Temporal evolution of a model Co-Al-W superalloy aged at 650 °C and 750 °C

The temporal evolution of a γ(f.c.c.)/γ’ (L1₂) Co-8.8Al-7.3 W superalloy aged at 650 °C (10 min–4096 h) and 750 °C (10 min–256 h) is studied utilizing atom-probe tomography (APT), scanning electron microscopy, and Vickers microhardness testing. The evolution of the phase compositions, γ’ (L1₂) volume fraction, and mean precipitate radius, <R_2-D(t)> are determined. Coarsening rate constants and temporal exponents are calculated for <R_-D(t)> of the γ’ (L1₂)-nanoprecipitates. The temporal exponents are found to be generally close to 1/p = 1/3 as required for diffusion-limited coarsening. Tungsten solid-solubility is significantly reduced in the γ(f.c.c.)-matrix at 650 °C (0.54 ± 0.04 at. %) and 750 °C (1.35 ± 0.06 at. %) when compared with aging at 900 °C (5.5 at. %). The value of <R_2D(t)> of the γ’ (L1₂)-nanoprecipitates increases with increasing aging time corresponding to an increase in the Vickers microhardness; the peak strength was not, however, achieved for the aging times investigated. The morphology of the γ’ (L1₂)-nanoprecipitates begins as spheroids but transitions to cuboids at longer aging times, with final the γ’ (L1₂) volume fractions for aging at 650 °C and 750 °C being ϕ = 53% and 54%, respectively. The effect of quench-rate (either furnace-cooled, air-cooled, oil quenched, or water quenched) from a supersolvus temperature of 1050 °C on the microstructure of the alloy is also investigated. Slow cooling (furnace and air-cooling) is shown to result in a uniform distribution of nanometer sized γ’ (L1₂)-nanoprecipitates, unlike Ni-based superalloys in which the γ’ (L1₂)-nanoprecipitates form in a non-uniform or multimodal distribution.