Investigation of the microstructure of single-crystal nickel-based-superalloys under directional solidification

Nickel-based superalloys are mainly used in the turbines of aircraft engines because of their excellent high-temperature properties. Due to the partially very high temperature loads, directionally or single-crystalline solidified turbine blades are used, which are produced by the Bridgman-process. The alloys consist of multi-component systems with more than ten alloying elements, resulting in a dendritic microstructure under industrial process conditions during the solidification process. The microstructure consists of γ-dendrites, γ/γ'-eutectics and γ'-precipitates. In carbon-containing alloys, additional carbides form in the interdendritic region. In the case of unfavourable alloy compositions or process parameters, solidification defects also form, which lead to a decrease in mechanical strength. In this work, defects on the microstructure scale, which deviate from the crystal orientation of the single crystal, are characterised. The influence of different cooling rates on carbon-free as well as carbon-containing alloys is investigated. In addition to the castings using the Bridgman technique under industrial conditions, quenching tests are also carried out to analyse the mushy zone. In order to understand the nucleation and growth of the misoriented components, the formation temperatures, area fractions, morphologies, chemical composition and crystal orientation of the microstructure components are determined. In the industry round molds are generally used to produce directionally solidified or single crystal components. Due to the shadow effect, it is confirmed that there is a higher freckle tendency on the opposite side to the heater, as a larger mushy zone is created in this area due to the lower heating and cooling performance. Additionally, it has been proved that a larger mold leads to a lower temperature gradient and consequently to a higher freckle formation. Within the scope of this work, it was found that γ/γ'-structures form homogeneously from the residual melt at certain process parameters and alloy composition. Furthermore misoriented γ/γ' can also form by closure of residual melt in chinese-script shaped carbides. If the carbide is open in the growth direction, these can also expand into the interdendritic region. Eutectics can also nucleate on these misoriented phases and consequently have a different crystal orientation compared to the single crystal. Furthermore, it was confirmed that eutectics can assume a random crystal orientation due to homogeneous nucleation in the residual melt. In carbon-containing alloys, eutectics also nucleate at low surface-to-volume ratios of carbides on these intermetallic phases. This occurs due to the accumulation of eutecticforming elements around the carbide, resulting in undercooling that overcomes the different lattice parameter of these two phases.