A new Ni-base superalloy VDM-780 Premium was developed for higher service temperatures (above 650°C), keeping the good processing characteristics of alloy 718. This article presents, for the first time, the morphology and the microstructure characterization of this newly developed superalloy VDM-780 by means of scanning electron microscopy (SEM) and neutron and X-ray diffraction (XRD), after three different aging treatments performed for setting up different microstructures. Results show the presence of the c-matrix, c¢-hardening phase, and a high-temperature phase whose structure is compatible with d and g phases but whose exact crystal structure or the possibility of two different high-temperature phases still remains open. Rietveld refinements have allowed phase identification, determination of the lattice constants, and the weight fractions of constituting phases and shown that the presence of the different phases, amount, and morphology highly depend on the aging treatments. No traces of the c¢¢ phase are observed regardless of the heat treatment. In situ neutron diffraction (ND) studies at high temperature have allowed the determination of the solvus temperatures of the different phases present in each material after the corresponding aging treatment as well as the study of the evolution of their lattice parameters with temperature. The Vickers hardness (HV) of the three different samples was measured, and the results are correlated with the amount and particle size of the c¢-hardening phase of each sample.
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.
Abstract. Alloy718 has been used for many years in the aerospace industry due to its unique mechanical properties and good processing characteristics, especially its workability. However, the temperature limit of Alloy718 is about 650• C because of the thermal instability of the main strengthening phase γ -Ni 3 (Nb,Ti,Al). Numerous attempts have been made to develop a new wrought 718-type alloy for high temperature applications. The approach was to increase the stability, i.e. the solvus temperature of the γ -phase (T γ ,s ). However, this affected workability as the solvus temperature of the δ-phase (T δ,s ) did not increase accordingly so that the window for fine grain forging T δ,s -T γ ,s became smaller. In this paper the development of a new γ /γ -alloy on the basis of Alloy718 is presented, where the microstructure is stable at 800• C, mechanical properties are similar to Alloy718, yet do not deteriorate beyond 650• C, and the forging window is wider than the one of Alloy718, allowing for good workability. This was essentially achieved by the addition of about 17%-30% Co in combination with an Al/Ti-ratio of more than 5.0 and an Al-content of about 1.6%-2.2%. The key role of cobalt is to stabilize the δ-phase, allowing for solvus temperatures in excess of 1100• C. Consequently, the stability of the γ -phase can be increased by further addition of aluminium. At the same time the Ti-content is reduced to prevent formation of the η-(Ni,Co) 3 (Ti,Al,Nb) phase. Besides discussion of the alloy development concept, information on microstructure evolution and mechanical properties will be given.
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