5 Werkstofftechnik
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Trace elements play an important role in the fine-tuning of complex material properties. This study focuses on the correlation of microstructure, lattice misfit and creep properties. The compositionally complex alloy Al10Co25Cr8Fe15Ni36Ti6 (in at. %) was tuned with high melting trace elements Hf and W. The microstructure consists of a γ matrix, γ' precipitates and the Heusler phase and it is accompanied by good mechanical properties for high temperature applications. The addition of 0.5 at.% Hf to the Al10Co25Cr8Fe15Ni36Ti6 alloy resulted in more sharp-edged cubic γ′ precipitates and an increase in the Heusler phase amount. The addition of 1 at.% W led to more rounded γ′ precipitates and the dissolution of the Heusler phase. The shapes of the γ' precipitates of the alloys Al9.25Co25Cr8Fe15Ni36Ti6Hf0.25W0.5 and Al9.25Co25Cr8Fe15Ni36Ti6Hf0.5W0.25, that are the alloys of interest in this paper, create a transition from the well-rounded precipitates in the alloy with 1% W containing alloy to the sharp angular particles in the alloy with 0.5% Hf. While the lattice misfit has a direct correlation to the γ' precipitates shape, the creep rate is also related to the amount of the Heusler phase. The lattice misfit increases with decreasing corner radius of the γ' precipitates. So does the creep rate, but it also increases with the amount of Heusler phase. The microstructures were investigated by SEM and TEM, the lattice misfit was calculated from the lattice parameters obtained by synchrotron radiation measurements.
A new race for high performance structural materials has started since the discovery of high entropy alloys at the beginning of the 21st century. The possible combination of several elements in an, until then, unknown composition space opened the ground for discovering new materials. Solid solution strengthening remains the most prominent mechanisms that is active in this family of materials, but it is supported by all other strengthening mechanism on the path to better and better performing materials. Chemical, thermal and mechanical approaches are combined to optimize these alloys. Optimum performances can be reached by using a high number of different strengthening mechanisms, induced both by composition and processing. The most prominent with composition induced mechanism is precipitation hardening, and on the processing side it is cold working such as cold-rolling, torsion or extrusion. The contribution uses tensile test data at room temperature solely for comparison – high temperature and cryogenic data are omitted because it would lead beyond the scope of this manuscript.