Resistance sintering of CoCrFeNiAlx (x = 0.7, 0.85, 1) high entropy alloys: Microstructural characterization, oxidation and corrosion properties

•Attributes of CoCrFeNiAlx (x = 0.7, 0.85, 1) HEAs varied significantly with x value.•Rise in Al content resulted in increase in BCC phase fraction.•High Al content encouraged the formation of a protective Al2O3 layer.•CoCrFeNiAl HEA exhibited the best cyclic oxidation resistance.•Al increased the cyclic oxidation resistance but reduced the electrochemical resistance. The cyclic oxidation and electrochemical corrosion behaviors of CoCrFeNiAlx (x = 0.7, 0.85 and 1 mol) high entropy alloys (HEAs) produced by resistance sintering were explored. In accordance with the purpose of this work, CoCrFeNiAlx HEAs with different Al content were cyclically oxidized at temperatures of 800, 875 and 950 °C for 120 h, and their corrosion performances in 0.5 M H2SO4 and 3.5 wt% NaCl solutions were assessed using the potentiodynamic polarization. X-ray diffraction analysis results revealed that BCC and FCC phases along with a minor B2 phase formed in all three HEAs. When the Al content increased to 1 mol from 0.7 mol, the phase fraction of BCC increased by 34.7%, accompanied by an increase in microhardness by 22.6%. The increased Al content had a significant effect in enhancing the cyclic oxidation resistance of CoCrFeNiAlx HEAs. The oxidation rate of all the oxidized HEAs increases with either increasing temperature or decreasing Al content. The best oxidation resistance at temperatures of 800 °C, 875 °C and 950 °C was observed with CoCrFeNiAl HEA, which had minimum weight change data during 120 h of exposure. XRD analysis results revealed that Al2O3 and Cr2O3 were the main oxides that formed on surfaces of the HEAs after oxidation for 120 h. Cross-sectional micrographs showed that the thickness and continuity of oxide scales that formed on HEAs were strongly dependent on the Al content. The improved oxidation behavior of HEAs was attributed to the rise in Al content, which causes the increasing Al2O3 content in oxide scale. Over the performed oxidation duration, neither cracking nor spalling of the oxide scales was witnessed. The effect of Al exhibited a contrary pattern in terms of the oxidation and corrosion behaviors of HEAs. In both 3.5 wt% NaCl and 0.5 M H2SO4 solutions the HEA with the highest Al showed worse corrosion resistance among the HEAs, reflected in the more negative corrosion potential and higher corrosion current density as well as higher corrosion rate. The worsening of the corrosion behavior has been attributed to the increased phase fraction of BCC due