Beryllium as a fast diffuser in bulk metallic glasses
Final Report Abstract
Since the discovery of bulk amorphous alloys, they are of considerable interest not only for technical applications, but also for basic research. These alloys allow for the first time the investigations in the deeply undercooled metallic melt and hence to test theories of glass transition and the mode coupling theory. In particular, the connection between the mobility of the constituents of the alloy and the general mobility, as expressed by the viscosity, is of particular interest. This connection is given in ordinary melts and liquids by the Stokes Einstein equation, whereas it is known that in glasses this equation does not hold true. The aim of the present project was to find out, which - if any - of the elements of a glass forming alloy determines the viscosity, and where, whith decreasing temperature the diffusivities decouple from each other and from the viscosity. We used neutron irradiated Pd-102 and radiotracer technique to determine simultaneously the diffusivities of palladium and chromium, where the latter resembles that of nickel and cobalt. We found that Palladium diffusivity is equal to the viscosity and chromium equal to cobalt. For the first time we experimentally observed the decoupling of the diffusivities of the components in the undercooled melt which has so far only been indirectly observed by comparing viscosity and and other dynamic quantities. As the diffusivity of palladium is equal to the viscosity, this shows that palladium as majority component mainly determines the viscosity and therefore the main dynamics of the system. An explanation might be the high concentration, which allows to form a closed network of atoms, which is hence rather rigid and therefore determines the dynamics of the whole system. This might also explain, why it is by far the slowest element at a given temperature. This formation of a percolating rigid network could be a key feature for the excellent glass forming ability of these alloys. The fact that at and above Tc the palladium diffusivity is similar to all other elements investigated so far shows that the dynamics of all constituents becomes rather similar. This means that there is a significant change in dynamics, not macroscopic like at the caloric glass transition temperature, but microscopic, so instead of thermal activated hopping over barriers all elements in the alloy move equally, like predicted by the Mode Coupling Theory above Tc, whereas below, only thermally activated jumps are possible. In summary, despite experimental difficulties, the main goals oft he present project could be reached and the results significantly contribute to the understanding of dynamics in undercooled metallic melts.
Publications
- Codiffusion of 32P and 57Co in glass-forming Pd-Cu-Ni-P alloy and its relation to viscosity A. Bartsch, K. Rätzke, F. Faupel, A. Meyer, AppL. Phys. Lett. 89 (2006) 121917
- Diffusion in bulk glass forming alloys - from the glass to the equilibrium melt K. Rätzke, V. Zöllmer, A. Bartsch, A. Meyer, F. Faupel, invited contribution, TMS February 2007, Defect and Diffusion Forum, 66 (2007) 109.
- Diffusion in bulk metallic glass forming Pd-Cu-Ni-P alloys: from the glass to the equilibrium melt K. Rätzke, V. Zöllmer, A. Bartsch, A. Meyer, F. Faupel, invited review, Liquid and amorphous Metals 12, Metz, Journal of Non-Crystalline Solids, 353 (32), (2007) 3285.
- Diffusion in metallic glasses and undercooled metallic melts, K. Rätzke, F. Faupel, Z. Metallkunde, 95 (2004) 956
