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

Study of nano-scale diffusion in thin films and multilayers

“…The activation energy for self-diffusion of Fe in the present system comes out to be 0.42 ± 0.04 eV and preexponential factor D 0 = exp (−39 ± 1). This value is substantially lower as compared to that in Fe-Zr metallic glasses of similar composition [14]. This low activation energy can be attributed to the fact that the present results pertain to amorphous film which would have a higher density of defects, and that the measurements have been done in the un-relaxed state of the system.…”

“…The diffusion length is obtained Curves are shifted vertically with respect to each other for the sake of clarity. Continuous curves represent best fit to the data obtained using the computer code [14]. Inset shows the concentration profiles of 57 Fe layer as obtained from the fitting of nuclear reflectivity data of pristine ( [18], however are comparable to diffusivity in nanocrystalline FeZr [19], where diffusion occurs mainly through grain boundaries which are amorphous in nature.…”

A Finite-Deformation Constitutive Model of Bulk Metallic Glass Plasticity

“…The first term in the right hand side of (20), R _ e p , denotes the shear-induced free volume creation which is assumed to be proportional to the plastic shear strain rate, whereas the relaxation process is described by ðn À n T Þ=t where t ¼ r=ðl _ e p Þ is the relaxation time. The evolution of free volume may additionally be influenced by diffusion [9]. The diffusion of free volume, however, is poorly understood at present and will not be considered here.…”

Computational Materials Design

“…Calculated and experimental kinetics of formation of P phase and Ni 2 Cr phase in alloy C-22 [6] Basic and Applied Research: Section I current results for the mobility of B, C, Fe, Cr and Mo used in the current calculations [7] with the correlations developed by Faupel et al [23] shown in Fig. 10, which indicates that the descriptions that are required to describe the observed formation and devitrification of amorphous iron alloys are in general accordance with comparable observations on other alloy systems.…”