Synopsis

Observing Iron Under Pressure

Physics 14, s141
Femtosecond-resolved x-ray diffraction images of iron’s crystals as they deform under an extreme load show that the material’s elastic-plastic transition comes after a surprisingly long elastic phase.  
S. Merkel et al. [1]

Knowing how materials respond to extreme loads is vital to understanding phenomena from debris impacts in jet engines to planetary formation. But experimental challenges in capturing material responses mean empirical data are patchy; instead, material behaviors must generally be predicted theoretically. Now, Sébastien Merkel at the University of Lille, France, and colleagues have provided much-needed ground truth for theory by observing directly how the crystal structure of iron evolves as it deforms at high strain rates [1].

The team fixed a 50-𝜇m-thick polymer film to a 25-𝜇m-thick iron foil and blasted the polymer with a 12-ns laser pulse to send a shock wave into the iron. For each sample, the researchers used femtosecond x-ray diffraction to capture a single snapshot of the structure and orientation of the iron’s crystals and the stress imparted by the shock wave. By varying the pulse-measurement interval over a series of experiments, they built a time-resolved record of how the stress developed and the iron crystals deformed.

Initially, the shock wave changed the iron’s structure from body-centered-cubic to hexagonal-close-packed, something the team expected to happen. The hexagonal structure then deformed elastically for several nanoseconds before yielding, after which it accommodated strain by rearranging itself into pairs of twinned crystals—a process that continued even after the stress had fallen below the yield stress. Both the time to yielding and the mechanism were previously unknown. Merkel and his colleagues attribute the observed “elastic overshoot” to the relatively slow buildup of twinning nuclei during the elastic phase: only when enough had accumulated could deformation via twinning begin.

–Marric Stephens

Marric Stephens is a Corresponding Editor for Physics based in Bristol, UK.

References

  1. S. Merkel et al., “Femtosecond visualization of hcp-iron strength and plasticity under shock compression,” Phys. Rev. Lett. 127, 205501 (2021).

Subject Areas

Materials ScienceCondensed Matter Physics

Related Articles

How to Cut into a Material More Smoothly
Materials Science

How to Cut into a Material More Smoothly

A theory confirmed by experiments explains what has been an unpredictable cutting process. Read More »

New Moiré Landscapes for Atomic Spins
Spintronics

New Moiré Landscapes for Atomic Spins

The interactions of the spins of single atoms with a substrate can be controlled via the moiré lattice created by depositing a 2D material on top of the substrate. Read More »

Three-In-One X-Ray Imaging
Materials Science

Three-In-One X-Ray Imaging

Researchers have developed a technique for simultaneously monitoring the attenuation, phase shift, and dark-field scattering of an x-ray beam as it passes through a melting metal powder. Read More »

More Articles