Denser glasses relax faster: Enhanced atomic mobility and anomalous particle displacement under in-situ high pressure compression of metallic glasses

Despite that metallic glasses are among the most studied metallic materials, still very little is known on the evolution of their unique structural, dynamical and elastic properties under compression, owing to the difficulty to perform in-situ high pressure experiments. Coupling the brightest x-rays available in synchrotrons with cutting edge high pressure technologies, we provide direct evidence of the microscopic structural and dynamical mechanisms occurring under in-situ high pressure compression and decompression in the GPa range, from the onset of the perturbation up to a severely-deformed state. We show that while pressure promotes density increasing through quasi-elastic structural deformations, the atomic mobility exhibits a hysteresis and is enhanced up to a factor 15 even at temperatures well below the glass transition. This surprising behaviour results from a competition between fast avalanche-like atomic rearrangements and slow relaxation processes triggered by an anomalous super-diffusive collective particle displacement. These results provide new insights on the effect of deformation in non-ergodic materials and support the occurrence of string-like diffusion of liquid-like atoms in metallic glasses. They explain also the macroscopic impressive rejuvenation and strain hardening reported recently under ex-situ densifications.