It has been known for decades that subtle chemical patterns exist in metal alloys, but researchers believed they were too minute to matter – or that they were removed during production. However, recent research has shown that under laboratory conditions, these patterns can change the properties of the metal, including its mechanical strength, durability, heat capacity, radiation tolerance, and more.
Now, MIT researchers have discovered that these chemical patterns also occur in conventionally produced metals. A surprising discovery has revealed a new physical phenomenon that explains the persistent patterns.
In article published in Nature communication Todayresearchers describe how they tracked these patterns and discovered the physics that explain them. The authors also developed a simple model for predicting chemical patterns in metals and show how engineers can use this model to tune the effects of such patterns on metal properties for applications in aerospace, semiconductors, nuclear reactors, and more.
“The bottom line is: you can never completely randomly arrange atoms in a metal. It doesn't matter how you process it,” says Rodrigo Freitas, TDK assistant professor at the Department of Materials Science and Engineering. “This is the first work to show non-equilibrium states that persist in metal. Right now, this chemical order is not something we control or pay attention to when producing metals.”
For Freitas, an early-career researcher, the findings demonstrate the need to explore a crowded field that he says few believed would lead to unique or broadly significant results. This is thanks to the United States Air Force Office of Scientific Research, which supported work under the Young Investigator program. He also credits the collaborative effort that made possible the paper, co-authored by three MIT graduate students: Mahmudul Islam, Yifan Cao, and Killian Sheriff.
“There was a question of whether I should even address this particular problem because people had been working on it for a long time,” Freitas says. “But the more I learned about it, the more I saw researchers thinking about it in idealized laboratory scenarios. We wanted to run simulations as realistic as possible to replicate manufacturing processes with high fidelity. My favorite part of this project is how non-intuitive the results are. The fact that you can't mix something completely is something people didn't expect.”
From surprises to theories
Freitas' research team started with a practical question: How quickly do chemical elements mix during metalworking? Conventional wisdom held that there is a point at which the chemical composition of metals becomes completely uniform through mixing during production. Finding this point, researchers thought they could develop a simple way to design alloys with different levels of atomic order, also known as short-range order.
Scientists used machine learning techniques to track millions of atoms as they moved and rearranged themselves under conditions that mimic metalworking.
“The first thing we did was deform a piece of metal,” Freitas explains. “This is a common manufacturing step: you roll the metal, deform it, reheat it, and deform it a little more to get the structure you want. We did this and traced the chemical order. The thought was that as you deform the material, its chemical bonds are broken, which causes the system to randomize. These rapid manufacturing processes essentially reshuffle the atoms.”
During the mixing process, the researchers encountered a hurdle: the rates never reached a fully random state. This was a surprise because no known physical mechanism could explain this result.
“This indicated a new element in metal physics,” the scientists write in the paper. “It was one of those cases where applied research led to a fundamental discovery.”
To discover new physics, researchers developed computational tools, including high-fidelity machine learning models that capture atomic interactions, as well as new statistical methods that quantify changes in chemical order over time. They then applied these tools to large-scale molecular dynamics simulations to track how the arrangement of atoms changed during processing.
Scientists have found some standard chemical systems in processed metals, but at higher temperatures than would normally be expected. Even more surprisingly, they discovered completely new chemical patterns that had never been seen outside of manufacturing processes. This is the first time such patterns have been observed. Scientists have called these patterns “far-from-equilibrium states.”
The researchers also built a simple model that reproduced key features of the simulation. The model explains how chemical patterns arise from defects called dislocations, which resemble three-dimensional scribbles in metal. As the metal deforms, the scribbles warp, mixing nearby atoms along the way. Previously, researchers thought that shuffling completely blurred the order in metals, but they discovered that dislocations favor some atomic swaps over others, resulting not in randomness but in subtle patterns that explain their findings.
“These defects have chemical preferences that direct their movement,” Freitas says. “They look for low-energy pathways, so given the choice between breaking chemical bonds, they tend to break the weakest bonds, and it's not completely random. This is very exciting because it's a state of imbalance: it's not something you see naturally in materials. In the same way, our bodies live in a state of imbalance. The temperature outside is always higher or lower than inside our body, and we maintain this state of balance to stay alive. That is why these states exist in metal: a balance between the internal tendency towards disorder and this ordering tendency to break certain bonds that are always weaker than others.
Application of new theory
Scientists are currently investigating how these chemical patterns develop under a wide range of production conditions. The result is a map linking different metal processing steps with different metal chemical formulas.
Until now, this chemical order and the properties it tunes have been largely considered an academic subject. The researchers hope that with this map, engineers will be able to start thinking about these patterns as design levers that can be pulled during production to achieve new properties.
“Scientists have been looking at the ways in which arrangements of atoms change the properties of metals — the most important of which is catalysis,” Freitas says of the process that drives chemical reactions. “Electrochemistry takes place on the metal surface and is very sensitive to the local atomic arrangements. There are other properties that are not affected by these factors. Another major problem is radiation damage. This affects the performance of these materials in nuclear reactors.”
Scientists have already told Freitas that the paper could help explain other surprising discoveries about the properties of metals, and he looks forward to the field moving from basic chemical research to more applied work.
“You can think about areas where you need very optimized alloys, such as in the aerospace industry,” Freitas says. “They care about very specific compositions. Advanced manufacturing now makes it possible to combine metals that would not normally mix through deformation. Understanding how the atoms actually mix and mingle in these processes is crucial because it is the key to gaining strength while maintaining low density. So this could be a huge deal for them.”
This work was supported in part by the U.S. Air Force Office of Scientific Research, MathWorks, and the MIT-Portugal program.
