Effect of Al and W Contents on the Solidification and Solution Microstructure of Novel γ/γ′ Cobalt‐Base Superalloys

The solidification and heat treatment characteristics of superalloys are key to engineering and practical applications. The solidification and solid solution behavior of γ/γ′ cobalt‐base superalloys Co–30Ni–xAl–(15 − x)W–5Cr–1Ta–4Ti (x = 7, 9, 11 abbreviated as 7Al8W, 9Al6W, and 11Al4W, respectively) are studied using directional solidification experiments. The results indicate that there are Laves phase, (γ + γ′)_e eutectics, and (β + γ′)_e eutectics in the interdendritic regions for the 7Al8W alloy. The solidification sequence of the 7Al8W alloy is L → L₁ + γ → L₂ + γ + Laves → L₃ + γ + Laves + (β + γ′)_e →L₄ + γ + Laves + (β + γ′)_e + (γ + γ′)_e → γ + Laves + (β + γ′)_e + (γ + γ′)_e. As the Al content increases and the W content decreases, the Laves phase fraction in the alloy decreases and the (β + γ′)_e eutectics fraction increases. The Laves phase completely disappears in the 11Al4W alloy. An unremovable μ phase forms in the 7Al8W alloy during solution treatment. The 9Al6W alloy and 11Al4W alloy can exhibit a γ single‐phase microstructure after single‐stage solution treatment (1250 °C for 1 h) and two‐stage solution treatment (1210 °C for 1 h + 1275 °C for 1 h), respectively. These experimental results are helpful for composition optimization, solidification, and solution treatment parameter determination of novel γ/γ′ cobalt‐base superalloys.