Metals retain hidden atomic order after processing

US-based researchers have shown that atomic organisation persists even after complex processing.

It has long been known that subtle patterns exist in metal alloys, although it was thought that manufacturing could remove their effects. However, recent studies have shown that in the lab these patterns can change a metal’s properties, including its mechanical strength, durability, heat capacity and radiation tolerance.

A paper published in Nature Communications describes how researchers at MIT have found persistence of the patterns in conventionally manufactured metals. The work revealed a new physical phenomenon that explains the persistent patterns.

‘The conclusion is: You can never completely randomise the atoms in a metal. It doesn’t matter how you process it,’ says Rodrigo Freitas, the TDK Assistant Professor in the 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.’

Freitas’ research team began by considering the speed at which elements mix during metal processing and when they became completely uniform. Thus, the researchers thought they could develop a simple way to design alloys with different levels of atomic order. They used machine-learning techniques to track millions of atoms under conditions that mimicked 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 randomises the system. These violent manufacturing processes essentially shuffle the atoms.’

However, the alloys never reached a fully random state and no known physical mechanism could explain the result.

Therefore, the researchers developed computational tools, including high-fidelity machine-learning models, to capture atomic interactions, along with statistical methods that quantify how chemical order changes over time. They then applied these tools in large-scale molecular dynamics simulations to track how atoms rearrange during processing.

The researchers found some standard chemical arrangements in their processed metals, but at higher temperatures than would normally be expected. Also, they found completely new chemical patterns never seen outside of manufacturing processes. The researchers referred to the patterns as ‘far-from-equilibrium states.’

A simple model was built that reproduced key features of the simulations. The model explains how the patterns arise from dislocations. It was found that dislocations favour some atomic swaps over others, resulting in the patterns seen.

The researchers are now exploring how these chemical patterns develop across a range of manufacturing conditions. The result is a map that links various metal processing steps to different chemical patterns in metal. The hope is that engineers can begin thinking of these patterns as levers in design that can be pulled during production to get new properties, examples include catalysis and radiation damage response.

Future applications could include areas where you need specific 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.’