Effect of heat treatment on the structure and properties of high-entropy alloy AlCoCrFeNiNb0.25

Introduction. Currently, one of the most studied high-entropy alloys (HEAs) is the CoCrFeNi system with the addition of a fifth component. An example of such an alloy is AlCoCrFeNi alloyed with additional elements. Nb alloying promotes the formation of a solid solution and a secondary Laves phase in the alloy, and leads to the formation of eutectics between these phases. The optimal combination of mechanical properties achieved in the hypoeutectic alloy AlCoCrFeNiNb0.25 was the basis for the choice of this alloy for further studies under heat treatment conditions. Purpose of the work. To investigate the effect of heat treatment, including heating to temperatures of 900°C, 1,000°C and 1,100°C with subsequent cooling in air, on the structure and properties of AlCoCrFeNiNb0.25. The methods of investigation were optical metallography, X-ray diffraction analysis, microhardness measurement, and compression tests. Results and Discussion. AlCoCrFeNiNb0.25 alloy retains the solid solution structure based on the BCC phase not only in the cast state, but also after heat treatment. Irrespective of heat treatment parameters, the alloy retains the hypoeutectic structure consisting of solid solution dendrites and eutectic with the Laves phase in the interdendritic space. Heat treatment leads to changes in the phase composition of the alloy and refinement of structural components. When heated to 900°C, along with the existing solid solution and Laves phase, σ-phase is released in the structure, which increases the microhardness of the alloy, but does not provide improvement of strength properties due to its low plasticity. The strength properties of the alloy are significantly improved by heat treatment with heating up to 1,000°C and 1,100°C. Heating up to 1,100°C is accompanied by an increase in residual strain. The main reasons for this effect may be transformations occurring both in the solid solution of the BCC phase (dissolution of the B2 phase, rearrangement of the substructure, increase in the lattice parameter) and in the eutectic (increase in the proportion of the Laves phase, refinement of eutectic cells).