Papers by Author: Axel Griesche

Abstract: A novel X-ray radiography method is used to measure chemical diffusion in a long-capillary in liquid Al95Ni5 at.-%. Molecular dynamics simulations provide interdiffusion coefficients and thermodynamic factors for the whole composition range in Al-Ni. The data are compared to literature data in Sb-Sn and Ag-Sn. The relation between interdiffusion coefficient and thermodynamic forces is discussed in the context of the Darken equation. In systems with common ordering tendency (Al-Ni, Sb-Sn) the thermodynamic factor is larger than one and enhances interdiffusion. In systems with common demixing tendency (Ag-Sn) the thermodynamic factor is smaller than one and reduces interdiffusion.

Abstract: We make use of a novel X-ray radiography method to measure chemical diffusion in capillaries in binary Al-Ni melts. Data are compared to self diffusion coefficients of Ni obtained by quasielastic neutron scattering as well as diffusion and thermodynamic data obtained by molecular dynamic simulations. Interdiffusion compared to self diffusion is enhanced with a maximum at Al40Ni60. We show that this enhancement is caused by thermodynamic forces as described by the Darken-Manning equation. In liquid Al-Ni alloys the Manning factor that is smaller than one can be attributed to collective cross correlations.

Abstract: We present diffusion measurements in metallic melts measured by capillary techniques and results of molecular dynamic simulations. The investigated systems are the binary alloy AlNi20 and the multicomponent bulk glass-forming alloy Pd43Cu27Ni10P20. The temperature range of interest reached from the glassy state to the equilibrium melt. In the glassy as well as in the deeply supercooled state, below the critical temperature Tc of the mode-coupling, theory (MCT), diffusion is a highly collective atomic hopping process. Both investigated systems show around Tc a change in the diffusion mechanism. Above the liquidus temperature, diffusion in Pd43Cu27Ni10P20 is a collective process whereas in AlNi20 the atoms diffuse probably by uncorrelated binary collisions. The influence of thermodynamic forces on diffusion in the liquid state of AlNi20 can be described by the Darken equation with an additional temperature independent correction factor (“Manning”- factor).

Abstract: The long-capillary method was used to measure chemical diffusion in molten AlNiCe alloys. The interdiffusion coefficients were determined for a mean concentration of Al87Ni10Ce3 at 1273 K and for a mean concentration of Al77Ni20Ce3 at 1373 K. The absence of major convection disturbances and of macro-segregation was demonstrated by time-dependent diffusion measurements. An in-situ x-ray monitoring technique for real-time concentration profile determination is presented.