Nowadays, the trend goes to better and more efficient gas turbine engines with lower emissions, greater durability and lower cycle costs. To this end, new materials such as Allvac 718PlusTM, should enhance the high temperature performance. The appearance, morphology and control of the δ-phase are of special interest because of their critical influence on grain structure, grain size and mechanical properties. In this work, the evolution and the morphology of the δ-phase during heat treatment between 900 and 1000°C for annealing times of up to 8 h are investigated in order to determine the time – temperature – precipitation diagram and to understand the phase morphology. For this, different analysis methods are applied, such as light microscopy analysis with computer-aided quantitative metallography, dual beam focused ion beam and both scanning electron and transmission electron microscopy.
The nickel-based superalloy Allvac 718Plus was developed in order to combine formability and cost advantages of Alloy 718 with high temperature strength of Alloy 720. Typical applications for this new material are turbine disks that are produced by closed die forging, e. g. by screw pressing. The final mechanical properties are strongly related to the microstructure, which forms during hot forming. It is therefore of great interest to model both the dynamic and static grain structure development during processing. In this work, dynamic and static recrystallization as well as normal grain growth models are described for this material. The calculated grain structure evolution in Allvac 718Plus is compared to that of Alloy 718 in the typical process range of disk forging. Additionally, both the experimental program and structure analysis methodology are specified.
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