High-entropy alloys have been in development for about two decades [1,2]. The ever-increasing interest of researchers in these materials is due to the broad possibilities of obtaining unique mechanical and functional properties [3]. The selection of alloying elements and heat treatments allows properties to be shaped for a wide variety of applications. In the AlCoCrFeNi alloy, the greatest effects are observed when the aluminum content is changed, which is due to the difference in atomic radii of the constituent elements [4]. One of the most important topics is to clarify the mechanisms of strengthening of these materials.

The authors of this paper analyzed the deformation and fracture processes of AlxCoCrFeNi-based alloys (x=0¸1.0). They studied the effect of aluminum content and titanium addition on changes in microstructure and mechanical properties.  Materials were produced by arc melting of pure metals in an argon atmosphere. Crystal structures of the samples were analyzed by means of X-ray diffraction (XRD) using an Empyrean Panalytical powder diffractometer (Malvern Panalytical). Microstructure studies were carried out on a SEM-FIB DualBeam Scios 2 scanning electron microscope equipped with an EDS chemical composition analyzer. Keyence optical microscope was used to analyze the fractures. Tensile tests were carried out during a tensile test of microsamples on an Instron machine using an Aramis vision system for strain assessment. Hardness was measured by the Vickers method under a load of 98 N using an INNOVATEST hardness tester.

XRD studies showed that the crystal structure of the alloys is dependent on the aluminum content. At x=0 there is a homogeneous structure of a solid solution, crystallizing in the fcc system, increasing the content of Al causes the appearance of a bcc phase, the proportion of which increases with increasing Al content, while the addition of titanium caused the appearance of numerous intermetallic phases. Similar changes were observed in the microstructure of the materials. The x=0 alloy is characterized by a homogeneous structure with no separations. In contrast, at x=0.5, the microstructure shows a large proportion of dendrites of the solid solution of the alloying elements. A mixture of phases is visible in the interdendritic spaces, with the structural elements of this mixture having dimensions of less than 1 mm. Segregation of alloying elements within the phases is observed. As the aluminum content increases, the proportion of the phase mixture increases. At x=1.0, the microstructure consists of regular grains inside which a substructure at the nanometer level is visible. In the case of an alloy with the addition of titanium, numerous separations of intermetallic phases additionally appear. Mechanical properties also change decisively with the aluminum content in the alloy. An increase in Al content resulted in an increase in Rm (Rm=560¸1200 MPa for x=0¸0.7 respectively) and hardness, and a decrease in ductility. With up to x=0.7 content, the samples showed high ductility within e=15¸30%. At x=1 and for the alloy with the addition of titanium, brittle fracture of the alloy took place. Analysis of the obtained results allows us to conclude that the mechanical properties are influenced by both the mechanism of solution strengthening and the microstructure of the alloys.