Effects of processing heterogeneities on the micro- to nanostructure strengthening mechanisms of an alloy 718 turbine disk. (15th December 2021)

• Record Type: Journal Article
• Authors: Rielli, Vitor Vieira; Godor, Flora; Gruber, Christian; Stanojevic, Aleksandar; Oberwinkler, Bernd; Primig, Sophie
• Is Part Of: Materials & design. Volume 212(2021)
• Publication Date: 2021-12-15
• Subjects: Superalloy, forging -- Direct ageing -- Recrystallization -- Strengthening; Materials -- Periodicals; Engineering design -- Periodicals; Matériaux -- Périodiques; Conception technique -- Périodiques; Electronic journals; 620.11
• DOI: 10.1016/j.matdes.2021.110295
• Languages: English
• ISSNs: 0264-1275

Abstract

Graphical abstract: Highlights: Direct ageing mitigates excessive hardness heterogeneities in Alloy 718. Local meta -dynamic recrystallization generates larger grains and lower hardness. γ′′ precipitation during air cooling in strained areas leads to higher coherency strengthening. Coarsening of precipitates during ageing reduces strengthening effects by coherency loss. Modelling of strengthening factors and processing parameters provides data for designing stronger industrially forged aerospace parts. 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.