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

Co-precipitation of gamma ' and gamma '' 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 gamma ' and gamma '' in materials free of the delta-phase. However, the combined effects of geometrically necessary dislocation density and lower Nb content due to higher delta-phase fractions on coprecipitation behaviour and strengthening remain unknown. We verify these effects by hardness testing as a proxy for high-temperature strength in materials with 4.1 % delta-phase fraction. Deformation at 950 degrees C yields a remarkable increase of 12 % in hardness after direct ageing, explained by the prevalence of complex coprecipitate configurations. Deformation at 1000 degrees C decreases the delta-phase fraction and geometrically necessary dislocation density but achieves up to 19 % volume fractions of gamma '', 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 delta-annealing treatment before the final forging step for manufacturing stronger Alloy 718 aerospace parts.