To improve the high-temperature capability of 718-type wrought nickel-base superalloys, the γ-phase (Ni 3 Al) can be stabilized. However, this also reduces the size of the forging window because forging has to be done above the γ-and below the solvus temperature of the phase that is used to enable fine-grain forging, i. e. the δ-phase of Ni 3 Nb type or the η-phase of Ni 3 Ti-type. Understanding the influence of alloying elements on the formation of these phases is therefore important. In this paper, density functional theory calculations at 0K are performed to determine the stabilizing effect of aluminium and of the transition group elements on the stability of the δ-phase and η-phase. Most of the transition group elements of 5th and 6th period stabilize the δ-phase, whereas the stabilizing effect on the η-phase is weaker. According to the calculations, Mo, Tc, W, Re, and Os may be expected to stabilize the δ-phase but not the η-phase, whereas Al and Zn strongly stabilize the η-phase.

The superalloy 718 stands out for its excellent manufacturability and strength at ambient temperature. However, its application temperature is limited to about 650 °C due to the instability of the γ’’ precipitates. Here, we provide an in-depth account of an alloy development concept, allowing for the design of superalloys with 718-type properties, yet with a significantly improved microstructural stability. The article begins with a detailed discussion on how the microstructural and chemical composition must be altered to achieve this objective. Then, model alloys were used to explore and validate the outlined strategy. Finally, it is shown how these considerations ultimately led to a new 718-type superalloy with far more improved microstructural stability— namely, VDM Alloy 780. The introduction of a large amount of Co as a substitute for Fe (and partially Ni) is the most important element of our alloy development concept in terms of chemical composition. The most important microstructural feature is the introduction of low solvus temperature, high misfit γ´-strengthening, replacing γ´´-hardening.

No abstract