High temperature oxidation behaviour of non-equimolar AlCoCrFeNi high entropy alloys

Oxidation properties of AlCoCrFeNi high entropy alloys, i.e. 5%Al-35%Co-25%Cr-15%Fe-20%Ni (Al5Co35Cr25Fe15Ni20) and 5%Al-25%Co-20%Cr-15%Fe-35%Ni (Al5Co25Cr20Fe15Ni35), were investigated in air atmosphere at 1273 K. Thermogravimetry (TG) investigations determined that, under these conditions, the samples demonstrate scale growth according to the parabolic rate law with rate constants the order of 10(-11) g(2) cm(-4) s(-1). Combined scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) surface analysis and X-ray diffraction (XRD) studies revealed that the formed protective scales consisted primarily of chromium oxide. Underneath the external Cr2O3 scale, a thin layer of the metallic substrate can be found. Below that, an internal chromium oxide scale with discontinuous Al2O3 can be observed. Al-rich precipitates can also be found in the internal layers, as well as the metallic substrates after oxidation. XRD analysis also indicated that the initial materials are single FCC phase solid solutions. From all these studies it can be concluded that both Co and Ni-rich non-equimolar AlCoCrFeNi high entropy alloys exhibit similar high temperature oxidation behaviour. Furthermore, each of the alloys has promising anti-corrosion properties that can possibly be further improved on, e.g. with higher aluminium content inside the materials.