Direct visualization of the existence of surface local chemical order in a high-entropy CoCrFeMnNi alloy

The impacts of local chemical order (LCO) on the physical properties of high-entropy alloys (HEAs) have been widely discussed. However, the difficulty in unambiguously observing LCO with high precision poses a great challenge in establishing microscopic mechanisms regarding the impacts of LCO on physical properties. Furthermore, it is still unclear whether the LCO extends to HEA surfaces, which may impact surface-based properties, such as corrosion, oxidation, and catalytic activities. Through the utilization of scanning tunneling microscopy (STM), two surface LCO domains with corresponding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msqrt><mml:mrow><mml:mn>5</mml:mn></mml:mrow></mml:msqrt><mml:mo>×</mml:mo><mml:msqrt><mml:mrow><mml:mn>5</mml:mn></mml:mrow></mml:msqrt><mml:mi>R</mml:mi><mml:mo>±</mml:mo><mml:mn>26</mml:mn><mml:mo>.</mml:mo><mml:msup><mml:mrow><mml:mn>6</mml:mn></mml:mrow><mml:mrow><mml:mo>∘</mml:mo></mml:mrow></mml:msup></mml:math> quasi-long-range orderings (QLRO) are directly observed on a CoCrFeMnNi surface. Density functional theory (DFT) calculations identify the LCO within QLRO supercells. The findings provide evidence of the existence of the surface LCO and demonstrate a method to directly observe the surface LCO of HEAs. With the ability to unambiguously resolve elemental configuration at atomic scale, the understanding of how LCO influences surface-based properties can be achieved, facilitating the design of HEAs with tailored functionalities.