Ma En, a professor of the School of Materials Science and Engineering at Xi'an Jiaotong University (XJTU), published a paper entitled "Direct observation of chemical short-range order in a medium-entropy alloy" in collaboration with Wu Xiaolei, a researcher from the Institute of Mechanics of the Chinese Academy of Sciences and Zhu Jing, an academician from the Beijing National Center for Electron Microscopy at Tsinghua University, in Nature on April 28.

High (medium)-entropy alloys (HEAs), also known as multi-principal element alloys, have become a hot issue for scientists conducting multidisciplinary research in physical metallurgy in recent years. Different from the structure of chemical disorder exhibited in traditional alloys, HEAs are a solid solution composed of multi-principal alloying elements in either equi-atomic or subequi-atomic proportions.

It is the first time scientists have found chemical short-range order (CSRO) in HEAs, or have directly observed the dislocation interactions of CSRO from analog experiments and calculation.

Frequent interactions between constituent elements in the solid solution show that HEAs have selective preferences that contribute to the formation of CSRO in some areas to varying degrees. CSRO is arguably difficult to decipher because it is often measured on a subnanometer-length scale and occupies certain lattice planes or sites in the first and second nearest-neighbor atomic shells. 

During their latest studies, Ma and his teammates utilized a double aberration-corrected transmission electron microscope (TEM) equipped with energy-filtered imaging to successfully find as appropriate zone axis. 

They also adopted multiple techniques like electron diffraction associated with comprehensive and micro zones, high-resolution imaging of high-angle annular dark-field (HAADF), Fast Fourier Transformation (FFT), Inverse FFT, energy-filtered dark-field imaging and atomic EDS-Mapping to explicitly reveal CSRO in a face-centered-cubic VCoNi concentrated solution, found evidence of electron backscatter diffraction of CSRO, and collected relevant information about its size, position of constituent elements and three-dimensional structure.

The chemical element distribution test shows that on the adjacent {113} plane and between the adjacent atomic columns on the (111) plane, there are alternating element distribution and occupancy characteristics of rich V/rich (Co/Ni)/rich V.

The researchers designed the spatial distribution correlation coefficient to analyze whether the elements tend to cluster (where the correlation coefficient is positive) or remain mutually exclusive (negative) at different distances. They successfully confirmed that the driving force for the formation of CSRO comes from the avoidance of V-V pairs and energy reduction induced by neighboring V-Co and V-Ni pairs.

In addition, the researchers also used atomic strain mapping to demonstrate the dislocation interactions enhanced by CSRO, shedding light on its effects on plasticity mechanisms and mechanical properties upon deformation. 

An image shows the selective zones, micro electron diffraction, energy-filtered dark-field scenario, high-resolution imaging of high-angle annular dark-field, FFT, Inverse FFTand distribution size in CSRO of VCoNi medium-entropy alloys.

Elements distribution, positioning and spatial distribution of correlation coefficient in CSRO, and an illustration of Monte Carlo analog computation.

Dislocation interaction of CSRO and strain transformation of ordered short-range zones before and after the occurrence of dislocation interaction.

In conclusion, their research was the first confirmation of the existence of chemical short-range order in medium-entropy alloys and its dislocation interaction, and provided a new way of understanding the characteristics of the basic microstructure of HEAs and the design of high-performance HEAs.

Link to the article: https://www.nature.com/articles/s41586-021-03428-z