期刊
ACTA MATERIALIA
卷 188, 期 -, 页码 492-503出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.actamat.2020.02.034
关键词
Superalloy; Nanostructures; Precipitation; Atom probe tomography (APT); High-resolution transmission electron microscopy (HRTEM)
资金
- Australian Research Council via her DECRA Fellowship program [DE180100440]
- UNSW Scientia Fellowship Scheme
- Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation programme [714043]
- H2020 Societal Challenges Programme [714043] Funding Source: H2020 Societal Challenges Programme
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 gamma'-and gamma ''-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 degrees 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 gamma' and gamma '' 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 gamma'- and gamma ''-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 gamma'- and gamma ''-precipitates nucleate homogeneously and grow coherently. However, gamma ''-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. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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