Imagine dropping a tennis ball onto a bedroom mattress. The tennis ball will bend the mattress a bit, but not permanently – pick the ball back up, and the mattress returns to its original position and strength. Scientists call this an elastic state.
On the other hand, if you drop something heavy – like a refrigerator – the force pushes the mattress into what scientists call a plastic state. The plastic state, in this sense, is not the same as the plastic milk jug in your refrigerator, but rather a permanent rearrangement of the atomic structure of a material. When you remove the refrigerator, the mattress will be compressed and, well, uncomfortable, to say the least.
But a material’s elastic-plastic shift concerns more than mattress comfort. Understanding what happens to a material at the atomic level when it transitions from elastic to plastic under high pressures could allow scientists to design stronger materials for spacecraft and nuclear fusion experiments.
Up to now, scientists have struggled to capture clear images of a material’s transformation into plasticity, leaving them in the dark about what exactly tiny atoms are doing when they decide to leave their cozy elastic state and venture into the plastic world.
Now for the first time, scientists from the US Department of Energy’s SLAC National Accelerator Laboratory have captured high-resolution images of a tiny aluminum single-crystal sample as it transitioned from an elastic to a plastic state. These images will allow scientists to predict how a material behaves as it undergoes plastic transformation, within five trillionths of a second of the phenomena occurring. The team reports its results in Nature Communications.
To capture images of the aluminum crystal sample, the scientists first needed to apply a force – although a refrigerator was obviously too large. So instead, they used a high-energy laser, which hammered the crystal hard enough to push it from elastic to plastic.
As the laser generated shockwaves that compressed the crystal, the scientists sent a high-energy electron beam through it with SLAC’s speedy ‘electron camera’, or Megaelectronvolt Ultrafast Electron Diffraction (MeV-UED) instrument. This electron beam scattered off the aluminum nuclei and electrons in the crystal, allowing the scientists to measure its atomic structure precisely. They took multiple snapshots of the sample as the laser continued to compress it, and this string of images resulted in a sort of flip-book video – a stop-motion movie of the crystal’s dance into plasticity.
More specifically, the high-resolution snapshots showed the scientists when and how line defects appeared in the sample – the first sign that a material has been hit with a force too great to recover from.
Line defects are like broken strings on a tennis racket. For example, if you use your tennis racket to lightly hit a tennis ball, your racket’s strings will vibrate a bit, but return to their original position. However, if you hit a bowling ball with your racket, the strings will morph out of place, unable to bounce back. Similarly, as the high-energy laser struck the aluminum crystal sample, some rows of atoms in the crystal shifted out of place.
Tracking these shifts – the line defects – using MeV-UED’s electron camera showed the crystal’s elastic-to-plastic journey. The high-resolution images of the line defects revealed how fast the defects grow and how they move once they appear.
“Understanding the dynamics of plastic deformation will allow scientists to add artificial defects to a material’s lattice structure,” said SLAC scientist Mianzhen Mo. “These artificial defects can provide a protective barrier to keep materials from deforming at high pressures in extreme environments.”
Key to the rapid, clear images was MeV-UED’s high-energy electrons, which allowed the team to take sample images every half second.
“Most people are using relatively small electron energies in UED experiments, but we are using 100 times more energetic electrons in our experiment,” said Xijie Wang, a distinguished scientist at SLAC. “At high energy, you get more particles in a shorter pulse, which provides three-dimensional images of excellent quality and a more complete picture of the process.”
The researchers hope to apply their new understanding of plasticity to diverse scientific applications, such as strengthening materials used in high-temperature nuclear fusion experiments. A better understanding of material responses in extreme environments is urgently needed to predict the performance of materials in a future fusion reactor, said Siegfried Glenzer, director for high energy density science at SLAC.
“The success of this study will hopefully motivate implementing higher laser powers to test a larger variety of important materials,” he added.
The team is interested in testing materials for experiments that will be performed at the ITER Tokamak, a facility that hopes to be the first to produce sustained fusion energy.
This story is adapted from material from SLAC National Accelerator Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
“Understanding the dynamics of plastic deformation will allow scientists to add artificial defects to a material’s lattice structure. These artificial defects can provide a protective barrier to keep materials from deforming at high pressures in extreme environments.”Mianzhen Mo, SLAC National Accelerator Laboratory