Development of the γ′ Stability in Co–Al–W Alloys at 800 °C by Alloying with Carbon

The microstructures and hardnesses of Co–10Al–9W–1C, Co–7Al–5W–1C, and Co–7Al–5W (at. pct) alloys are reported. Homogenization of the Co–10Al–9W–1C alloy was unsuccessful at 1300 °C and both B2–CoAl and η carbide phases remained in the interdendritic regions. However, the lower-solute Co–7Al–5W–1C and Co–7Al–5W alloys did homogenize at 1300 °C. Upon aging the Co–10Al–9W–1C alloy at 800 °C, the undissolved η carbide transformed into W-supersaturated D0 19 –Co 3 W phase and subsequently into γ ′-Co 3 (Al,W) phase, indicating the stability of the γ ′ phase in the C-doped alloy in contrast with ternary Co–Al–W alloys where the γ ′ is metastable. Also, high γ ′ volume fractions and low γ ′ coarsening rates were revealed by transmission electron microscopy in the various Co–Al–W–C alloys. These observations are explained by the effects of C on the phase equilibria and the γ / γ ′ composition in the Co–Al–W system. Finally, the microhardness is increased while the density is decreased by alloying with C, which could result in higher specific strengths in the C-doped alloys.