Nov 15 2017
Materials can change their shape when hit by a powerful shock wave – a property called plasticity – yet they keep their lattice-like atomic structure. Scientists have now used the X-ray laser at the Department of Energy’s SLAC National Accelerator Laboratory to observe, for the first time, how a material’s atomic structure gets deformed when shocked by pressures almost as extreme as the ones at the center of the Earth.
This image depicts an experimental setup at SLAC’s Linac Coherent Light Source, where a tantalum sample is shocked by a laser and probed by an X-ray beam. The resulting diffraction patterns, collected by an array of detectors, show the material undergoes a particular type of plastic deformation called twinning. The background illustration shows a lattice structure that has created twins. (Ryan Chen/LLNL)
The researchers stated that this new way of watching plastic deformation as it takes place can help study an extensive range of phenomena, such as the effects of bullets, meteor impacts and other penetrating projectiles and superior-performance ceramics used in armor, and also how to protect spacecraft from high-speed dust impacts and even how dust clouds develop between the stars.
The experiments were performed at the Matter in Extreme Condition (MEC) experimental station at SLAC’s Linac Coherent Light Source (LCLS). They were headed by Chris Wehrenberg, a Physicist at the DOE’s Lawrence Livermore National Laboratory, and described in a recent paper published in Nature.
“People have been creating these really high-pressure states for decades, but what they didn't know until MEC came online is exactly how these high pressures change materials – what drives the change and how the material deforms,” said SLAC staff scientist Bob Nagler, a co-author of the report.
LCLS is so powerful, with so many X-rays in such a short time, that it can interrogate how the material is changing while it is changing. The material changes in just one-tenth of a billionth of a second, and LCLS can deliver enough X-rays to capture information about those changes in a much shorter time that that.
Bob Nagler, Co-author of the Report and Staff Scientist, SLAC
Elusive Atomic Deformations
The material they analyzed here was a thin foil made up of tantalum, a blue-gray metallic element whose atoms are organized in cubes. The team employed a polycrystalline form of tantalum that is naturally textured such that the orientation of these cubes changes little from place to place, making it easier to see specific types of disruptions from the shock.
When squeezing this type of crystalline material by a powerful shock, it is capable of deforming in two different ways: twinning, where tiny regions develop lattice structures that are the mirror images of the ones in surrounding areas, and slip deformation, where a section of the lattice shifts leading to spreading of the displacement, like a propagating crack.
However, while these two mechanisms are basically vital in plasticity, it is hard to observe them as a shock take place. Earlier research studied shocked materials after the fact, as the material recovered, which brought about complications and resulted in conflicting interpretations.
The Tremendous Shock of a Tiny Recoil
The scientists shocked a piece of tantalum foil with a pulse from an optical laser in this experiment. This vaporizes a tiny piece of the foil into a hot plasma that flies away from the surface. The recoil from this small plume produces remarkable pressures in the remaining foil – up to 300 gigapascals, which is three million times the atmospheric pressure present around us and comparable to the 350-gigapascal pressure at the center of the Earth, Nagler said.
While this was taking place, researchers investigated the state of the metal with X-ray laser pulses. The pulses are very short – just 50 femtoseconds, or millionths of a billionth of a second, long – and like a camera with an extremely fast shutter speed they can record the metal’s response in a thorough manner.
The X-rays bounce off the metal’s atoms and then into a detector, where they produce a “diffraction pattern” – a series of concentric, bright rings – that scientists examine in order to determine the atomic structure of the sample. X-ray diffraction has been employed for decades in order to discover the structures of biomolecules, materials and other samples and also to observe how those structures differ, but only recently it has been used for studying plasticity in shock-compressed materials, Wehrenberg said.
This time the researchers moved the technique one step further: They examined not just the diffraction patterns, but also how the scattering signals were distributed within individual diffraction rings and how their distribution differed over time. This deeper level of analysis exposed changes in the tantalum’s lattice orientation, or texture, happening in about one-tenth of a billionth of a second.
It also presented whether the lattice was experiencing twinning or slip over an extensive range of shock pressures – right up to the point where the metal melts. The team learnt that as the pressure increased, the dominant type of deformation differed from twinning to slip deformation.
Wehrenberg stated that the results of this study are applicable directly to Lawrence Livermore’s efforts to model both tantalum and plasticity at the molecular level.
These experiments, he said, “are providing data that the models can be directly compared to for benchmarking or validation. In the future, we plan to coordinate these experimental efforts with related experiments on LLNL’s National Ignition Facility that study plasticity at even higher pressures.”
In addition to SLAC and LLNL, researchers from the University of Oxford, the DOE’s Los Alamos National Laboratory and the University of York contributed to this study. The DOE Office of Science provided funding for the work at SLAC. LCLS is a DOE Office of Science User Facility.