From Blacksmiths to Beamlines: 3D Atomic Revelations Transform Alloy Engineering

The groundbreaking research conducted by scientists at the University of California, Los Angeles (UCLA) has ushered in a new era in alloy engineering. This pioneering study marks the first-ever three-dimensional mapping of medium and high-entropy alloys, a development that has the potential to revolutionize the field by enhancing the toughness and flexibility of these materials.

Alloys, such as steel, which result from the combination of two or more metallic elements, play a fundamental role in modern life. They are essential for constructing buildings, transportation, appliances, and tools, including the very device you are using to access this information. Engineers have long grappled with a classic trade-off in materials: hard alloys tend to be brittle and prone to breaking under strain, while flexible ones are susceptible to denting.

Approximately two decades ago, the possibility of circumventing this trade-off emerged with the development of medium and high-entropy alloys. These novel materials offer a unique blend of hardness and flexibility not found in traditional alloys. The term “entropy” in their name denotes the level of disorder in the mixture of elements within these alloys.

In a groundbreaking feat, a research team led by UCLA has provided an unprecedented glimpse into the structure and characteristics of medium and high-entropy alloys. Leveraging advanced imaging techniques, they accomplished the first-ever three-dimensional mapping of the atomic coordinates within these alloys. Furthermore, they successfully correlated the mixture of elements with structural defects, a scientific breakthrough in material science.

Medium-entropy alloys combine three or four metals in roughly equal proportions, while high-entropy alloys combine five or more metals similarly. This is in stark contrast to conventional alloys, which are predominantly composed of one metal with other elements present in lower proportions. To illustrate the significance of the research, consider a blacksmith forging a sword. Surprisingly, small structural defects make metals and alloys tougher. As the blacksmith repeatedly heats a malleable metal bar until it glows and then rapidly cools it, structural defects accumulate, transforming the bar into a robust sword.

The research team focused on a particular type of structural defect called a twin boundary, known to be a crucial factor in the exceptional combination of toughness and flexibility exhibited by medium and high-entropy alloys. Twinning occurs when strain causes a section of a crystal matrix to bend diagonally while the surrounding atoms maintain their original arrangement, resulting in mirror images on either side of the boundary.

Creating these innovative alloys involved an extraordinary and rapid process reminiscent of the blacksmith’s craft. The scientists melted the metal at temperatures exceeding 2,000 degrees Fahrenheit for a fraction of a second and then rapidly cooled it. The objective was to solidify the alloy with the same diverse mixture of elements as in its liquid state. This process induced twin boundaries in six out of ten nanoparticles, with four of them each having a pair of twins.

Identifying these defects required the development of a specialized imaging technique called atomic electron tomography, which utilizes electrons due to the atomic-level details being much smaller than the wavelengths of visible light. The resulting data was mapped in three dimensions by capturing multiple images as the sample was rotated. Tuning atomic electron tomography to handle the complex mixtures of metals was a meticulous endeavor.

The researchers meticulously mapped each atom within the medium-entropy alloy nanoparticles. However, some of the metals in the high-entropy alloy were so similar in size that electron microscopy could not distinguish among them. Consequently, the map of these nanoparticles grouped the atoms into three categories.

The research findings revealed that the more atoms of different elements (or categories of elements) are mixed, the greater the likelihood that the alloy’s structure will change in a way that enhances its toughness and flexibility. These findings have the potential to inform the design of medium and high-entropy alloys, imparting added durability and unlocking properties not yet realized in traditional steel and alloys through the strategic engineering of element mixtures.

Studying defective materials typically necessitates the examination of each individual defect to understand its impact on the surrounding atoms. Atomic electron tomography is the only technique with the necessary resolution to achieve this. The ability to observe intricate atomic arrangements within such minuscule objects is truly remarkable.

To expand on their research, Miao and his colleagues are now developing a novel imaging method that combines atomic electron microscopy with a technique for identifying a sample’s composition based on emitted photons. This will enable the differentiation of metals with atoms of similar sizes. Additionally, they are working on ways to investigate bulk medium and high-entropy alloys and to unravel the fundamental relationships between their structures and properties.

This groundbreaking study was published in the prestigious journal Nature on December 20, 2023. It was made possible with support from the U.S. Department of Energy and conducted at Berkeley Lab’s Molecular Foundry, also sponsored by the DOE. 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 include 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 the University of Maryland, College Park.

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More about Alloy Engineering

• UCLA Newsroom Article
• Nature Journal Article