γʹ and γ″ co-precipitation phenomena in directly aged Alloy 718 with high δ-phase fractions

• Published in: Materials & design Vol. 241; p. 112961
• Main Authors: Rielli, Vitor V., Farabi, Ehsan, Godor, Flora, Gruber, Christian, Stanojevic, Aleksandar, Oberwinkler, Bernd, Primig, Sophie
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.05.2024; Elsevier
• Subjects: Atom probe tomography; Direct ageing; Precipitation; Superalloy; Superalloy; Direct ageing; Precipitation; Atom probe tomography
• ISSN: 0264-1275; 1873-4197
• DOI: 10.1016/j.matdes.2024.112961

[Display omitted] •Deformation at 950 °C favours γʹ monolith formation; 1000 °C reduces δ-phase, increases Nb, promoting γ′′ monolith formation.•Deformation at 950 °C improves hardness by 12 % after direct ageing, due to complex γ′ and γ′′ co-precipitate structure.•Deformation at 1000 °C leads to up to 19 % γ′′ precipitation after direct ageing, with superior direct ageing response.•A δ annealing before forging to strengthen Alloy 718 is recommended. Co-precipitation of γ′ and γ′′ is the main strengthening mechanism that provides superior high-temperature strength in directly aged Alloy 718 aerospace parts. Control of their morphology, fraction, and configuration might allow exposure to more demanding operation environments in next-generation aircraft engines. The density of geometrically necessary dislocations introduced during hot deformation has been shown to significantly affect the co-precipitate morphology of γ′ and γ′′ in materials free of the δ-phase. However, the combined effects of geometrically necessary dislocation density and lower Nb content due to higher δ-phase fractions on co-precipitation behaviour and strengthening remain unknown. We verify these effects by hardness testing as a proxy for high-temperature strength in materials with 4.1 % δ-phase fraction. Deformation at 950 °C yields a remarkable increase of 12 % in hardness after direct ageing, explained by the prevalence of complex co-precipitate configurations. Deformation at 1000 °C decreases the δ-phase fraction and geometrically necessary dislocation density but achieves up to 19 % volume fractions of γ″, leading to a predominance of monoliths and duplet co-precipitates and a better direct ageing response. Atom probe microscopy reveals the flux of elements during co-precipitation. We recommend a δ-annealing treatment before the final forging step for manufacturing stronger Alloy 718 aerospace parts.