This atomic map of a high-entropy alloy nanoparticle shows different categories of elements in red, blue and green, and twinning boundaries in yellow. Image: Miao Lab/UCLA.

Alloys are materials such as steel that are made by combining two or more metallic elements. They are an essential components of buildings, transportation, appliances, tools and electronic devices. But in applying alloys, engineers have faced an age-old trade-off common to most materials: alloys that are hard tend to be brittle and break under strain, while those that are flexible under strain tend to dent easily.

Possibilities for sidestepping this trade-off arose about 20 years ago, when researchers first developed medium- and high-entropy alloys, stable materials that combine hardness and flexibility in a way that conventional alloys do not. (The ‘entropy’ in the name indicates how disorderly the mixture of the elements in the alloys is.)
 


Now, a team led by researchers at the University of California, Los Angeles (UCLA) has provided an unprecedented view of the structure and characteristics of medium- and high-entropy alloys. Using an advanced imaging technique, the team mapped, for the first time ever, the three-dimensional (3D) atomic coordinates of such alloys. In another scientific first for any material, the researchers correlated the mixture of elements with structural defects.

“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 Jianwei ‘John’ Miao, professor of physics in the UCLA College, member of the California NanoSystems Institute at UCLA and corresponding author of a paper on this work in Nature. “We found a new knob that can be turned to boost alloys’ toughness and flexibility.”
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.)