Eta Phase Precipitation at Equilibrium and in Strain Free ATI 718Plus

Abstract

As aerospace engines advance to obtain higher thermal efficiencies, it is imperative to develop high temperature materials. Inconel 718 is a nickel-based superalloy that has been used for decades in aero-engine parts as it allows for use in high temperature applications. ATI 718Plus is a newer nickel-based superalloy that has been developed with a 55°C higher temperature capability over Inconel 718. ATI 718Plus components are manufactured by forging a wrought billet in stages to obtain the desired geometry and microstructure. Parts are heat treated to optimised proportions of γ’ and η phases. η phase is an acicular phase that precipitates on the grain boundaries, whereas γ’ is the primary strengthening phase. η phase is an important phase to understand as it is utilised in controlling the grain size during hot working processes at temperatures below its solvus temperature. When η phase is fully solutioned, the grain size is free to increase and hence the material mechanical properties can become detrimental. The short-term precipitation kinetics of η phase in strain-free ATI 718Plus is still not completely understood. In this study, the aims and objectives were to study the η precipitation kinetics in strain-free material as well as studying η phase precipitation in equilibrium conditions. TTT diagrams were produced for the η phase in strain-free material and compared to the limited data available in the open literature. In addition, the equilibrium η phase content, aspect ratio, length and width were determined and compared to the very little data that is currently published.