First-Principles Calculations and Experimental Studies of Phase Structure and Mechanical Stability of AlxCoCrFeNi Series High-Entropy Alloys

High-entropy alloys are endowed with significant high-entropy effects，slow diffusion effects，lattice distortion effects，and cocktail effects due to their unique design composition，microstructure，and adjustable properties，and they also exhibit good mechanical stability properties. Among a wide variety of high-entropy alloys，CoCrFeNi series high-entropy alloys show their potential as a material with excellent performance due to their outstanding mechanical properties. The mechanical properties of high-entropy alloys，such as hardness，strength，and plasticity，will change with the addition of minor alloying elements. Therefore，the addition of different content of Al elements to the CoCrFeNi high-entropy alloy system is important for its structural and mechanical properties. In this paper，the first principles calculation based on density functional theory was used and combined with the virtual crystal approximation method，the phase structure and mechanical properties of AlxCoCrFeNi（x=0，0.1，0.3，0.5，0.7，1.0；mole fraction）series high-entropy alloys were focused. Meanwhile，X-ray diffraction analysis（XRD）and a metallographic microscope were further used to reveal the evolution of the microstructure of high-entropy alloys. Vickers hardness tester and nanoindentation test system were also used to investigate the influence of the structure of AlxCoCrFeNi series high-entropy alloys on the mechanical properties. The ductility of the high-entropy alloy was determined qualitatively by plasticity work. According to the high-entropy alloy composition criterion，it was proved that a stable solid solution structure could be formed in the alloy，and the solid solution structure would change with the increase of Al content. When 0≤x<0.3，the high-entropy alloy was mainly face-centered cubic（fcc）structure；while 0.3≤x≤1，the high-entropy alloy was more likely to form a mixed solid solution of body center cubic（bcc）and fcc at this time. The results of calculating the corresponding crystal structure showed that the lattice constant first decreased and then increased with the increase of Al element，mainly because the real lattice distortion of the high-entropy alloy was smaller，and there was a relaxation process of the arrangement of atoms in the lattice，i.e.，the atoms spontaneously adjusted the space occupation so that the free energy was the lowest，which led to such changes in the lattice constant. When low-density Al was added to the system，the overall density of the alloy decreased. However，when x=0.1，the density of the alloy slightly increased，because the total mass of the system increased，the lattice distortion decreased，and the volume decreased. According to the calculation of elastic constants，fcc structural alloy and bcc structural alloy both met the mechanical stability criterion. The mechanical stability of the alloy increased with the increase of Al content，the covalent bonding characteristics of the alloy increased with an increase in the alloy's negative Cauchy pressure C12-C44，and the alloy gradually became brittle. In contrast，CoCrFeNi high-entropy alloy was a ductile material. At the same time，calculations of the material modulus revealed that the resistance of the high-entropy alloy to volume deformation，shear deformation，and elastic deformation was subsequently increased. The experimental study found that with the increase of Al content，the crystal structure of the high-entropy alloy changed from a single fcc structure to a mixed fcc+bcc structure and then to a single bcc structure，and the solidification organization changed from columnar cytosol to columnar dendrite and equiaxed dendrite. When x=0.5，the initial NiAl type B2 phase started to grow in fcc matrix and was biased between the dendrites，which could play a strengthening role. The microscopic nanoindentation tests showed that the hardness and Young's modulus of the high-entropy alloy increased with the increase of Al content，and the hardness and elastic modulus of Al1.0CoCrFeNi alloy were the highest. This was because the addition of Al atoms increased the lattice distortion in the high-entropy alloy system，and the solid solution strengthening effect was enhanced，while the absolute value of alloy mixing enthalpy increased and the interaction force between elements within the crystal structure increased. The load-displacement curves showed that the deformation resistance of the alloy increased with the increase of Al content. Meanwhile，all alloys with different Al contents showed permanent deformation and some elastic recovery after unloading，and all creep plateaus appeared at the maximum load retaining stage. This phenomenon occurred due to the dynamic strain aging of the alloys，in which the interaction between Al atoms in solute atoms and dislocations had an important effect，while the sawtooth rheology gradually disappeared with increasing Al content. During the unloading phase，all alloys showed the same recovery behavior. The ductility of the high-entropy alloys was qualitatively determined by calculating the ratio of plastic work to total work of compression（Wp/Wt），and the ductility of the high-entropy alloys gradually deteriorated with increasing Al content，which corresponded to the computational determination that AlxCoCrFeNi series high-entropy alloys were mostly brittle materials. Accordingly，the analysis of the above calculation results and the experiments conducted could provide theoretical basis and experimental guidance for further development of the structure and mechanical properties of high-entropy alloys. © 2024 Editorial Office of Chinese Journal of Rare Metals. 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