MIT scientists find metals hold secret atomic patterns

Researchers at MIT have now discovered that these subtle chemical arrangements also persist in metals made through standard industrial processes. The unexpected finding points to a new physical principle that explains why these patterns remain.

In a study published in Nature Communications, the MIT team detailed how they identified and analyzed the patterns, uncovering the physics that drives them. They also developed a model that predicts how these patterns form, allowing engineers to potentially adjust them to fine-tune a metal's properties for use in aerospace, semiconductor, or nuclear applications.

"The conclusion is: You can never completely randomize the atoms in a metal. It doesn't matter how you process it," explains Rodrigo Freitas, the TDK Assistant Professor in the MIT Department of Materials Science and Engineering. "This is the first paper showing these non-equilibrium states that are retained in the metal. Right now, this chemical order is not something we're controlling for or paying attention to when we manufacture metals."

For Freitas, an early-career researcher, the discovery validates his decision to pursue a problem many others thought was already settled. He credits support from the U.S. Air Force Office of Scientific Research's Young Investigator Program and the collaborative effort of his team, which includes three MIT PhD students -- Mahmudul Islam, Yifan Cao, and Killian Sheriff -- as co-first authors.

"There was the question of whether I should even be tackling this specific problem because people have been working on it for a long time," Freitas says. "But the more I learned about it, the more I saw researchers were thinking about this in idealized laboratory scenarios. We wanted to perform simulations that were as realistic as possible to reproduce these manufacturing processes with high fidelity. My favorite part of this project is how non-intuitive the findings are. The fact that you cannot completely mix something together, people didn't see that coming."

From surprises to theories

Freitas and his team began with a simple question: how quickly do elements mix during the processing of metals? Conventional thinking suggested that there comes a point where metals become completely uniform at the atomic level during manufacturing. Finding that point, they believed, could help design alloys with varying levels of short-range atomic order.

Using advanced machine-learning tools, the researchers simulated how millions of atoms moved and rearranged during metal processing.

"The first thing we did was to deform a piece of metal," Freitas explains. "That's a common step during manufacturing: You roll the metal and deform it and heat it up again and deform it a little more, so it develops the structure you want. We did that and we tracked chemical order. The thought was as you deform the material, its chemical bonds are broken and that randomizes the system. These violent manufacturing processes essentially shuffle the atoms."

Yet the metals didn't behave as expected. Despite extreme processing, the alloys never reached a completely random state. The result puzzled the team since no existing theory could account for it.

"It pointed to a new piece of physics in metals," the researchers write in the paper. "It was one of those cases where applied research led to a fundamental discovery."

To explore further, they built high-precision computational models to capture how atoms interact and statistical methods to measure how order evolves over time. Through large-scale molecular dynamics simulations, they watched how atoms reorganized during deformation and heating.

The team observed that certain atomic arrangements appeared at unexpectedly high temperatures, and even more remarkably, entirely new patterns emerged that had never been seen outside of real-world manufacturing. They described these patterns as "far-from-equilibrium states."

They then developed a simplified model to reproduce the main features of the simulations. The model revealed that these patterns originate from defects in metals known as dislocations -- irregular, three-dimensional distortions in the atomic lattice. When the metal is deformed, dislocations twist and shift, nudging nearby atoms into preferred positions. Previously, researchers thought this process destroyed all atomic order, but the MIT team found the opposite: dislocations actually favor certain atomic exchanges, creating subtle but stable patterns.

"These defects have chemical preferences that guide how they move," Freitas says. "They look for low energy pathways, so given a 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 non-equilibrium state: It's not something you'd see naturally occurring in materials. It's the same way our bodies live in non-equilibrium. The temperature outside is always hotter or colder than our bodies, and we're maintaining that steady state equilibrium to stay alive. That's why these states exist in metal: the balance between an internal push toward disorder plus this ordering tendency of breaking certain bonds that are always weaker than others."

Applying a new theory

The researchers are now exploring how these chemical patterns develop across a wide range of manufacturing conditions. The result is a map that links various metal processing steps to different chemical patterns in metal.

To date, this chemical order and the properties they tune have been largely considered an academic subject. With this map, the researchers hope engineers can begin thinking of these patterns as levers in design that can be pulled during production to get new properties.

"Researchers have been looking at the ways these atomic arrangements change metallic properties -- a big one is catalysis," Freitas says of the process that drives chemical reactions. "Electrochemistry happens at the surface of the metal, and it's very sensitive to local atomic arrangements. And there have been other properties that you wouldn't think would be influenced by these factors. Radiation damage is another big one. That affects these materials' performance in nuclear reactors."

Researchers have already told Freitas the paper could help explain other surprise findings about metallic properties, and he's excited for the field to move from fundamental research into chemical order to more applied work.

"You can think of areas where you need very optimized alloys like aerospace," Freitas says. "They care about very specific compositions. Advanced manufacturing now makes it possible to combine metals that normally wouldn't mix through deformation. Understanding how atoms actually shuffle and mix in those processes is crucial, because it's the key to gaining strength while still keeping the 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.