V. Zöllmer

Diffusion and isotope effect in bulk-metallic glass-forming Pd-Cu-Ni-P alloys from the glass to the equilibrium melt

We report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd-Cu-Ni-P alloys. The Pd-Cu-Ni-P system, with its high stability against crystallization, allows diffusion measurements from the glassy state to the equilibrium melt for the first time. Serial sectioning was performed by grinding and ion-beam sputtering. The time and temperature as well as mass dependence, expressed in terms of the isotope effect E , of codiffusion were investigated. In the glassy state as well as in the deeply supercooled state below the critical temperature T c , where the mode-coupling theory predicts a freezing-in of liquidlike motion, the measured very small isotope effects indicated a highly collective hopping mechanism. Below T c , the temperature dependence showed Arrhenius-type behavior. Above T c , the onset of liquidlike motion was evidenced by a gradual drop of the effective activation energy, resulting from the decay of hopping barriers, and by the validity of the Stokes-Einstein equation, which was found to break down below T c . This strongly supports the mode-coupling scenario. Isotope effect measurements, which have never been carried out near T c in any material, showed atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions. The latter appears to be a prerequisite of excellent glass-forming abilities.

Silver diffusion in Fe40Ni38Mo4B18 metallic glass under high vacuum

Abstract In order to ascertain reports on unexpected dispersion of noble metals in Fe 40 Ni 38 Mo 4 B 18 glass the diffusion behaviour of 110m Ag was studied by the radio-tracer technique. Silver did not show a clear penetration profile in high vacuum, confirming the crucial role of the annealing atmosphere.

Diffusion in Bulk Glass Forming Alloys– from the Glass to the Equilibrium Melt

Since the discovery of bulk metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd-Cu-Ni-P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. The Co isotope effect measurements, which have never been carried out near Tc in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions. In the glassy state as well as in the deeply supercooled state below the critical temperature Tc, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below Tc the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2 eV. Above Tc the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes-Einstein equation, which is found to break down below Tc. Although having strong covalent bonding tendencies, Phosphorous diffusion is only slightly slower than Co diffusion, indicating that it does not determine the overall viscosity below Tc. The Stokes-Einstein equation is presently tested for other constituents of the alloy.

Diffusion in Metallic Glasses and Supercooled Melts

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.