS = 825 MPa < 965 MPa of the standard and EL = 70.3 % > 30 %) and reaches high strength levels after aging (σUTS = 1400 MPa > 1240 MPa and EL = 23.4 % > 12 % at room temperature, σUTS = 1100 MPa > 965 MPa and EL = 10.2 % > 5 % at 923 K). This alloy also shows the lowest coarsening rate of 4–7 × 10−3 nm3 s−1 after long-term aging at 923 K, lower than the prevailing coarsening rate of about 16 × 10−3 nm3 s−1.