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

Abstract

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 model for the microstructural evolution. It is shown that fine γ'-and γ"-precipitates nucleate homogeneously and coarsen according to the Lifshitz-Slyozov-Wagner-theory. However, γ"-precipitates also nucleate heterogeneously on dislocations and experience 3 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.

On Strong-Scaling and Open-Source Tools for High-Throughput Quantification of Material Point Cloud Data: Composition Gradients, Microstructural Object Reconstruction, and Spatial Correlations

“…Next, two sets of high-throughput studies were executed: One for characterizing γ and another one for characterizing γ precipitates. Previous high-resolution transmission electron microscopy studies [136] on a similar material and processing conditions supports our assumption to restrict the analyses in this work on γ and γ as the most important precipitate types. We assume that closed objects segmented from niobium iso-composition surfaces (ϕ N b ) represent γ precipitates while closed objects segmented from aluminium plus titanium iso-composition surfaces (ϕ Al+T i ) are γ precipitates.…”

Toward a Better Understanding of Phase Transformations in Additive Manufacturing of Alloy 718

“…However, during the precipitation and growth of ' and ″ , the local chemical composition near the vicinity of the precipitate could change due to difference in solubility of elements in these phases. This could affect the nucleation and growth of secondary precipitates of ' and ″ [56] . As seen in Fig.…”

Effect of Deformation Parameters of an Initial Aged GH4169 Superalloy on Its Microstructural Evolution during a New Two-Stage Annealing

“…In general, the large second-phase particles can impede the movement of dislocation, thereby promoting the formation of high-density dislocation cells. These dislocation cells accelerate SRX nucleation, which is also called particle stimulation nucleation (PSN) [ 51 , 52 ]. Therefore, the dispersed large δ phase was conducive to recrystallization nucleation during CRT.…”