Directional solidification of multiphase metallic high temperature materials beyond Ni-base superalloys

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Major goal of this project is to determine the influence of directional solidification (DS) on the mechanical properties of alloy systems exhibiting melting points far beyond Nickel based superalloys (e.g. Nb-Si-Cr and Mo-Si-Ti). As soon as the zone melting device had been put into service by using the reference binary eutectic alloy Nb-18 at% Si, the obtained knowledge was transferred to the three-phase system Nb-Si-Cr and to the two-phase system Mo-Si-Ti. A direct correlation between solidification parameters and the resulting microstructure was obtained. Regarding the high temperature creep properties, DS of Nb-18 at% Si led to a significantly increased creep resistance (about three orders of magnitude better), as compared to a powder metallurgically produced alloy. This difference can mainly be attributed to the different size of microstructural features, the predominant mechanism being diffusional creep in both materials. A comparison with a state of the art single-crystalline Nickel based superalloys (CMSX 4) underpins the outstanding improvement of the creep resistance of DS NbSi eutectics. First creep results for DS ternary Nb-Si-Cr alloys indicate dislocation creep to be the rate controlling mechanism. The observed increase of the stress exponent with decreasing stress can be rationalized by the formation of nanoscaled NbCr2 precipitates. Unexpectedly, the arc melted reference alloy, exhibits superior creep resistance. In the second phase of the project, phase field simulations will be continued at the binary Nb-Si in 3D, involving anisotropic interfacial energies derived from atomistic models. Through simulation studies, an understanding of the spatial composition of the lamellar structure is obtained in 3D and the influence of potential nucleation events is analyzed. In parallel, a deeper understanding of the creep behavior observed in the ternary system Nb-Si-Cr will be developed. Also, the determination of the ductile-brittle transition temperature (DBTT) and the oxidation properties is a major task of this project. Besides experimental identification of the ternary-eutectic point, the phase field modelling will be extended to the ternary Nb-Si-Cr system. For two thermodynamic models based on Pandat data and on experimental results, 3D microstructure simulations will be performed. Phase arrangements and microstructural parameters will be comparatively assessed, and the morphologies will be categorized. Based on own literature data, the alloying element Vanadium will be added to the Nb-Si-Cr system, as it potentially improves fracture toughness and oxidation resistance. In the system Mo-Si-Ti an optimum composition regarding the possibility of directional solidification, has to be identified. Finally, selected creep tests will be performed to compare the influence of the generated microstructures.