  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.
