“Medium- and high-entropy alloys had been previously imaged at the atomic scale in 2D projections, but this study represents the first time that their 3D atomic order has been directly observed,” said corresponding author Jianwei “John” Miao, a professor of physics in the UCLA College and member of the California NanoSystems Institute at UCLA. “We found a new knob that can be turned to boost alloys’ toughness and flexibility.”
Composition and Unique Qualities of Medium and High-Entropy Alloys
Medium-entropy alloys combine three or four metals in roughly equal amounts; high-entropy alloys combine five or more in the same way. In contrast, conventional alloys are mostly one metal with others intermixed in lower proportions. (Stainless steel, for example, can be three-quarters or more of iron.)
To understand the scientists’ findings, think of a blacksmith forging a sword. That work is guided by the counterintuitive fact that small structural defects actually make metals and alloys tougher. As the blacksmith repeatedly heats a soft, flexible metal bar until it glows and then quenches it in water, structural defects accrue that help turn the bar into an unyielding sword.
Miao and his colleagues focused on a type of structural defect called a twin boundary, which is understood to be a key factor in medium- and high-entropy alloys’ unique combination of toughness and flexibility. Twinning happens when strain causes one section of a crystal matrix to bend diagonally while the atoms around it remain in their original configuration, forming mirror images on either side of the boundary.
The Innovative Creation Process of New Alloys
The researchers used an array of metals to make nanoparticles, so small they can be measured in billionths of a meter. Six medium-entropy alloy nanoparticles combined nickel, palladium, and platinum. Four nanoparticles of a high-entropy alloy combined cobalt, nickel, ruthenium, rhodium, palladium, silver, iridium, and platinum.
The process to create these alloys resembles an extreme — and extremely fast — version of the blacksmith’s task. The scientists liquified the metal at over 2,000 degrees DOI: 10.1038/s41586-023-06785-z
The co-first authors of the study are Saman Moniri, a former UCLA postdoctoral scholar; Yao Yang, who earned a doctorate from UCLA in 2021; and Jun Ding of Xi’an Jiaotong University in China. Other co-authors are UCLA postdoctoral scholars Yuxuan Liao; former UCLA postdoctoral scholars Yakun Yuan, Jihan Zhou, Long Yang and Fan Zhu; and Yonggang Yao and Liangbing Hu of University of Maryland, College Park.
The study was supported by the U.S. Department of Energy. The experiment was performed at Berkeley Lab’s Molecular Foundry, also sponsored by the DOE.