Effects of processing heterogeneities on the micro- to nanostructure strengthening mechanisms of an alloy 718 turbine disk

Abstract

Advancements in thermo-mechanical processing of Ni-based superalloys enable manufacturing of engineering parts with superior mechanical properties. Forged aeroengine disks exhibit radial microstructural heterogeneities from both pre-materials and prior processing. Here we correlate microstructural variations in an industrially forged alloy 718 aeroengine disks to their processing history via finite element simulations. We examine a disk intentionally manufactured to exhibit hardness variations. Several radial positions are characterized to reveal twin boundaries and δ phase fractions prior to ageing and grain size, geometrically necessary dislocation (GND) densities, chemical compositions, γ′ and γ′′ precipitates in both as-forged and direct aged conditions. We find that localized meta-dynamic recrystallization causes accelerated grain growth in low hardness regions, while highly strained areas accelerate precipitation of strengthening phases. In these areas, the direct ageing effect is weakened by the presence of non-shearable coarsened γ′′ precipitates. Overall, direct ageing is shown to successfully mitigate radial property heterogeneities. We underpin this by a strengthening model elucidating the direct ageing effect on the reduction of hardness heterogeneity. Variations in grain boundary strengthening, work hardening, and solid solution strengthening after direct ageing have low impact on hardness variations, where precipitation strengthening provides as much as 44% of the strength after direct ageing.

Keywords

• Superalloy, forging
• Direct ageing
• Recrystallization
• Strengthening