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Optimizing the mechanical properties of variations of Al10Co25Cr8Fe15Ni36Ti6 compositionally complex alloy

Subject Area Metallurgical, Thermal and Thermomechanical Treatment of Materials
Mechanical Properties of Metallic Materials and their Microstructural Origins
Term from 2016 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 316699240
 
Final Report Year 2023

Final Report Abstract

This project shows the microstructure, the mechanical properties and the oxidation behaviour of eleven compositionally complex alloys (CCA) based on the high entropy alloy Al10Co25Cr8Fe15Ni36Ti6. Focus lay on the behaviour of the tow main phases, γ and γ’, which is characterized by a cuboid morphology, and that of the third phase, a Heusler phase. Formation of the latter can present itself in different shapes, depending on the composition, or, in case of the addition of W, avoided completely. It is known from superalloys that sharp edges of the γ’ phase often results in good creep properties. This fact could be confirmed in this project. The addition of Hf proved to be particularly effective: for a given creep rate in the order of 10-6 to 10-8 the alloy with Hf addition can withstand 13 % higher tension than the base alloy without Hf. In addition to γ and γ, the alloy with Hf is characterized by a round shaped Heusler phase. Its tensile properties are also improved compared to the base alloy. It shows the best mechanical properties of all investigated alloys. Another addition that proved positive was the element W: it improves the creep properties in a particular way. It does not rely on the shape of the γ’ particles (these are indeed rather round) but on the almost complete absence of the Heusler phase. A few rare examples can be found at the grain boundaries. This leads to a much higher Norton exponent than in all other investigated alloys. The third interesting addition is Zr. It improves the oxidation resistance of the alloy. Mass gaining the alloy with Zr is the lowest of all investigated alloys. Similar to Hf, but to a lesser extend, it improves the mechanical properties. A combination of the trace elements (Hf&W, Hf&Zr) shows an almost expected linearity. The properties of Al9.25Co25Cr8Fe15Ni36Ti6Hf0.5W0.25 and Al9.25Co25Cr8Fe15Ni36Ti6Hf0.25W0.5 lie between those of Al9.2Co25Cr8Fe15Ni36Ti6Hf0.5 and Al9Co25Cr8Fe15Ni36Ti6W1. Combining Zr and Hf leads to an even higher lattice misfit than in the alloys with only Hf or Zr, and thus even better mechanical properties are expected. Addition of B, Y and Mo and a compositional change of the base alloy Al10Co25Cr8Fe15Ni36Ti6 to Al6Co25Cr8Fe15Ni36Ti10 did not lead to an improvement of the alloys’ properties.

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