UCLA has made a new breakthrough in the field of alloy research, proposing for the first time three-dimensional mapping of medium-entropy alloys and high-entropy alloys. These materials have greater flexibility and have the potential to bring revolutionary changes to the field. Alloys are materials composed of two or more metallic elements, such as steel, and are one of the foundations of contemporary life. They are basic materials for construction, transportation, appliances and tools, most likely including the equipment you are using as you read this story.
In applying alloys, engineers face an age-old trade-off common in most materials: Harder alloys tend to break brittle under stress, while elastic alloys tend to dent under stress.
About 20 years ago, researchers first developed medium-entropy and high-entropy alloys, stable materials that combine hardness and flexibility not found in conventional alloys. (The "entropy" in the name refers to the degree of disorder in the mixture of elements in the alloy).
Now, a UCLA-led research team has provided an unprecedented look at the structure and properties of medium- and high-entropy alloys. The research team used advanced imaging technology to map the three-dimensional atomic coordinates of this type of alloy for the first time. In another scientific first for any material, researchers have linked mixtures of elements to structural defects.
"Medium-entropy and high-entropy alloys have been imaged at the atomic scale before in two-dimensional projections, but this study is the first to directly observe their three-dimensional atomic order," said corresponding author John Miao, professor of physics at UCLA and a member of the California NanoSystems Institute. "We have discovered a new 'knob' that improves the alloy's toughness and flexibility."
Medium-entropy alloys combine three or four metals in roughly equal amounts; high-entropy alloys combine five or more metals in equal amounts. In contrast, traditional alloys are primarily one metal mixed with other metals in lower proportions. (For example, stainless steel is three-quarters or more iron).
To understand what the scientists discovered, imagine a blacksmith forging a sword. Blacksmiths' forging work follows the counter-intuitive fact that tiny structural flaws actually make metals and alloys stronger. When a blacksmith repeatedly heats a soft, stretchy metal rod until it glows and then quenches it in water, structural flaws accumulate, helping to turn the rod into an incredibly hard sword.
Miao and his colleagues focused on a structural defect called a twin boundary, which is understood to be a key factor in the unique combination of toughness and flexibility of medium-entropy and high-entropy alloys. Twins occur when strain causes one part of the crystal matrix to bend diagonally, while the surrounding atoms maintain their original configuration, forming mirror images on either side of the boundary.
The researchers used a range of metals to create nanoparticles, which are so small that they can be measured in billionths of a meter. Six medium-entropy alloy nanoparticles combine nickel, palladium and platinum. Four high-entropy alloy nanoparticles combine cobalt, nickel, ruthenium, rhodium, palladium, silver, iridium and platinum.
The process of creating these alloys is similar to an extreme and speedy version of a blacksmith's task. The scientists liquefied the metal at temperatures of more than 2,000 degrees Fahrenheit for five-hundredths of a second, then cooled it down in less than a tenth of a second. The purpose of this is to anchor the solid alloy in the same mixture of elements as the liquid alloy. During this process, 6 out of 10 nanoparticles were impacted and twinned boundaries were created; 4 of them each had a pair of twin particles.
Identifying the defects required researchers to develop an imaging technique called atomic electron tomography. The technique uses electrons because atomic-level details are much smaller than visible light wavelengths. Because multiple images are captured as the sample rotates, three-dimensional data can be plotted. Adapting atomic electron tomography to map complex mixtures of metals is painstaking work.
"Our goal is to find the truth about nature, and our measurements must be as precise as possible," said Miao, who is also deputy director of the STROBE National Science Foundation Science and Technology Center. "We work slowly, push the limits, make each step as perfect as possible, and then move on to the next step."
Scientists have mapped every atom in a medium-entropy alloy nanoparticle. Some of the metals in high-entropy alloys are too similar in size for electron microscopy to distinguish them. The map of these nanoparticles therefore divides the atoms into three categories.
The researchers observed that the more atoms of different elements (or different classes of elements) were mixed together, the more likely the alloy's structure was to change, helping to match toughness with flexibility. The findings could inform the design of medium- and high-entropy alloys with greater durability, and could even engineer blends of certain elements to unlock potential properties currently undiscovered in steel and other traditional alloys.
"The problem with studying defective materials is that you have to look at each defect individually to really understand its impact on surrounding atoms," said co-author Peter Ercius, a scientist at Lawrence Berkeley National Laboratory's Molecular Foundry. "Atomic electron tomography is the only technique with this kind of resolution. It's amazing that we can see the chaotic arrangement of atoms at this scale inside something so small."
Miao and his colleagues are currently developing a new imaging method that combines atomic electron microscopy with a technique that identifies the makeup of a sample based on photon emission to distinguish metals with similar atomic sizes. They are also developing methods to examine bulk medium- and high-entropy alloys to understand the fundamental relationship between their structure and properties.
The research was published today (December 20) in the journal Nature.
DOI:10.1038/s41586-023-06785-z
Compiled source: ScitechDaily