Precipitation kinetics and morphology evolution of the Co3(Al, W) phase in a medium supersaturation Co-Al-W alloy

The effects of aging temperature on the precipitation of the gamma '-Co-3(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 gamma ' phase in the medium supersaturation Co-9Al-8W (at.%) alloy from early precipitation to later coarsening. The results showed that the gamma ' 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 gamma ' 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 gamma ' 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 gamma ' 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 gamma ' phase in a medium supersaturation Co-Al-W alloy.