Microstructure-property relationships in directly aged Alloy 718 turbine disks

Direct ageing (DA) of forged Alloy 718 turbine disks enables the design of more efficient aircraft engines due to high-temperature yield strength increments of around 10%. This 'DA effect' is related to the dislocation density, 8-phase content and nanoscale gamma'- and gamma ''-precipitate morphology. However, in real turbine disks, local differences in the thermo-mechanical history often deteriorate the DA effect below customers specifications. Thus, the aim of this paper is to unravel the complex microstructural evolution in low-versus high-yield strength regions. We use hardness mapping, macro-etching, thermo-kinetic and FEM modelling to identify the regions of interest. EBSD, TEM and atom probe microscopy (APM) enable micro- and nanostructure characterization. Low-hardness regions exhibit larger grains, lower 6-phase contents, and lower geometrically necessary dislocation densities. Nanoscale gamma'- and gamma ''-precipitates are coarser, with lower number density, and more complex coprecipitate morphology. We assign the origins of these changes to local temperature and strain conditions during forging, and inhomogeneities in pre-materials. A proposed microstructural model explains the underlying mechanisms. Local strain-induced delta-phase dissolution results in recovery, recrystallisation and grain growth during forging, reducing the number of nucleation sites for precipitation. Thus, the DA effect deteriorates due to accelerated gamma ''-precipitate coarsening and less uniform particle dispersions.