X-ray photon correlation spectroscopy revealing the change of relaxation dynamics of a severely deformed Pd-based bulk metallic glass

The influence of severe plastic deformation on the relaxation dynamics of a Pd40Ni40P20 (at.%) bulk metallic glass was investigated on the atomic length scale by X-ray photon correlation spectroscopy and fluctuation electron microscopy. At a series of isothermal temperatures adjusted by step heating below the glass transition, the relaxation times were obtained by X-ray photon correlation spectroscopy and the corresponding activation energies were evaluated. It was found that the relaxation dynamics was accelerated by about a half order of magnitude after plastic deformation by high-pressure torsion processing, which indicates that effective rejuvenation has occurred. At relatively low temperatures the relaxation process was stress-dominated and deformation could improve the atomic mobility without changing the energy barrier. At higher temperatures, a second regime of relaxation times occurred. It is concluded that with increasing temperature, a crossover from a stress-dominated to a diffusion-dominated relaxation process occurs. Moreover, the medium range order was measured using variable resolution fluctuation electron microscopy indicating that deformation-induced changes in topology are responsible for the rejuvenation and the accelerated dynamics.