Researchers recreate deep earth conditions to see how iron resists stress

Far below you is a sphere of solid iron and nickel about as wide as the widest part of Texas: the inner core of the Earth. The metal in the inner core is subjected to a pressure approximately 360 million times greater than that which we experience in our daily lives and at temperatures approximately as high as the surface of the Sun.

Fortunately, the planetary core of the Earth 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 nuclei after catastrophic impacts.

Measuring what happens during the collision of celestial bodies or at the heart of the Earth is obviously not very practical. As such, 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 National SLAC Accelerator Laboratory have now observed for the first time how the atomic structure of iron deforms to accommodate the stresses of the pressures and temperatures that occur just outside the body. inner core.

The results appears in Physical examination letters, where they were highlighted as an editor’s suggestion.

To manage stress

Most of the iron that you encounter in your daily life has its atoms arranged in nanoscopic cubes, with one iron atom at each corner and one in the center. If you squeeze these cubes using extremely high pressure, they rearrange themselves into hexagonal prisms, allowing the atoms to pile up 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 the iron in the heart of Earth or upon reentry from space. “We haven’t quite created the conditions for the inner core,” says co-author Arianna Gleason, a scientist in the High-Energy Density Science (HEDS) division of SLAC. “But we have reached the conditions of the outer core of the planet, which is truly remarkable.”

No one had ever directly observed iron’s response to stress under such high temperatures and pressures before, so researchers were unsure 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 find the most effective mechanism to do it.”

The coping mechanism iron uses to deal with this extra stress is called ‘pairing’. The arrangement of the atoms drifts to the side, rotating all of the hexagonal prisms almost 90 degrees. Twinning is a common pressure response in metals and minerals: quartz, calcite, titanium, and zirconium all undergo twinning.

“Pairing allows the iron to be incredibly strong – stronger than we initially thought – before it begins to plastically flow over much longer timescales,” Gleason said.

The story 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) free-electron laser, which allowed researchers to observe iron at an atomic level. “At the time, LCLS was the only institution in the world where you could do this,” explains lead author Sébastien Merkel from 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 directly above the experiment room,” Merkel explains. “When you trigger the discharge, you hear a loud pop. “

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 take a measurement in a billionth of a second,” says Gleason. “Freezing atoms where they are in this nanosecond is really exciting.”

The researchers collected these images and put them together in a flipbook that showed deformation of the iron. Until the end of the experiment, they weren’t sure whether the iron would react too fast for them to measure or too slow for them to see. “The fact that the match is happening on a timescale that we can measure is an important outcome in and of itself,” Merkel said.

The future is bright

This experiment serves as a bookend to understand the behavior of iron. Scientists had gathered experimental data on the structure of iron at lower temperatures and pressures and used it 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 some physical models a boost for some really fundamental strain mechanisms,” Gleason said. “It helps to strengthen some of the predictive capabilities we lack in modeling the reaction of materials under extreme conditions.”

The study provides interesting information on the structural properties of iron at extremely high temperatures and pressures. But the results are also a promising indicator that these methods could help scientists understand the behavior of other materials under extreme conditions as well.

“The future is bright now that we’ve developed a way to do these measurements,” says Gleason. “The recent upgrade of the X-ray inverter as part of the LCLS-II project allows for higher X-ray energies, allowing studies on thicker alloys and materials that have lower symmetry and more complex x-ray prints. “

The upgrade will also allow researchers to observe larger samples, which will give them a more complete view of the atomic behavior of iron and improve their statistics. In addition, “we are going to get more powerful optical lasers with approval to proceed with a new flagship petawatt laser facility, known as MEC-U,” Gleason said. “This will make future work even more exciting, as we will be able to access the conditions of the Earth’s inner core without any problems.”

– This press release was originally posted on the SLAC National Accelerator Laboratory website

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