Scientists Substantiate Chemical Medium-range Order in High-entropy Alloys
Chinese scientists have made prediction and successful observation of the nanoscale chemical medium-range order (CMRO) in a high-entropy alloy (HEA). This finding undisputedly fills the gap of the ordering evolution in this kind of alloy.
This work, published in Nature Communications on February 23, was conducted by Prof. Wu, Xiaolei from the Institute of Mechanics of the Chinese Academy of Sciences (CAS).
The high (medium)-entropy alloys (HEAs/MEAs) are rapidly becoming a hot and frontier topic in multidisciplinary fields, especially recently showing a few breakthrough performances. In these HEAs/MEAs, a signature feature is the development of ordering structure, markedly differing from the conventional metallic materials. For an ordering, the starting point is the chemical short-range order (CSRO), two small in size, usually as a gene with no sign of periodicity, while the ending is known as the long-range order of crystal periodicity. In view of the structural size and periodicity, CMRO is expected as an indispensable intermediate link.
“Unfortunately, as such an important structural link, CMRO remains an entirely green field thus far in the H/MEAs, let alone to catch sight of it. These questions are open as to if, and what kind of,CMROwould be produced and if CMRO is mechanically stable during plastic deformation.” said Prof. Wu, Xiaolei, leader of the research team.
“Here, we show compelling evidences for CMRO in an Al9.5CrCoNi MEA. Specifically, the electron diffractions under both  and  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.” said Ms. Wang, the first author.
“Both the CMRO and CSRO are co-existent, with the same structure motif of atomic configuration. This strongly signals that CMRO is growing CSRO, but at medium range.” added Ms. Jiang.
“From a general perspective, it is anticipated that CMRO will enhance mechanical property. CMRO of large sizes will be more effective to reinforce the interaction with dislocations upon straining. CMRO offers a new knob to tailor the macroscopic properties, i.e., by providing an opportunity to tune the degree of chemical orders.” said Prof. Yuan.
“The present results suggest a potential strategy to tailor the macroscopic properties via the heterogeneous CMRO engineering in the H/MEAs.” said Prof. Wu.
This work was supported by the Ministry of Science and Technology of China, the Basic Science Center Program of the National Natural Science Foundation of China, and CAS.
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