Effect of solutionizing temperature on the microstructural evolution during double aging of powder bed fusion-additive manufactured IN718 alloy

Quantitative microstructural analysis was carried out to determine the size and volume fraction of strengthening phases: gamma double prime (γ″) and gamma prime (γ′); in additively manufactured (AM) Inconel 718 (IN718) with an emphasis on the utilization of Nb for γ″ formation during aging treatment. Powder bed fusion AM (PBF-AM) technique was used to generate coupons using indigenously synthesized IN718 alloy powder. Microstructural studies using transmission electron microscope (TEM) and small angle x-ray scattering (SAXS) were carried out on powder bed fusion-additive manufactured (PBF-AM) coupons in as-built condition (as processed) and in three different heat-treated conditions: solutionizing at three different temperatures (980, 1030 and 1080 °C) followed by double aging treatment. As-built microstructure consists of dendritic structure accompanied with Nb segregation along the interdendritic regions resulting in the formation of a Nb rich interdendritic phases dispersed within the interdendritic Nb-rich γ matrix. The inhomogeneous Nb distribution resulted in preferential precipitation of coarse δ during solutionization at 980 °C and further coarse γ″during direct aging and aging subsequent to solutionizing at 1030 °C. As-cast structure and related elemental segregation got modified during solutionization at 1080 °C and resulted homogeneous γ″ precipitation. Though the average size of γ″precipitates remained same in all aged samples, volume fraction increased with increasing solutionization temperature. SAXS studies were carried out on PBF-AM as well as conventionally produced wrought samples with the similar heat treatment conditions and confirmed that PBF-AM built IN718 aged samples solutionized at 1030 °C and 1080 °C resulted in γ″ precipitates with average precipitate size and volume fraction similar to that of the wrought-IN718 sample solutionized at 1080 °C.