Evolution of nanoscale precipitates during common Alloy 718 ageing treatments

Abstract

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.

Keywords

• Superalloy
• Atom probe tomography (APT)
• Direct ageing
• Conventional ageing
• Pipe diffusion
• Early stages of precipitation