Defects can make a material stronger or cause it to fail catastrophically. Understanding how quickly defects propagate helps researchers understand problems such as seismic rupture, structural failure and precision manufacturing. After half a century of debate, researchers have discovered that tiny linear defects can propagate through materials faster than sound waves.
Illustration: An intense laser pulse hits a diamond crystal from the upper right, creating elastic and plastic waves (curved lines) in the material. The laser pulse creates linear defects, called dislocations, where it hits the crystal. They travel through the material faster than the transverse speed of sound, leaving behind accumulation surfaces - lines that fan out from the point of impact. Source: Greg Stewart/SLAC National Accelerator Laboratory
These linear defects, or dislocations, give the metal its strength and workability, but they can also cause the material to fail catastrophically—something that happens every time you open the tab on a can of soda.
The research was led by Leora Dresselhaus-Marais, a professor at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, and Norimasa Ozaki, a professor at Osaka University.
As the shock waves travel through the material, they create defects called dislocations -- tiny displacements in the material's crystals that propagate through them and leave behind what are called stacked faults. In the picture on the left, the regular arrangement of the material's atoms is undisturbed. In the image on the right, dislocations travel through the material from left to right, creating stacked faults (purple) where adjacent crystal layers do not line up the way they should. Image credit: Greg Stewart/SLAC National Accelerator Laboratory
So far, no one has been able to directly measure how quickly these dislocations spread through the material. Her team used X-ray radiography - similar to the medical X-rays used to reveal the inside of the human body - to time how quickly dislocations propagate in diamond, lessons learned that should apply to other materials as well. They described this result in a paper published in Science on October 5.
Chasing the speed of sound
For nearly 60 years, scientists have debated whether dislocations can propagate through materials faster than sound. Many studies conclude that they cannot. But some computer models suggest they can - if they start moving faster than sound.
It takes a huge amount of impact to get them to that speed instantly. First, sound travels much faster in solid materials than in air or water, depending on factors such as the properties of the material and temperature. The speed of sound in air is generally 761 miles per hour, while the speed of sound in water is 3,355 miles per hour. In diamond, the hardest material, the speed of sound reaches an astonishing 40,000 miles per hour.
To make things more complicated, there are two types of sound waves in solids. Longitudinal waves are the same as sound waves in the air. But because solids offer some resistance to the propagation of sound, they also carry slower-moving sound waves, known as transverse sound waves.
It is important from both a basic science and a practical perspective to understand whether ultrafast dislocations can break both sound barriers. When dislocations move faster than the speed of sound, they behave very differently and cause unexpected failures that have so far only been modeled. Without measurements, no one knows how damaging these ultrafast dislocations can be.
"It wouldn't be great if a structural material failed more catastrophically than people expected because of its high failure rate," said Taketo Katagiri, a postdoctoral scholar in the research group and the paper's lead author. "For example, if a fault breaks through the rock during an earthquake, that could cause more damage to everything. We need to know more about these types of catastrophic failures."
Dresshaus-Marais added that the study's results "may suggest that we were wrong about what we thought we knew about the fastest possible material failure."
Top effect
To obtain the first direct image of the speed at which dislocations move, Dresshaus-Marais and her colleagues carried out experiments at Japan's SACLAX free-electron laser. They conducted experiments on tiny crystals of artificial diamond.
To obtain the first direct image of the speed at which dislocations propagate, the researchers used an intense laser beam to drive a shock wave through a diamond crystal. They then used an X-ray laser beam to take a series of X-ray images of dislocation formation and diffusion on a time scale of billionths of a second. The images, similar to medical X-rays that reveal the inside of the human body, are recorded on a detector. Image credit: K.Katagiri/Stanford University
Katagiri said diamond provides a unique platform for studying how crystalline materials fail. "To understand the damage mechanism, we need to identify clear dislocation signatures in the images, rather than other types of defects," he said.
When two dislocations meet, they attract or repel each other and create more dislocations. Open a can of soda made from an aluminum alloy, and the many dislocations already present in the lid—dislocations created when it was molded into its final shape—interact and create trillions of new dislocations, which cascade into absolutely critical failure when the top of the can bends and the lid snaps open. These interactions and the way they behave determine all the mechanical properties of the material that we observe.
"In diamond, there are only four types of dislocations, whereas in iron, for example, there are 144 different types of dislocations," Dreshaus-Maris said. Diamonds may be harder than metals, the researchers said. But like a soda can, if it receives a big enough impact, it will still bend by forming billions of dislocations.
Producing X-ray images of shock waves
At SACLA, the research team used powerful lasers to generate shock waves in diamond crystals. They then took a series of ultrafast X-ray images of dislocation formation and diffusion on a time scale of one billionth of a second. Only X-ray free electron lasers can deliver X-ray pulses short and bright enough to capture this process.
The initial shock wave splits into two types of waves that continue through the crystal. The first type of wave, called an elastic wave, temporarily deforms the crystal; the atoms in the crystal immediately snap back to their original positions, like a rubber band that is stretched and then released. The second wave, called a plastic wave, permanently deforms the crystal by creating tiny errors in the repeating patterns of atoms that make up the crystal structure.
This X-ray radiographic image (similar to medical X-rays but taken at ultra-fast speeds using an X-ray laser) shows shock waves passing through a diamond crystal. The initial wave is elastic. Plastic waves follow, creating defects in the material called dislocations, which travel through the material faster than the speed of sound. The arrow shows the path and direction of a dislocation that leaves behind a linear defect called a stack fault. The dislocation itself can be seen at the top of the arrow. Other accumulation faults are visible from the location of the laser strike. Source: K.Katagiri/Stanford University
These tiny shifts, or misalignments, create "stacking faults," in which adjacent layers of the crystal shift away from each other so they don't line up the way they should. Stacking faults propagate outward from where the laser hits the diamond, with a mobile dislocation at the front of each stacking fault.
Using X-rays, the researchers discovered that dislocations propagate in diamond faster than slower sound waves - shear waves - a phenomenon that has never been seen in any material before.
Now, Katagiri said, the team plans to return to X-ray free electron facilities, such as SACLA or SLAC's Linac Coherent Light Source (LCLS), to see whether dislocations can propagate in diamond faster than higher longitudinal sound velocities, which would require more powerful laser strikes. If they broke the sound barrier, he said, they would be considered truly supersonic.