Phase transformation-induced TGO rumpling failure of an AlCoCrFeNi high-entropy alloy after isothermal oxidation at 1200 C

We compare the oxidation behavior of two AlCoCrFeNi-based high-entropy alloys (HEAs) at 1200 degrees C. The thermally grown oxide (TGO) on an AlCoCrFeNi HEA undergoes significant surface rumpling, which causes TGO cracking and spallation. However, a similar HEA with a lower Cr content, AlCoCr0.8FeNi, shows no TGO rumpling or failure even after 1000 h oxidation. Our analysis suggests that TGO rumpling on the AlCoCrFeNi HEA is predominantly driven by the body-centered cubic (BCC) to tetragonal Cr-rich sigma phase transformation within the Al-depleted layer during cooling. A reduction of the Cr content in the HEA or fast water quenching prevents the phase transformation and, therefore, TGO rumpling. The findings in this study provide scientific guidance for designing more durable, oxidation-resistant AlCoCrFeNi HEAs and coatings.