Mechanische Eigenschaften, Rissfortschrittsverhalten und Langzeitstabilität der polykristallinen Nickelbasis-Superlegierung Allvac 718Plus

The operating temperature and thus the efficiency of jet engines are significantly affected by the temperature capability of turbine disc alloys. Hence one objective of this theses was to characterize in detail the thermal stability of the polycrystalline γ‘-hardened nickel-based superalloy Allvac 718Plus. Contrary to Inconel 718, the most common representative of this alloy type, long term aging at temperatures up to 700 °C does not degrade the microstructure to a large extend. Using transmission electron microscopy only a slight coarsening of the γ‘ particles in the order of a few nanometer could be detected. While the creep strength decreases continuously with increasing γ‘-size, the yield strength is initially not affected.

Analysis of the single process steps revealed that the strong local dependency of the microstructure is a consequence of the forging process. The main factors which influence the volume fraction and morphology of the grain boundary η-precipitates are the degree of deformation and the adiabatic heat dissipated during forging. Within forgings below the η-solvus temperature different fractions of blocky η-phase can be identified which can be clearly distinguished from the typical thin lamellae morphology. Furthermore, these blocky η-precipitates are not randomly oriented, but show a certain degree of parallel alignment. The correlation with finite element simulation confirms that the alignment is connected to the direction of the local material flow during forging.

A strong alignment of the η-precipitates additionally causes an extraordinarily strong, so far unknown, effect on the crack propagation behavior and dominates all other microstructural aspects. However, the crack propagation rate depends on the orientation of the η-plates with respect to the crack plane, which causes strong anisotropic material behavior. Micro-cantilever experiments proved that the effect of the η-phase can be attributed to the oxidation behavior of these precipitates and their phase boundary to the surrounding γ-matrix. Additionally, focused ion beam tomography demonstrated the deflection of a crack caused by an η-plate and its subsequent propagation along the weak oxidized η/γ-interface. Consequently, a tailored alignment of secondary grain boundary precipitates with respect to the local stress state during operation has a very high potential to achieve a significant improvement of the crack propagation behavior of nickel-based superalloys.