Mechanical properties and water vapour corrosion behaviour of AlxCoCrFeNi high-entropy alloys

Abstract

Al_xCoCrFeNi (x = 0.1, 0.5 and 1) high-entropy alloys (HEAs) were prepared using a spark plasma sintering (SPS) technique combined with aerosol powder. Their microstructure and phase constituents were characterized using an X-ray diffractometer and SEM, and their tensile properties, hardness and compactness were tested. The results show that the crystal structure of the Al_xCoCrFeNi HEAs changed significantly with the Al content, from the original single face-centered cubic FCC phase (Al_0.1CoCrFeNi) to an FCC + BCC structure (Al_0.5CoCrFeNi), and then to FCC + BCC + sigma (σ) phase structures (AlCoCrFeNi). Chemical composition analysis showed that the crystal structure transformation was related to the segregation caused by the increased Al content. The hardness of the Al_xCoCrFeNi HEAs increases with increasing Al content, and the hardness of AlCoCrFeNi reaches a maximum of 507.3 HV. The tensile properties of the alloy show a trend of first increasing and then decreasing with increasing Al content, and the yield strength, ultimate tensile strength and elongation of the Al_0.5CoCrFeNi alloy reach maximum values of 527.4 MP, 943.3 MPa and 28.2%, respectively. The fracture mechanism of the Al_0.1CoCrFeNi and Al_0.5CoCrFeNi alloys is typical ductile fracture, while that of the AlCoCrFeNi alloy is cleavage fracture. The compactness of the alloy increases with the Al content. The samples were also subjected to high-temperature water vapour corrosion, and corrosion products such as Al₃Fe₅O₁₂, CoCr₂O₄ and NiCr₂O₄ were found in the Al_0.1 and Al_0.5 alloys, whereas no oxide peaks were detected using XRD for the Al₁ alloy. It was also presumed that a very thin alumina film was generated on the surface of the Al₁ alloy, preventing the oxidation of the sample, in combination with the analysis of SEM, EDS and XPS behaviour.