Effect of electron beam treatment on the structure and properties of AlCoCrFeNi high-entropy alloy

Abstract

In the last decade the attention of scientists in the field of physical material science has been drawn to the study of high entropy alloys. Using wire arc additive manufacturing (WAAM) technology the high-entropy alloy (HEA) AlCoCrFeNi of nonequiatomic composition has been produced (wt. %: 15.64 Al; 7.78 Co; 8.87 Cr; 22.31 Fe; 44.57 Ni). It is shown by methods of modern material science that the HEA is a polycrystalline material with a grain size 4–15 μm along whose grain boundaries the second-phase particles are detected. It is shown by the mapping methods that bulks of grains are enriched in aluminium and nickel while grain boundaries contain chromium and iron. Cobalt is quasi-uniformly distributed in crystal lattice of the manufactured HEA. The material ultimate strength in testing for compression depends on production mode and varies within the interval from 1300 to 1800 MPa. The HEA wear parameter amounts to 1.4∙10−4 mm3/N∙m, friction coefficient — 0.65. In testings for tension the material failure occurred by the mechanism of intragrain cleavage. The formation of brittle cracks along boundaries and in grain boundary junctions i.e. in sites containing the inclusions of second phases is revealed. One of the reasons of HEA increased brittleness is the revealed nonuniform distribution of elements in alloy structure. The HEA’s irradiation by pulsed electron beam with energy density of 10–30 J/cm2 (pulse duration 200 μs, number of pulses 3) results in material homogenization. High-velocity crystallization of molten surface layer of HEA samples is accompanied by the formation of columnar structure having a submicro-nanocrystalline structure. The HEA irradiation results in the increase in hardness and plasticity of the material. The largest increase (1.6 times) in ultimate strength is obtained in the alloy after electron beam processing with energy density of 30 J/cm2. Electron beam processing leads to decrease in microhardness of alloy surface layer up to 90 μm thick.