On the early stages of precipitation during direct ageing of Alloy 718

• Published in: Acta materialia Vol. 188; pp. 492 - 503
• Main Authors: Theska, F., Nomoto, K., Godor, F., Oberwinkler, B., Stanojevic, A., Ringer, S.P., Primig, S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.04.2020
• Subjects: Atom probe tomography (APT); High-resolution transmission electron microscopy (HRTEM); Nanostructures; Precipitation; Superalloy; High-resolution transmission electron microscopy (HRTEM); Superalloy; Precipitation; Nanostructures; Atom probe tomography (APT)
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2020.02.034

The Ni-based superalloy Alloy 718 is used in aircraft engines as high-pressure turbine discs and must endure challenging demands on high-temperature yield strength, creep-, and oxidation-resistance. Nanoscale γ′- and γ″-precipitates commonly found in duplet and triplet co-precipitate morphologies provide high-temperature strength under these harsh operating conditions. ‘Direct ageing’ of Alloy 718 is an attractive alternative manufacturing route known to increase the yield strength at 650 °C by at least +10%, by both retaining high dislocation densities and changing the nanoscale co-precipitate morphology. However, the detailed nucleation and growth mechanisms of the duplet and triplet co-precipitate morphologies of γ′ and γ″ during the direct ageing process remain unknown. We provide a correlative high-resolution microscopy approach using transmission electron microscopy, high-angle annular dark-field imaging, and atom probe microscopy to reveal the early stages of precipitation during direct ageing of Alloy 718. Quantitative stereological analyses of the γ′- and γ″-precipitate dispersions as well as their chemical compositions have allowed us to propose a qualitative model of the microstructural evolution. It is shown that fine γ′- and γ″-precipitates nucleate homogeneously and grow coherently. However, γ″-precipitates also nucleate heterogeneously on dislocations and experience accelerated growth due to Nb pipe diffusion. Moreover, the co-precipitation reactions are largely influenced by solute availability and the potential for enrichment of Nb and rejection of Al+Ti. [Display omitted]