Multiphase Solidification and Multi-crystal Microstructure in Single Crystal Superalloys
Mechanical Properties of Metallic Materials and their Microstructural Origins
Final Report Abstract
The aim of this project was to analyze the polycrystalline microstructure of single-crystalline nickel-based superalloys and to investigate the influences of the chemical composition of the alloys as well as the process parameters. In the first step, the formation of the microstructure was considered by quenching tests. The solidification sequence of γ-dendrites, carbides, γ/γ'-eutectic and γ'-precipitates was detected. In the next step, the contents, morphologies and compositions were measured in quenching tests at laboratory scale as well as in casting plants under technical production conditions. These results were the basis for assessing the nucleation mechanisms from the subsequent polycrystalline mircostructures detected by EBSD measurements: In carbon-free alloys, γ/γ'-structures form in the interdendrite region from the CMSX-4 and CMSX-4-Plus alloy grains, which can be attributed to the higher γ'-forming elements and low critical supercoolability. - Furthermore, homogeneously formed γ/γ'-eutectic as well as Ti nitrides are detected in all alloys with no and low carbon content. - In carbon-containing alloys, carbides will form that have a different orientation than the single crystal. In branched chinese-script shaped structures, the carbide can enclose a residual melt. As a result, the residual melt nucleates on the carbide. This misorientation with respect to the single crystal can also, if the carbide is open in the growth direction, grow into the interdenritic region. On this, γ/γ'-eutectic can nucleate and assume the same crystal orientation. - Furthermore, the eutectic can nucleate on a carbide by so-called co-growth. Due to the accumulation of eutecticforming elements around the carbide, eutectic nucleation occurs on the carbide despite the different lattice parameters of these microstructure components. This has been observed on different carbide compositions. This effect is mainly influenced by the carbide morphology and thus, in this correlation, the carbon content and the process parameters. In summary by adjusting the alloy composition and process parameters, the polycrystalline phases in nickel-based superalloys can be reduced, or eliminated. For example in CMSX-4 and CMSX-4-Plus alloys, lower withdrawal rates tend to be preferred with respect to the misoriented contents. For carbon containing CMSX-6 series, high temperature gradients are more optimal with respect to enclosure of the melt in a carbide and nucleation of eutectic on a carbide. However when considering homogeneus γ/γ’-eutectic, low withdrawal rates are more optimal. For the high carbon alloys CM-247-LC and MAR-M-247 high withdrawal rates are necessary to avoid nucleation of eutectic on carbides, or cogrowth. Consequently, the process parameters must be adapted by the chemical composition of the alloys.
Publications
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Microstructural Investigations of Ni-Based Superalloys by Directional Solidification Quenching Technique. Materials, vol. 13, no. 19, p. 4265, 2020
Wittenzellner, T.; Sumarli, S.; Schaar, H.; Wang, F.; Ma, D.; Bührig-Polaczek, A.
