Precipitation kinetics and morphology evolution of the Co(Al, W) phase in a medium supersaturation Co–Al–W alloy

The effects of aging temperature on the precipitation of the γ′-Co(Al, W) phase were studied by utilizing transmission electron microscopy, scanning electron microscopy and atom probe tomography, with the aim of exploring the continuum kinetics of the γ′ phase in the medium supersaturation Co–9Al–8W (at.%) alloy from early precipitation to later coarsening. The results showed that the γ′ phases with small size and large number density caused a rapid increase in alloy microhardness at 1073 K when the aging time was less than 136 h, implying that precipitation strengthening clearly occurred. Due to the high thermodynamic driving force and small critical radius of nucleation, the γ′ phases expeditiously precipitated within 0.08 h of aging. Additionally, the partitioning coefficients of the elements showed that the partitions of Al and W in the γ′ phase were more obvious at 1073 K. The time exponents of the average particle radius were close to the value of 1/3 predicted by the Lifshitz–Slyozov–Wagner theory. The particle size distribution of the γ′ phase was more consistent with the Brailsford–Wynblatt model in the early growth-dominated stage, whereas it matched well with the trans-interface diffusion-controlled model in the later concomitant growth and coarsening stages. This research revealed the intimate relationship among the morphology, mechanical properties and evolution kinetics of the γ′ phase in a medium supersaturation Co–Al–W alloy.