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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

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The tracer-diffusion coefficient for 'Ge has been measured in Ge single crystals as a function of pressure, temperature, and doping. Ion-beam sputtering was used for microsectioning. The activation volume in intrinsic Ge increases slightly with temperature from 0.240 at 876 K to 0.410 at 1086 K (0 is the atomic volume). The fairly small values of the activation volume show that the defect or defects which act as diffusion vehicles must be either strongly relaxed and/or spread out.Measurements of the doping dependence performed at 973 K show that the diffusivity increases with n doping and decreases with p doping. This supports the view that self-diffusion in Ge proceeds by a vacancy mechanism and that the vacancy acts as an acceptor. As a consequence the contribution of negatively charged vacancies, which is about 77% for intrinsic material, increases (decreases) with n doping (p doping). The measurements of the pressure dependence of the diffusivity in doped materials, also performed at 973 K, show that the activation volume is larger for p-than for n-doped material. We deduce 0.560 for the activation volume for the neutral vacancy and 0.28Q for the negatively charged vacancy.

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Self-diffusion in nickel single crystals is measured between 813 and 1193K using ion-beam sputtering as e microsectioning technique. Gaussian activity-depth profiles are observed over about three orders of magnitude in the activity decrease. The bulk diffusivities obtained from these profiles are analysed in conjunction with Bakker's high temperature data. The monovacancy contribution to the lattice diffusivity is given by DIV = 0.92 exp (-2.88 eV/kT) cmZ/s. Preliminary isotope effect experiments are also performed at two temperatures in the high-temperature region. The results support the view that a divacancy contribution to self-diffusion exists near the melting point.