Phase-field simulation of early-stage kinetics evolution of γ′ phase in medium supersaturation Co-Al-W alloy

The early precipitation of gamma'-Co-3 (Al, W) phase affects the spatial distribution and kinetic evolution of precipitates for the morphology transmission effect, but the nucleation and concomitant growth are not studied still by aging experiments due to the expeditious precipitation of gamma' phase in Co-Al-W alloy. By using the phase-field simulation with sublattice free energy, the early-stage kinetics evolution of gamma'-Co-3 (Al, W) phase in a medium supersaturation Co-9Al-8W (at.%) alloy aged from 1023 K to 1173 K is investigated. The influences of aging temperature on the evolution of morphology and composition of gamma' phase, and the kinetics of nucleation and growth to coarsening are clarified. It is found that the rates of composition evolution of W in gamma phase are two or three times that of gamma' phase, and the W compositions in gamma and gamma' phases show a linear relationship with time t(1/3), which means that the coarsening takes place earlier at high temperature. In addition, the equilibrium partitioning ratios indicate Al and W partition into the gamma' phase and the ratios decrease with elevated temperature. The compositional variations across the gamma/gamma' phase interfaces suggest that low aging temperature makes the stoichiometric ratio closer to 3:1. Moreover, the precipitation evolutions from early nucleation to growth and coarsening in Co-Al-W alloy are distinguished, and the rate constants of square and cube of average particles radius increase with temperature. In later growth stage, the relationship of the square of average particles radius and time is obeyed, while the steady-state coarsening stage follows the cube law. The time exponents of particles number density at the coarsening stage are close to -1 of Kuehmann-Voorhees (KV) theory. The study demonstrates that the early-stage evolution of gamma' phase which is undiscovered in the experiment can be captured by the phase-field simulation, and the resultant kinetics laws agree well with the experimental and theoretical results. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.