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

TL;DR

This study investigates the early precipitation mechanisms of gamma prime and gamma double prime phases in Alloy 718 during direct ageing, revealing nucleation sites, growth behaviors, and microstructural evolution using advanced microscopy techniques.

Contribution

It provides the first detailed microstructural model of early precipitation stages in Alloy 718 during direct ageing, combining high-resolution microscopy and quantitative analysis.

Findings

Gamma prime and gamma double prime precipitates nucleate homogeneously and coherently.

Gamma double prime also nucleates heterogeneously on dislocations with accelerated growth.

Precipitation is influenced by solute availability and Nb enrichment.

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 $\gamma^{\prime}$- 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.