Diffusion in metallic glasses and supercooled melts

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. CONTENTS

Evolution of crystallite size, lattice parameter and internal strain in Al precipitates during high energy ball milling of partly amorphous Al87Ni8La5 alloy

“…[22]). Furthermore, the diffusion of alloying elements in metallic glasses is known to be sensitive to the relative size of the diffusing atoms and the host atoms [23]. Higher diffusivity of the smaller Ni compared to the larger La in the amorphous Al-based matrix supports the hypothesis of preferred uptake of Ni from the amorphous phase intothe Al crystallites during ball milling.…”

Softening of phonon spectra in metallic glasses

“…20 At lower temperatures, hopping over barriers in the energy landscape requires large atomic amplitudes that can be provided by localised soft vibrations. 18 At higher temperatures, there is an increase in both the atomic displacements and the number of atoms participating in the jumps. 21 It is pertinent to note that, although crystallisation is universally known to be a first-order thermodynamic transformation, 22,23 glass transition has been prevailingly considered as a largely dynamical phenomenon 24 and soft phonons are known to be related to the dynamics of amorphous alloys.…”

“…21 It is pertinent to note that, although crystallisation is universally known to be a first-order thermodynamic transformation, 22,23 glass transition has been prevailingly considered as a largely dynamical phenomenon 24 and soft phonons are known to be related to the dynamics of amorphous alloys. 18,25 Although the energy landscape curvature and mean-squared atomic displacements (MSD) are two different quantities, they are inter-connected often in complicated relations. The calculated MSD of Mg atoms in the vicinity of glass transition temperature 15 Figure 5.…”

Vibrations and relaxations in a soft sphere glass: boson peak and structure factors

“…Such jumps give e. g. rise to the telegraph noise in point contacts and are thought to be the elementary process in diffusion [50]. In our previous study of relaxations in the soft sphere glass we found them to be closely correlated to the quasi-localized vibrations.…”