Microstructure and creep properties of dendritically solidified nickelbase alloys over a wide range of cooling rates

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Hollow turbine blades with wall thicknesses below 1 mm lead to an increase in efficiency, weight and raw material savings. Currently, wall thicknesses down to 0.2 mm are used at the trailing edges of the blades. In the first phase of the project, a process route was established for the production of single-crystalline samples from a nickel-based alloy with wall thicknesses down to 0.4 mm. A dependence of the primary dendrite spacing on the geometry of the castings was observed, as well as a pronounced directional dependence of the dendrite spacing. The goal of the second phase of the project is to investigate and understand the dendritic solidification of thin-walled single crystals through a combination of experimental work and a multiscale simulation chain. In addition, the effect of dendritic segregation on creep properties at 980°C will be investigated. Two research assistants are requested for 36 months each to carry out this project. In the Bridgman process, single-crystalline thin-walled (up to 0.4 mm) and cylindrical samples (diameter 15 mm) will be grown at different withdrawal rates. Dendrite spacings of less than 100 µm to greater than 1000 µm are to be achieved. By accompanying finite element simulations of the casting process, the local temperature distribution in the component, as well as the shape of the solidification zone will be determined. These parameters determine the local distribution of the dendrites. The segregation microstructure is determined experimentally by optical, scanning and occasionally transmission electron microscopy, as well as by energy dispersive X-ray spectroscopy. Phase field studies, based on the simulated temperature fields, will be used to describe the microstructure formation. The dendrite spacing and the segregation coefficients of the elements from experiment and simulation will be compared. Based on existing knowledge about the formation of dendrite spacing, a model for inhomogeneous temperature distributions will be established. Creep specimens will be fabricated from the cast samples, which will be tested at 980°C under vacuum. The effect of residual segregations with different dendrite spacing on the creep behavior of the alloys will be investigated. Selective transmission electron microscopy studies of the local dislocation density will clarify how the creep properties differ between the dendritic and interdendritic regions. The creep properties of the cast specimens will be explained by finite element simulations of the creep tests. For this purpose, the local creep properties of dendritic and interdendritic region are approximated by varying the parameters of a phenomenological creep model. The local deformation of the microstructure of dendrite and interdendritic region is validated in the simulation with the experimentally determined local dislocation densities.