Chemical short-range orders in high-/medium-entropy alloys

Abstract

High (or medium)-entropy alloys (H/MEAs) are complex concentrated solid solutions prone to develop the chemical short-range orders (CSROs), as an indispensable structural constituent to make H/MEAs essentially different from the traditional alloys. The CSROs are predicted to play roles in dislocation behaviors and mechanical properties. So far, the image of CSROs is built up by the theoretical modeling and computational simulations in terms of the conventional concept, i.e., the preference/avoidance of elemental species to satisfy the short-ranged ordering in the first and the next couple of nearest-neighbor atomic shells. In these simulated CSROs, however, the structural image is missing on the atomic scale, even though the lattice periodicity does not exist in the CSROs. Further, it is pending as to the issues if and what kind of CSRO may be formed in a specific H/MEA. All these are ascribed to the challenge of experimentally seeing the CSROs. Until recently, the breakthrough does not appear to convincingly identify the CSROs in the H/MEAs by using the state-of-the-art transmission electron microscope. To be specific, the electron diffractions provide solid evidence to doubtlessly ascertain CSROs. The structure motif of CSROs is then constructed, showing both the lattice structure and species ordering occupation, along with the stereoscopic topography of the CSRO. It is suggested that the CSROs, as the first landscape along the path of development of the local chemical ordering, offer one more route to substantially develop the ordered structure on the atomic scale in the H/MEAs, parallel to the existing grain-leveled microstructure. The findings of CSROs make a step forward to understand the CSROs-oriented relationship between the microstructure and mechanical properties. This review focuses on the recent progress mainly in the experimental aspects of the identification, structure motif, and mechanical stability in CSROs, along with the chemical medium-range orders as the growing CSROs.