Effects of aluminum and molybdenum content on the microstructure and properties of multi-component γ′-strengthened cobalt-base superalloys

Five novel Co-30Ni-(6+x)Al-(3-y)W-yMo-1Ta-3.5Ti-12Cr-0.1Hf-0.025 C (x = 0, y = 0, 1, and x = 2, y = 0, 1, 2 at%) multi-component polycrystalline γ′-strengthened Co-base superalloys are designed to investigate the effects of Al and Mo content on the microstructure and properties in this study. Based on the 6Al3W alloy, the addition of 2 at% Al has the effects of increasing the γ′ solvus temperature, reducing the density, increasing the γ′ volume fraction and coarsening rate, reducing the Vickers hardness but increasing the yield strength at 750 ℃. The replacement of 1 at% W by 1 at% Mo remarkably reduced the density with a negligible effect on the γ′ coarsening rate; Meanwhile, this slightly reduced the γ′ solvus temperature and volume fraction causing a slight decrease of Vickers hardness and yield strength at 750 ℃. The experimental alloys all show relatively low density, large processing window, good microstructure stability at 750 ℃ and mechanical properties, especially for 6Al2W1Mo alloy which has density of 8.69 g cm−3, processing window of 247 ℃, room-temperature Vickers hardness of 5.1 GPa, stable microstructure without detrimental phase after a heat treatment of 900 ℃ @8 h+ 750 ℃ @1000 h. These results may provide guidance for the development of the novel wrought Co-base superalloys with high performance and low density.