Evolution of nanoscale precipitates during common Alloy 718 ageing treatments

[Display omitted] •Larger amount of γ′′ during early stages of direct ageing leads to higher hardness.•No clusters of Al or Nb are formed in the bulk of the material prior to ageing.•Rejection of Nb from γ′ drives the formation of γ′′-γ′-γ′′ in conventional ageing.•γ′′ precipitation on dislocations occurs faster than predicted by CCT diagrams.•Pipe diffusion has low efficiency in forming compositionally stable precipitates. Improving mechanical properties of the Ni-based superalloy Alloy 718, commonly used in turbine discs, is of vital importance to commercial aviation. The need for higher efficiency will push materials to the limit. Modifications in the processing routes of this alloy can be more easily implemented in industry than changes in the alloying elements. Variations in ageing treatment or cooling rate from hot forging can produce a wide variety of mechanical properties. These are mostly caused by changes in the nanoscale precipitates responsible for strengthening. The effects of different common ageing treatments such as conventional and direct ageing on clustering, size, volume fraction, morphology, configuration, and chemical composition of γ′-and γ′′-precipitates are still largely unknown. We investigate these aspects of the precipitates with atom probe microscopy and correlate them with hardness. The flux of the main chemical elements in the formation of co-precipitates such as duplets and triplets is remarkably different in conventionally and directly aged materials. Dislocation rich structures also inflict in unexpected rapid formation of γ′′-precipitates at temperatures more than 90 °C higher than predicted by time–temperature-transformation diagrams. Water quenching from forging impedes precipitates formed along dislocations to coarsen during ageing, considerably impacting hardness, in comparison to air cooling.