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تسجيل مستخدم جديدOn the early stages of precipitation during direct ageing of Alloy 718
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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 $gamma^{prime}$- and $gamma^{prime prime}$-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 $^{deg}$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 $gamma^{prime}$ and $gamma^{prime prime}$ 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 $gamma^{prime}$- and $gamma^{prime prime}$-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 $gamma^{prime}$- and $gamma^{prime prime}$-precipitates nucleate homogeneously and grow coherently. However, $gamma^{prime prime}$-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.
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