Scientists study metals at atomic level

Hoping to gain insights into processes that would create longer-lasting batteries and lighter vehicles, scientists at Pacific Northwest National Laboratory in Washington are investigating the effects of physical forces on metals at the atomic level as they undergo shear deformation.

Shear deformation occurs when an object changes shape when forces are applied to it but not in the way of just becoming longer or shorter.

The PNNL researchers are taking a direct look at changes in the metals at the atomic level as they are subjected to forces. These changes rearrange the atoms in metals, but not in the same way for every metal or alloy.

Atomic arrangement can affect a metal's properties, such as strength, formability, and conductivity. Thus, gaining a better understanding of how atoms in metals move when they are subjected to physical forces during shear has become a key part of ongoing efforts by materials scientists to custom design next-generation metals with specific properties from the atom up.

In a recent statement, the researchers said these visualizations form the foundation for understanding how shear deformation creates the improved characteristics observed in metals produced using shear assisted processing and extrusion (ShAPE), a PNNL innovation in metals manufacturing. During ShAPE manufacturing, metals are processed using shear forces to produce high-performance metal alloys for use in vehicles, batteries, and other applications.

Chongming Wang, PNNL Laboratory fellow and leader of the research team studying the forces of induced shear deformation, explained that the forces that are purposefully applied during metal manufacturing to create alloys are the same forces that can damage structures inside batteries to cause eventual failure.

Researchers also know that shear deformation can fundamentally alter the microstructure of metals in ways that can improve the material – making metals stronger, lighter, and more flexible.

But how that happens is still a mystery.

"If you were to snap a picture of a track runner at the start and end of their run, you might think they didn't move at all," explained Arun Devaraj, PNNL materials scientist. "But if you film the runner while they are going around the track, you will know just how far they traveled. It is the same here. If we understand exactly what happens to metals on the atomic level during shear deformation, we could apply that knowledge strategically to design materials with specific properties."

Looking at gold, copper

To watch how shear deformation rearranges metal atoms, the researchers used a specialized probe inside a transmission electron microscope. PNNL is one of only a handful of laboratories with this capability in the world. The microscope is used to record how individual rows of atoms within metals move during shear deformation.

The team began by looking at gold – the standard because it is easiest to visualize on an atomic level.

As the researchers watched gold undergoing shear, they saw that crystals of gold split into smaller grains. They also noticed that natural defects in the arrangement of gold atoms changed how shear deformation moved the atoms. This is useful information because defects are common in metals during deformation but do not behave the same in all metals, which can directly affect metal properties.

"The defects in crystal, grain size and microstructure in a metal can affect the metal's characteristics, like strength and toughness. That's why it's important to understand how shear deformation moves metal atoms around and affects the overall microstructure of the metal," said Shuang Li, postdoctoral research associate at PNNL and first author on three studies sharing results of the study.

Next, the research team experimented with copper. They observed how shear deformation creates nanotwins or structural features that make metals stronger. Observing an alloy of copper and niobium, they found that shear deformation affects atoms differently inside the copper and niobium phases of the metal mixture.

This is a valuable insight that can inform how to manufacture alloys with specific properties using shear deformation, they said.

The information gained from studying how these forces affect metals during controlled manufacturing processes can be directly translated and applied wherever metal experiences the same physical forces, the scientists explained.

"From the real-time observation, we discover that the phase boundary with a zigzag morphology effectively blocks stacking faults nucleated in a copper grain from slipping into a niobium grain," the researchers observed. "This work sheds light on understanding the shear deformation and the behavior of phase boundaries in multiphase alloys during shear deformation."

Countless applications

The atomic-level visualization capability at PNNL is also useful for understanding how materials used in extreme conditions (e.g., nuclear reactors) or clean energy applications (e.g., hydrogen transmission lines and storage tanks) will respond to external stresses.

Longer-lasting batteries, lighter alloys for more efficient vehicles, and custom design of next-generation metals with improved strength and conductivity could all be possible by better understanding the atomic physics of metals manufacturing, according to the PPNL researchers.

"If we understand what happens to metals on an atomic level during shear deformation, we can use that knowledge to improve countless other applications where metals experience those same forces, from improving battery life to designing metals with specific properties, like lighter, stronger alloys for more efficient vehicles," said Wang

The work was supported by the Laboratory Directed Research and Development program at PNNL as part of the Solid Phase Processing Science Initiative. A portion of this research was performed using facilities at EMSL, the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy, Office of Science, Biological and Environmental Research program also located at PNNL.

Scientific articles on the studies appear in three research publications: In-situ TEM observation of shear induced microstructure evolution in copper-niobium alloy in the journal Scripta Materialia, Nanotwin assisted reversible formation of low angle grain boundary upon reciprocating shear load in the journal Acta Materialia, and In-situ observation of deformation twin associated sub-grain boundary formation in copper single crystal under bending in Materials Research Letters.