Andreas Meyer

Self diffusion in liquid aluminium

Here we report temperature dependent self-diffusion coefficients of liquid aluminium measured on absolute scale by using incoherent quasielastic neutron scattering at temperatures of 980K, 1020K, and 1060K. Aluminium self-diffusion coefficients follow an Arrhenius law with an activation energy of 280±70 meV. The Sutherland-Einstein equation relating viscosity to the diffusion coefficient well captures the temperature dependence and absolute values of the here reported aluminium self-diffusion coefficients using the covalent radius of aluminium. A comparison to published molecular dynamics simulation data helps to further narrow down the choice of the potential.

Liquid Al80Cu20: Atomic diffusion and viscosity

Properties of mass transport in liquid Al80Cu20 were measured over a broad temperature range of more than 500 K by means of oscillating cup viscometry and quasielastic neutron scattering. The shear viscosity and the coefficient of the Cu self-diffusion exhibit an Arrhenius-type temperature dependence. The activation energy for the viscous flow is 2.4 times smaller than that of the Cu self-diffusion. Below 1400 K, the Cu self-diffusion becomes increasingly smaller than expected from the viscosity data rescaled via the Stokes–Einstein relation.

Atomic diffusion mechanisms in a binary metallic melt

The relation between static structure and dynamics as measured through the diffusion coefficients in viscous multicomponent metallic melts is elucidated by the example of the binary alloy Zr64Ni36, by a combination of neutron-scattering experiments and mode-coupling theory of the glass transition. Comparison with a hard-sphere mixture shows that the relation between the different self diffusion coefficients strongly depends on chemical short-range ordering. For the Zr-Ni example, the theory predicts both diffusivities to be practically identical. The kinetics of concentration fluctuations is dramatically slower than that of self-diffusion, but the overall interdiffusion coefficient is equally large or larger due to a purely thermodynamic prefactor. This result is a general feature for non-demixing dense melts, irrespective of chemical short-range order.

In situ studies of mass transport in liquid alloys by means of neutron radiography

When in situ techniques became available in recent years this led to a breakthrough in accurately determining diffusion coefficients for liquid alloys. Here we discuss how neutron radiography can be used to measure chemical diffusion in a ternary AlCuAg alloy. Neutron radiography hereby gives complementary information to x-ray radiography used for measuring chemical diffusion and to quasielastic neutron scattering used mainly for determining self-diffusion. A novel Al(2)O(3) based furnace that enables one to study diffusion processes by means of neutron radiography is discussed. A chemical diffusion coefficient of Ag against Al around the eutectic composition Al(68.6)Cu(13.8)Ag(17.6)xa0at.% was obtained. It is demonstrated that the inxa0situ technique of neutron radiography is a powerful means to study mass transport properties in situ in binary and ternary alloys that show poor x-ray contrast.

Atomic dynamics in binary Zr-Cu liquids

We studied Cu self-dynamics and liquid density of Zr66.7Cu33.3, Zr35.5Cu64.5, Zr36Cu64, and Zr38.2Cu61.8 liquids using a combination of containerless processing techniques and quasielastic neutron scattering. We show that the composition dependence of the Cu self-dynamics is qualitatively controlled by the liquid packing density. It is hence monotonic within a narrow composition range and not sensitive to small composition variations. Similarly, replacing Cu by Ni results in a slower atomic dynamics in the Zr-Ni liquids, as a consequence of the more dense packing. This is in contrast to the strong composition dependence of the glass-forming behaviour, and the better glass-forming ability of the Zr-Cu over Zr-Ni alloys, which usually favours sluggish liquid dynamics. Thus, in the Zr-Cu case, the glass-forming ability is not directly correlated with liquid dynamics and packing density.

Atomic Diffusion and its Relation to Thermodynamic Forces in Al-Ni Melts

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.

Self Diffusion in Liquid Titanium: Quasielastic Neutron Scattering and Molecular Dynamics Simulation

Self diffusion in liquid titanium was measured at 2000K by quasielastic neutron scattering (QNS) in combination with container less processing via electromagnetic levitation. At small wavenumbers q the quasielastic signal is dominated by incoherent scattering. Up to about 1.2 °A−1 the width of the quasielastic line exhibits a q2 dependence as expected for long range atomic transport, thus allowing to measure the self diffusion coefficient DTi. As a result the value DTi = (5.3± 0.2)× 10−9 m2s−1 was obtained.With a molecular dynamics (MD) computer simulation using an embedded atom model (EAM) for Ti, the self diffusion coefficient is determined from the mean square displacement as well as from the decay of the incoherent intermediate scattering function at different q. By comparing both methods, we show that the hydrodynamic prediction of a q2 dependence indeed extends up to about 1.2 °A−1. Since this result does not depend significantly on the details of the interatomic potential, our findings show that accurate values of self diffusion coefficients in liquid metals can be measured by QNS on an absolute scale.

Coupled relaxation processes in a glass forming ZrTiNiCuBe liquid

We studied liquid Zr46.5Ti18.2Cu7.5Ni10Be27.5 in the temperature range from 1060 K to 1255 K using a combination of quasielastic neutron scattering and newly developed electrostatic levitation. We show that not only the microscopic incoherent and coherent relaxation times, and the derived mean self-diffusion coefficient of Ni/Cu/Ti, but also the macroscopic melt viscosity exhibit very similar temperature dependences. Thus, independent of the experimental time/length scales and the different natures of the relaxation times, the Maxwell relation of linear viscoelasticity holds, as a result of highly coupled structural relaxation processes in the melt.

Ni self-diffusion in Zr-Ni(-Al) melts

We report on investigations on the atomic dynamics in melts of different binary Zr-Ni alloys and of the ternary glass-forming Zr60Ni25Al15 alloy. The liquids are containerlessly processed in an electromagnetic levitator that is combined with quasielastic neutron scattering at the time of flight spectrometer TOFTOF of the FRM II. Ni self-diffusion coefficients are determined that exhibit an Arrhenius-type temperature dependence with comparatively large activation energies ranging between 0.64 and 0.90 eV. Although glass forming abilities and melting temperatures for these alloys exhibit large differences, the absolute values of the self-diffusion coefficients are similar at same temperature.

Viscosity measurements of metallic melts using the oscillating drop technique

By means of benchmarking reduced gravity experiments, we have verified the measured viscosity of binary Zr-Ni glass forming liquids utilizing the oscillating drop technique combined with ground-based electrostatic levitation (ESL). Reliable viscosity data can be obtained as long as internal viscous damping of a single oscillation mode of a levitated drop dominates external perturbations. This can be verified by the absence of a sample mass dependence of the results. Hence, ESL is an excellent tool for studying the viscosity of metallic glass forming melts in the range of about 10–250u2009mPau2009s, with sample masses below 100u2009mg. To this end, we show that, for binary Zr-Ni melts, the viscosity is qualitatively controlled by the packing density.