On the Development Concept for a New 718-Type Superalloy with Improved Temperature Capability

Abstract

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.

Near-Surface and Bulk Dissolution Behavior of γ′ Precipitates in Nickel-Based VDM® Alloy 780 Studied with In-Situ Lab-Source and Synchrotron X-ray Diffraction

“…Furthermore, the coherency strain also influences the coarsening behavior of the precipitates and thus the long-term stability of the microstructure and the mechanical strength of the material at elevated temperature [12][13][14]. In addition to the γ/γ -microstructure, micro-sized high-temperature stable phases (δ, Ni 3 Nb-based, DO a or η, Ni 3 Ti-based, DO 24 or carbides, A1) are present in these alloys to impede grain growth during the forging process (δ or η) or as a by-product of the processing history (carbides) [15][16][17].…”

“…In recent decades, many efforts have been made to develop new polycrystalline Ni-based alloys with operating temperatures above 650 • C, while retaining the good mechanical and processing characteristics of alloy 718 [20][21][22]. One very promising candidate is the new VDM ® Alloy 780 [23,24]. The most important difference in this new alloy, as compared with alloy 718, is the altered chemical composition, where Fe is replaced by Co and there is a higher Al-content in combination with a lower Ti-content [24].…”

“…One very promising candidate is the new VDM ® Alloy 780 [23,24]. The most important difference in this new alloy, as compared with alloy 718, is the altered chemical composition, where Fe is replaced by Co and there is a higher Al-content in combination with a lower Ti-content [24].…”

Effect of Co and Cr on the Stability of Strengthening Phases in Nickelbase Superalloys

“…Usually, formation energies of the precipitation phases are calculated with respect to the energy of either the pure materials (e. g. Al in the case of γ ) or with respect to the solution energy of the element under consideration in a nickel matrix. However, wrought alloys like VDM Alloy 780 [8] may contain a high amount of Co and Cr. The reference energy for the formation of one of the precipitate phases should thus be the energy of the element under consideration in a matrix that is rich in Co and Cr.…”

Deformation Mechanisms in Ni-Based Superalloys at Room and Elevated Temperatures Studied by In Situ Neutron Diffraction and Electron Microscopy

“…Many Metals 2021, 11, 719 2 of 17 efforts have been put into the research of new polycrystalline Ni-based alloys with operating temperatures above 650 • C, while keeping the advantageous processing characteristics of Alloy 718 during the last few decades [6,7]. One very promising candidate is the newly developed VDM ® Alloy 780 (VDM 780) [8,9]. The most important differences between these two alloys are essentially the replacement of Fe by Co and a higher Al-content in combination with a lower Ti-content in VDM 780.…”