Mechanical properties and water vapour corrosion behaviour of Al x CoCrFeNi high-entropy alloys.

Al x CoCrFeNi ( 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 x CoCrFeNi HEAs changed significantly with the Al content, from the original single face-centered cubic FCC phase (Al 0.1 CoCrFeNi) to an FCC + BCC structure (Al 0.5 CoCrFeNi), 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 x CoCrFeNi 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 0.5 CoCrFeNi alloy reach maximum values of 527.4 MP, 943.3 MPa and 28.2%, respectively. The fracture mechanism of the Al 0.1 CoCrFeNi and Al 0.5 CoCrFeNi 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 Al 3 Fe 5 O 12 , CoCr 2 O 4 and NiCr 2 O 4 were found in the Al 0.1 and Al 0.5 alloys, whereas no oxide peaks were detected using XRD for the Al 1 alloy. It was also presumed that a very thin alumina film was generated on the surface of the Al 1 alloy, preventing the oxidation of the sample, in combination with the analysis of SEM, EDS and XPS behaviour.