High-resolution characterisation of strengthening effects in nickel-based superalloy Alloy 718

More than 37,000 new aircraft are required by 2037 as air traffic has been predicted to double every two decades. Improved fuel efficiency is crucial for next generation aircraft engines to extend aircraft range and speed. Aircraft engine turbine discs are commonly made of Alloy 718, a polycrystalline, precipitation-strengthened nickel-based superalloy. High-temperature strength, creep resistance and corrosion resistance allow operational temperatures of 650 °C for service cycles of several 100,000 h under high mechanical workloads.Conventional processing of Alloy 718 turbine discs involves multi-step forging, solution annealing, and dual ageing. Direct ageing is an alternative process to achieve yield strength increments of > 10 % by omitting solution annealing. However, the detailed processing-microstructure-property relationships during direct ageing of Alloy 718 are not fully understood.The microstructure of Alloy 718 contains a γ- (FCC) matrix with a grain size of ASTM 10 for grain boundary and solid solution strengthening. Micronscale δ- (Ni3(Nb), D0a) phase platelets decorate grain- and twin-boundaries for optimal creep strength. High dislocation densities in the grain interior provide work hardening. Nanoscale γ'- (Ni3(Al,Ti), L12) and γ''- (Ni3(Nb),D022) precipitates in duplet and triplet morphology provide the high-temperature strength of Alloy 718.This study provides a correlative high-resolution microscopy approach to reveal the microstructural evolution in Alloy 718. Comparing conventional versus direct ageing demonstrates that high retained dislocation densities and Nb concentrations in solid solution yield inverted precipitate morphologies. Atom probe microscopy techniques such as the enhanced iso-surface method and the interface method are developed to quantify γ'- and γ''-precipitate volume fractions, particle radii, number densities, and co-precipitate morphologies. Slices of forged turbine disc materials are characterized to clarify the occurrence