Multi-phase-field simulation of D019-χ transformation in Co-Al-W superalloy with L12-γ′ + fcc-γ phases

The L12-γ′-Co3(Al, W) phase in Co-based superalloys substantially improves the creep properties and oxidation resistance. However, the cubic γ′ phase in Co-Al-W superalloy becomes unstable at high temperature and transforms into the strip-shaped D019-χ-Co3W phase with prolonged aging. The transformation mechanism and evolution kinetics of the χ phase in Co-8.25Al-10 W (at.%) superalloy with γ + γ' phases are studied by establishing the multi-phase-filed model with the sublattice free energies of fcc-L12 and hcp-D019 phases. Aged at 1173 K, the γ' phase transforms into χ phase through in-situ transformation by superlattice intrinsic stacking faults (SISF), the strip-shaped χ phase grows in direction of [0001]χ[111]γ and forms an angle of 60° with the alignment orientations [100]γ′ and [010]γ′ of γ′ phase. The strip-shaped χ phase causes the stress concentration and the large difference of diffusion potential, promoting the continuous transformation of the γ' phase to χ phase, the Ostwald ripening of γ′ phase happens simultaneously at the regions with low diffusion potential. The chemical potential shows a dominated effect on driving the transformation of γ′ phase to χ phase. The study reveals the mechanisms and driving force of multi-phase transformation from fcc to D019 structure with the stress change in these phases.