New observations of the atomic structure of iron reveal that it undergoes “twinning” under extreme stresses and pressures.
Far below you is a sphere of solid iron and nickel about as wide as the widest part of Texas: the Earth’s inner core. The metal in the inner core is under pressure about 360 million times greater than what we experience in our daily lives and at temperatures about as hot as the surface of the Sun.
Fortunately, Earth’s planetary core is intact. But in space, similar nuclei can collide with other objects, causing the crystalline materials in the nucleus to deform rapidly. Some asteroids in our solar system are massive iron objects that scientists suspect are the remnants of planetary cores after catastrophic impacts.
Measuring what happens when celestial bodies collide or in the Earth’s core is obviously not very practical. Thus, much of our understanding of planetary cores is based on experimental studies of metals at less extreme temperatures and pressures. But researchers at the Department of Energy’s SLAC National Accelerator Laboratory have now observed for the first time how iron’s atomic structure deforms to accommodate the stress of pressures and temperatures occurring just outside. of the inner core.
The results appear in Physical examination letterswhere they have been highlighted as an editor’s suggestion.
To manage stress
Most of the iron you encounter in your daily life has its atoms arranged in nanoscopic cubes, with an iron atom at each corner and one in the center. If you squeeze these cubes by applying extremely high pressures, they rearrange into hexagonal prisms, allowing the atoms to pack together more tightly.
The SLAC group wanted to see what would happen if you continued to apply pressure to this hexagonal arrangement to mimic what happens to iron in the Earth’s core or during atmospheric re-entry from space. “We haven’t quite created the inner core conditions,” says co-author Arianna Gleason, a scientist in SLAC’s High Energy Density Science Division (HEDS). “But we’ve reached outer core conditions on the planet, which is really remarkable.”
No one had ever directly observed iron’s response to stress under such high temperatures and pressures before, so the researchers didn’t know how it would react. “As we keep pushing it, the iron doesn’t know what to do with that extra stress,” says Gleason. “And he has to relieve that stress, so he’s trying to figure out the most effective mechanism to do that.”
The coping mechanism iron uses to deal with this added stress is called “twinning”. The arrangement of atoms drifts sideways, rotating all of the hexagonal prisms nearly 90 degrees. Twinning is a common pressure response in metals and minerals – quartz, calcite, titanium, and zirconium all undergo twinning.
“The twinning allows the iron to be incredibly strong – stronger than we initially thought – before it starts to flow plastically over much longer time scales,” Gleason said.
A tale of two lasers
Achieving these extreme conditions required two types of lasers. The first was an optical laser, which generated a shock wave that subjected the iron sample to extremely high temperatures and pressures. The second was SLAC’s Linac Coherent Light Source (LCLS) X-ray laser, which allowed researchers to observe iron at the atomic level. “At the time, LCLS was the only facility in the world where you could do this,” says lead author Sébastien Merkel of the University of Lille in France. “It opened the door to other similar facilities around the world.”
The team fired the two lasers at a tiny sample of iron the width of a human hair, hitting the iron with a shock wave of heat and pressure. “The control room is just above the experiment room,” Merkel explains. “When you trigger the discharge, you hear a loud noise.”
When the shock wave hit the iron, the researchers used the X-ray laser to observe how the shock changed the arrangement of the iron atoms. “We were able to make a measurement in a billionth of a second,” says Gleason. “Freezing atoms where they are in that nanosecond is really exciting. »
The researchers collected these images and assembled them into a flipbook showing iron deformation. Before the end of the experiment, they didn’t know if the iron would react too fast for them to measure or too slow for them to see. “The fact that the twinning is happening on a timescale that we can measure is an important result in itself,” Merkel said.
The future is bright
This experiment serves as a bookend to understanding the behavior of iron. Scientists had gathered experimental data on the structure of iron at lower temperatures and pressures and used them to model the behavior of iron at extremely high temperatures and pressures, but no one had ever experimentally tested these models.
“Now we can give a thumbs up, a thumbs down to some of the physical models for the really fundamental deformation mechanisms,” Gleason says. “It helps develop some of the predictive ability we lack to model how materials react under extreme conditions.”
The study provides exciting insights into the structural properties of iron at extremely high temperatures and pressures. But the results are also a promising indicator that these methods could also help scientists understand how other materials behave under extreme conditions.
“The future is bright now that we’ve developed a way to make these measurements,” says Gleason. “The recent upgrade to the X-ray undulator as part of the LCLS-II project enables higher X-ray energies – enabling studies of alloys and thicker materials that have lower symmetry and indentations. more complex X-ray fingerprints.”
The upgrade will also allow researchers to observe larger samples, giving them a more complete view of the atomic behavior of iron and improving their statistics. Additionally, “we will be getting more powerful optical lasers with approval to proceed with a new flagship petawatt laser facility, known as MEC-U,” says Gleason. “This will make future work even more exciting because we can achieve Earth’s inner core conditions without any problems.”
Reference: “Femtosecond visualization of hcp iron strength and plasticity under shock compression” by Sébastien Merkel, Sovanndara Hok, Cynthia Bolme, Dylan Rittman, Kyle James Ramos, Benjamin Morrow, Hae Ja Lee, Bob Nagler, Eric Galtier, Eduardo Granados, Akel Hashim, Wendy L Mao and Arianna E Gleason, November 9, 2021, Physical examination letters.
Researchers from Los Alamos National Laboratory (LANL) contributed to this study. Funding was provided by the University of Lille, a research and development grant led by the LANL Reines laboratory and the DOE Office of Science, including Gleason’s DOE Early Career Award in Fusion Energy Science . LCLS is a user facility of the DOE Office of Science.