Diffusion in metallic glasses and supercooled melts

TLDR

Amorphous metallic alloys, or metallic glasses, are technologically important dense random packings whose metastability induces temperature‑dependent rearrangements linked to low‑frequency excitations. The authors review experimental results and computer simulations on diffusion in metallic glasses and supercooled melts. The review examines experimental techniques and the temperature, structural relaxation, atom‑size, pressure, isotope, irradiation effects, and molecular‑dynamics simulations of diffusion. Diffusion in metallic glasses differs from crystalline metals, proceeding via thermally activated collective atomic processes that also govern supercooled liquid diffusion above the caloric glass transition, thereby supporting the mode‑coupling scenario of glass transition.

Abstract

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance. The metastability of these systems, which gives rise to various rearrangement processes at elevated temperatures, calls for an understanding of their diffusional behavior. From the fundamental point of view, these metallic glasses are the paradigm of dense random packing. Since the recent discovery of bulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid state and to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article the authors review experimental results and computer simulations on diffusion in metallic glasses and supercooled melts. They consider in detail the experimental techniques, the temperature dependence of diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, the isotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown that diffusion in metallic glasses is significantly different from diffusion in crystalline metals and involves thermally activated, highly collective atomic processes. These processes appear to be closely related to low-frequency excitations. Similar thermally activated collective processes were also found to mediate diffusion in the supercooled liquid state well above the caloric glass transition temperature. This strongly supports the mode-coupling scenario of the glass transition, which predicts an arrest of liquidlike flow already at a critical temperature well above the caloric glass transition temperature.