Contribution of Al₂O₃ nanoparticles to increasing thermomechanical properties, wear- and oxidation resistance of CrFeCoNiCu-Al₂O₃ composites

Using high-energy ball milling of metallic powders and Al₂O₃ nanopowder followed by spark plasma sintering, CrFeCoNiCu-x%Al₂O₃ (x = 0.1, 0.5, 1 and 3 wt%) composites with a thermally-stable hierarchical microstructure were prepared, in which micrometer-sized regions consisting of FCC1 (high-entropy alloy (HEA), a = 0.35 nm) and FCC2 (Cu-based oxide, a = 0.51 nm) nanograins and reinforcing Al₂O₃ nanoparticles are surrounded by Cu-rich layers (FCC3). The introduction of Al₂O₃ nanoparticles into CrFeCoNiCu HEA significantly altered the microstructure and improved its chemical, mechanical, and tribological properties. Doping with Al₂O₃ nanoparticles resulted in: (i) an increase in the dispersion of structural components, (ii) an increase in the volume fraction of the Cu-rich phase, (iii) a significant decrease in the oxygen content in the Cu-rich phase, and (iv) a high dislocation density and an increase in the local stresses level. Al₂O₃ NPs located at grain boundaries hinder oxygen diffusion, which leads to the rapid formation of a dense Cr₂O₃ oxide layer during oxidation and a decrease in the oxide scale thickness. The addition of 1 wt% Al₂O₃ resulted in an increase in hardness by 27.5 %, compressive strength by approximately 29 % (both at 25 °C and at 750 °C), crack resistance and dynamic impact resistance by 64.3 (500 N) and 40.4 % (700 N), wear resistance by 6.2 (25 °C) and 3.4 (500 °C) times, and a 2-fold decrease in the oxidation rate at 750 °C. After annealing at 750 °C, which resulted in complete relaxation of internal stresses, the composites exhibited fully plastic behavior under compression at 750 °C. The HEA-1 %Al₂O₃ composite achieved UCS₂₅° = 2197 MPa (annealed) and UCS₇₅₀° = 1328 MPa (not annealed).