Microstructure-property relationships in directly aged Alloy 718 turbine disks

摘要

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 γ‘- and Y"-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 8-phase contents, and lower geometrically necessary dislocation densities. Nanoscale γ’- and γ"-precipitates are coarser, with lower number density, and more complex copre-cipitate 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 8-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 -γ"-precipitate coarsening and less uniform particle dispersions.