Abstract:Nickel-based superalloys for heavy-duty gas turbines usually have a high Cr content, but the high Cr content makes it difficult to optimize the composition design of the alloy. In particular, in order to avoid the precipitation of harmful topologically close-packed (TCP) phases, the content of solution-strengthening elements W and Mo is limited. In this work, the effects of W and Mo content changes on the γ/γ′ two-phase state and TCP phase precipitation of nickel-based directional superalloy DZ409 for gas turbines aged at 900 °C for 1000 h were studied by multi-component diffusion multi-junction technique. The results show that when the Mo content remains unchanged, the volume fraction of the γ′ phase decreases slightly as the W content increases from 3.8wt% to 4.3wt%, the size of the γ′ phase decreases, and its morphology remains spherical. When the W content exceeds 4.3wt%, σ and P phases begin to precipitate in the alloy. When the Mo content increases from 1.4wt% to 1.6wt%, and the W content decreases from 4.0wt% to 3.3wt%, the volume fraction of the γ′ phase increases slightly, the size of the γ′ phase decreases, and the morphology remains square. After the Mo content exceeds 1.6wt%, the σ phase and P phase are precipitated in the alloy. According to the APT tip reconstruction diagram and the ion distribution map of each major element, it can be seen that the increase in W content will promote the precipitation of TCP phase, and the addition of Mo while reducing W content will also promote the precipitation of TCP phase of the alloy, mainly because the enrichment of W, Cr, and other elements in the γ matrix makes the total amount of refractory elements in the γ phase exceed the solid solution limit of γ matrix.