Chemical medium-range order in a medium-entropy alloy

High-/medium-entropy alloys (H/MEA) have the inherent local chemical order. Yet, as a structural link between the incipient short-range order and mature long-range counterpart, the chemical medium-range order (CMRO) is conjectural and remains open questions as to if, and what kind of, CMRO would be produced and if CMRO is mechanically stable during plastic deformation. Here, we show compelling evidences for CMRO in an Al9.5CrCoNi MEA. Specifically, the electron diffraction under both [112] and [013] zone axis show the definite spots for CMRO of lattice periodicity. CMRO entities are seen directly of medium-ranged in sizes by using dark-field imaging, along with the tendency towards like-pair avoidance and unlike-pair preference based on atomic-resolution EDS mapping. These findings substantiate CMRO with a realistic structural picture in view of crystal periodicity and chemical species occupation, shedding light on understanding the microstructural link at an extended length scale beyond the short-range order.

Automated summary

This study demonstrates the existence of chemical medium-range order (CMRO) in an Al9.5CrCoNi high-/medium-entropy alloy (H/MEA) and observes CMRO entities with lattice periodicity. Dark-field imaging and atomic-resolution EDS mapping reveal the pairing tendencies of atoms in the medium range and the avoidance of like-pair and preference for unlike-pair. These findings provide insights into the microstructural link at an extended length scale beyond the short-range order.