γ’-(L1₂) precipitate evolution during isothermal aging of a Co–Al–W–Ni superalloy

The coarsening kinetics and elemental partitioning behavior of γ’-(L1₂) precipitates in a γ(f.c.c.)-matrix for a model quaternary Co-8.8Al-8.9W-9.9Ni at.% superalloy are investigated utilizing isothermal aging conditions at 650, 800 and 900 °C. The γ′-precipitate mean radius, number density, and volume fraction, at 800 and 900 °C, were studied using scanning electron microscopy; the calculated temporal exponents associated with coarsening of γ′-precipitates display good agreement with model predictions for quasi-stationary coarsening. An atom probe tomographic (APT) investigation of the aged γ/γ′ microstructure at 650 °C demonstrates that the compositions and volume fractions of both phases vary continuously up to 4096 h. The aged microstructure at 650 °C consists of interconnected nanoscale γ′-precipitates, corroborated utilizing SEM for the 4096 h aged-specimen. The activation energy for coarsening is estimated for the temperature range 650–900 °C to be 283 kJ mol⁻¹, in reasonable agreement with activation energies for diffusion of Al, W, and Ni in Co, suggesting that coarsening of γ′-precipitates is limited by bulk-diffusion. APT measurements of specimens aged for 1024 h at 800 and 900 °C demonstrate that the isothermal aging temperature has a significant effect on the compositions and partitioning behavior of Co, Al, W and Ni between the γ- and γ′-phases. The partitioning ratio of the concentrations between the γ′- and γ-phases is largest for W, decreasing linearly from 5.3 ± 0.1 at 650 °C to 2.1 ± 1.2 at 900 °C, and smallest for Co, decreasing from 0.86 ± 0.01 at 650 °C to 0.73 ± 0.01 at 900 °C.