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The defence will be in English.
Abstract: Glasses are disordered solids formed when liquids are cooled rapidly enough to avoid crystallization, a process that leads to a dramatic slowdown of atomic motion at the glass transition temperature, Tg. Although glasses are ubiquitous in everyday life and play a central role in many technologies, the microscopic mechanisms governing vitrification and relaxation remain only partially understood. This long-standing challenge is largely due to the difficulty of directly accessing atomic-scale dynamics, particularly under extreme thermodynamic conditions. This PhD thesis addresses this issue by focusing on metallic glasses, a class of amorphous materials that combine a simple atomic structure with remarkable mechanical and chemical properties. Because they lack the additional degrees of freedom present in molecular or polymeric glasses, metallic glasses provide an ideal platform for investigating fundamental aspects of glassy dynamics. While the influence of temperature on relaxation processes has been extensively studied, the role of pressure, especially in metallic glass formers, has remained largely unexplored experimentally.
To overcome this limitation, a novel experimental approach was developed within this work, combining wide-angle X-ray Photon Correlation Spectroscopy (XPCS) in the hard X-ray regime with a high-pressure sample environment. This technique makes it possible to directly probe atomic-scale dynamics over a wide range of pressures, up to several gigapascals, and temperatures extending from room temperature to 700 K. Complementary X-ray diffraction measurements allow structural changes induced by pressure to be correlated with the observed dynamical behavior.
Using this approach, the combined effects of temperature and pressure on the relaxation dynamics of metallic glasses are explored across the glass transition, from the deep glassy state to the equilibrium supercooled liquid. Depending on the ergodic state of the system, pressure is found to induce two contrasting effects on the atomic dynamics. In the glassy state, where atomic motion is largely governed by internal stresses accumulated during vitrification, compression leads to an unexpected acceleration of the dynamics, in agreement with theoretical predictions. In contrast, in the supercooled liquid state, the structural or α-relaxation, here experimentally probed for the first time under in situ high-pressure conditions, progressively slows down as the system becomes denser. This pressure-induced dynamical slowdown reflects the system’s evolution toward deeper energy states and highlights the fundamentally different role played by compression in equilibrium and non-equilibrium regimes.
