We have measured the pressure and temperature dependence of 57 Co diffusion in structurally relaxed Co 81 Zr 19 glass. The activation enthalpy and the preexponential factor are 1.4 eV and 8.4 3 10 29 m 2 s 21 . The pressure dependence is extremely small and yields an activation volume of V act ͑0.08 6 0, 1͒V, where V is the mean atomic volume. These results allow us to rule out that diffusion at this alloy composition is mediated by vacancylike thermal defects, as recently suggested for Zr-rich metal-metal glasses [A. Grandjean et al., Phys. Rev. Lett. 78, 697 (1997)]. The validity of the Keyes relationship is critically discussed. [S0031-9007(98)05821-9]

The tracer diffusion coefficient of 44Ti in a-Fe is determined by the serial sectioning method in the temperature range of 948 to 1174 K. In paramagnetic a-Fe the diffusion coefficient of Ti shows a linear Arrhenius relationship from the a-y transformation temperature to about 30 K above the Curie temperature T, = 1043 K. Below 1076 K, the temperature dcpendence of the diffusion coefficient deviates from the linear Arrhenius relationship due to the magnetic spin ordering. The temperature dependence of the diffusion coefficient D in the whole temperature range of a-Fe can be expressed by (293.2 f 8.8) kJ mol-' R T DTi(T) = (0.21?::;3 exp (1 + 0.079M2(T))) mz s -' ,where M is the magnetic long-range order parameter. The frequency factor Do = (0.21 T ! , ; ; ) m2 s -' and the activation enthalpy Q = (293.2 f 8.8) kJ mol-' describe Ti diffusion in paramagnetic a-Fe and correspond to a normal vacancy diffusion mechanism. Ti diffusion is faster by a factor of about 5 when compared to Fe self-diffusion. This can be understood by a vacancy-solute binding due t o the larger atomic radius of Ti. A comparison with the diffusion behaviour of other transition elements in x-Fe confirms thc correlation between diffusion velocity and solute atomic radius.

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

We have measured the pressure dependence of 95 Zr self-diffusion in structurally relaxed Co 92 Zr 8 glass at 698 K. The resulting activation volume of ͑0.9 6 0.1͒V, where V is the mean atomic volume, is similar to vacancy diffusion in crystalline solids. This suggests that, while Co does not diffuse via thermal defects in Co-rich Co-Zr glasses [P. Klugkist et al., Phys. Rev. Lett. 80, 3288 (1998)], Zr selfdiffusion is mediated by vacancylike defects. We thus provide evidence of the existence of an opposite Kirkendall effect in interdiffusion between certain amorphous alloys as proposed by K. N. Tu and T. C.

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