Development of High-Performance Co-Al-W Alloys by Alloying with Carbon

The microstructures and hardnesses of Co-10Al-9W-1C, Co-7Al-5W-1C and Co-7Al-5W (at %) 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 D019-Co3W phase and subsequently into γ'-Co3(Al,W) phase, consistent with the higher stability of the γ' phase in the C-doped alloy. 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 effect of C on the phase equilibria and γ/γ' interfacial energy 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.