The effects of short-range chemical and structural ordering related to oxygen interstitials on mechanical properties of CrCoFeNi high-entropy alloys: A first-principles study

In this work, the effects of oxygen interstitials on the structural and mechanical properties of face-centered cubic (FCC) CrCoFeNi high-entropy alloys (HEAs) are investigated by first-principles calculations. It is found that oxygen interstitials are more energetically favorable to locate at the octahedral sites where their coordinated constituent alloy elements consist of more Cr and less Ni, and their formation energy decreases with increasing resultant lattice distortions. Meanwhile, the interstitials-induced lattice distortions result in strong non-symmetrical local strains that are closely related to the local chemical ordering of Cr. An enhancement of yield strength for the HEAs containing 0.926 at.% oxygen interstitials is observed, in consistent with that estimated by the interstitial strengthening mechanisms, while further increases in interstitials content lead to the severely deteriorated fracture strength and strain. The ordering of oxygen interstitials occurred in the HEAs with an oxygen concentration exceeding 3.7 at.% could result in the formation of oxides with a rock salt crystal structure consistent with the high entropy oxides. The mechanical strength of HEAs containing those sub-nanometer ordered clusters are found to be higher than those containing randomly distributed oxygen interstitials, when the oxygen concentration is not higher than 3.57 at.%.