A groundbreaking study by UCLA scientists has for the first time mapped medium and high-entropy alloys in 3D, revealing their unique combination of toughness and flexibility. This advancement could transform the way alloys are engineered and utilized. Credit: SciTechDaily.com
Atomic map of a high-entropy alloy nanoparticle shows different categories of elements in red, blue and green, and twinning boundaries in yellow. Credit: Miao Lab/UCLA
“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
” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>Fahrenheit for five-hundredths of a second, then cooled it down in less than one-tenth that time. The idea is to fix the solid alloy in the same varied mixture of elements as a liquid. Along the way, the shock of the process induced twin boundaries in six of the 10 nanoparticles; four of those each had a pair of twins.
Revolutionary Imaging Technique: Atomic Electron Tomography
Identifying the defects required an imaging technique the researchers developed, called atomic electron tomography. The technique uses electrons because atomic-level details are much smaller than wavelengths of visible light. The resulting data can be mapped in 3D because multiple images are captured as a sample is rotated. Tuning atomic electron tomography to map the complex mixtures of metals was a painstaking endeavor.
“Our goal is to find the truth in nature, and our measurements have to be as accurate as possible,” said Miao, who is also deputy director of the STROBE National Science Foundation Science and Technology Center. “We worked slowly, pushing the limit to make each step of the process as perfect as possible, then moved on to the next step.”
The scientists mapped each