Effects of Thermo-Mechanical Processing on the Mechanical Properties and the Nanoscale Precipitates in a Ni-Based Superalloy

Ni-based superalloys are critical to manufacture aeroengine parts. Alloy 718 is one the main representatives of this type of material due to its excellent combination of properties, including elevated strength up to 650°C. Changes in the thermo-mechanical processing routes affect micro- to nanoscale strengthening factors. Direct ageing, for instance, generates different configurations of γ′ and γ′′ precipitates when compared to conventional ageing, contributing to a 10% increment in yield strength [1]. Other processing conditions, such as cooling rate from forging also impact the microstructure, where air cooling enables accelerated formation of γ" precipitates along dislocation channels [2]. However, a systematic approach to compare differences between ageing treatments, and processing heterogeneities within industrially forged materials was lacking.<br/>Atom probe microscopy, as a sub-nanometre resolution characterization technique, that provides a plethora of information regarding precipitates and atomic clustering [3]. By systematically examining the evolution of the nanoscale precipitates and hardness during conventional versus direct ageing [4], we determine that highly strained Alloy 718 enables faster γ" precipitation at temperatures higher than predicted by existing time-temperature-transformation diagrams. We also provide a thorough microstructure-properties-processing analysis in multiple regions of a forged turbine disk at varying length scales, allowing future developments in modelling and microstructural engineering.<br/>[1] F. Theska, A. Stanojevic, B. Oberwinkler, S.P. Ringer, S. Primig, Acta Mater. 156 (2018) 116–124.<br/>[2] F. Theska, K. Nomoto, F. Godor, B. Oberwinkler, A. Stanojevic, S.P. Ringer, S. Primig, Acta Mater. 188 (2020) 492–503.<br/>[3] B. Gault, M.P. Moody, J.M. Cairney, S.P. Ringer, Atom Probe Microscopy, Springer Science & Business Media, 2012.<br/>[4] V.V. Rielli, F. Theska, F. Godor, A. Stanojevic, B. Oberwinkler, S. Primig, Mater. Des. 205 (2021) 109762.