LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. Around 60 years ago a theoretical limit was set for the movement of dislocations in materials. Modelling and atomistic simulation work has suggested that this limit may not be strictly true. Now, for the first time, experimental work has shown that the simulations are correct.
KENTO KATAGIRI: The question was: Can dislocations propagate faster than the soundwave speed? Even in textbooks it is written that these dislocation velocities do not go over the transverse wave speed of a material.
LAURA LEAY: That was Dr. Kento Katagiri from Stanford University. His interest is in laser shock compression where a high intensity laser causes compression of the material. This leads to extreme pressures, greater than a terrapascal. His research has shown that the sudden shock imparted by the laser means that dislocations are transonic: in diamond they travel faster than the slowest transverse sound speed.
KENTO KATAGIRI: It is known: dislocations cannot travel at soundwave speed. If you slowly increase the speed, the soundwave speed becomes a barrier that we cannot overcome. But if you use a shockwave, because the shockwave front is a discontinuous wave with a strong energy jump, it can jump to the transonic region.
LAURA LEAY: Dr Katagiri’s team looked at diamond, a highly stable material that doesn’t undergo phase transformation, allowing them to look at purely plastic deformation. His team used an X-ray free electron laser, also known as an XFEL, to collect data from the diamond as it was hit with an intense shock from an additional drive laser oriented at 90 degrees to the XFEL beam. The XFEL was important as it produces intense, short pulses of X-rays to collect frames of data from a single shot of the laser providing the shock. The femtosecond-pulses of the XFEL are necessary to see the ultrafast phenomena that occur on a tiny scale.
KENTO KATAGIRI: Laser shock community has usually used something called velocimetry, which uses optical laser—green light laser—and irradiated from the other side of the drive laser to track the speed of these wavefronts. In this case, we are pretty much the first ones to apply X-ray radiography on laser shock compression experiments to visualize something behind the wavefront.
LAURA LEAY: When the diamond was oriented so that the shock from the drive laser travels down a certain crystallographic axis, remarkably clear V-shaped bands were observed. These shapes correspond to stacking faults. Compiling data collected with a time resolution of femtoseconds allowed the team to determine the velocity of propagation of these features and figure out that they are moving at 16-18 km/s, firmly putting them in the transonic region. To collect these data, the team made some innovations. Usually a CCD—or Charge-Coupled Device—is used to collect images that result from the XFEL beam passing through the sample but this type of detector would be damaged because it needs to be located close to sample, and therefore close to the shock laser. Instead, a new detector that uses a lithium fluoride crystal was developed which makes use of color centers that form in its structure as a result of the XFEL beam emerging from the sample of diamond. To help visualize the results, the sample also had to be carefully aligned.
KENTO KATAGIRI: I wasn’t expecting to see these lines. Somehow I just came up with the idea of rotating the crystal 90 degrees. It was lucky to find these V-shaped lines. The idea was we aligned these slip planes along the XFEL beam.
LAURA LEAY: The propagation of waves through a material is familiar to disciplines such as geology and seismology but Dr. Katagiri’s work is most applicable to extreme shock events such as missile strikes and shuttle launches where pressures of one terapascal or more might apply. These pressures can also occur inside stars or during meteor strikes. The results are relevant to a type of nuclear fusion known as Inertial Confinement Fusion, which uses intense lasers to compress the fuel. Now that one theoretical limit has been exceeded, Dr. Katagiri wants to push things even further.
KENTO KATAGIRI: The question is, where’s the top limit? Is it actually the longitudinal wave or is there another thing above the longitudinal wave in the supersonic region or not?
LAURA LEAY: This work was published in a recent issue of Science. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.