Axel Griesche

Self-diffusion and interdiffusion in Al80Ni20 melts : Simulation and experiment

A combination of experimental techniques and molecular dynamics (MD) computer simulation is used to investigate the diffusion dynamics in Al 80 Ni 20 melts. Experimentally, the self-diffusion coefficient of Ni is measured by the long-capillary (LC) method and by quasielastic neutron scattering. The LC method yields also the interdiffusion coefficient. Whereas the experiments were done in the normal liquid state, the simulations provided the determination of both self-diffusion and interdiffusion constants in the undercooled regime as well. The simulation results show good agreement with the experimental data. In the temperature range 3000 K ≥ T ≥ 715 K, the interdiffusion coefficient is larger than the self-diffusion constants. Furthermore the simulation shows that this difference becomes larger in the undercooled regime. This result can be refered to a relatively strong temperature dependence of the thermodynamic factor Φ, which describes the thermodynamic driving force for interdiffusion. The simulations also indicate that the Darken equation is a good approximation, even in the undercooled regime. This implies that dynamic cross correlations play a minor role for the temperature range under consideration.

Shear cell development for diffusion experiments in foton-satellite missions and on the ground with consideration of shear-induced convection

A shear cell technique was developed to obtain exact diffusion data. The shear cell in this study was designed for the utilization under μg-conditions, especially in the FOTON-M2 mission, but also under 1g-conditions. To minimize the influence of the shear convection, the cell size, the rotation system and the speed of the discs were optimized. To minimize free surfaces, which can cause Marangoni convection, a reservoir system providing pressure on the liquid was introduced. Using this FOTON shear cell we performed short-time diffusion experiments in the In-Sn system in a parabolic flight and under 1g conditions to investigate the influence of the shear convection quantitatively. As a result, the influence of the shear convection was so small that the mean square diffusion depth caused by the shear convection was in the order of10− 7m2, which is smaller than 1% of the typical value Xdiff2 ≈ 10− 4m2 in a standard diffusion experiment using the FOTON shear cell. By using this result a correction method for the evaluation of the diffusion coefficient was established. In several ground experiments, the FOTON shear cell showed the same diffusion coefficients as from μg reference experiments within the range of errors and no obvious indication of Marangoni convection was detected. From these results we confirmed that the FOTON shear cell can be applied to μg-experiments and ground-based experiments as well.

Chemical Diffusion Experiments in AlNiCe-Melts

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.

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.

Imaging of hydrogen in steels using neutrons

Abstract We investigated the hydrogen distribution spatially and temporally in technical iron at room temperature. Samples were charged electrochemically and subsequently analysed by means of neutron radiography and tomography. The radiographic images allowed for a time-resolved analysis of hydrogen fluxes. The three-dimensional distribution of hydrogen measured by neutron tomography delivered valuable information for the damage analysis of hydrogen-induced cracks. For the first time hydrogen concentration gradients inside the material could be detect directly together with the cracks. The neutron radiography and tomography results were gained at the Research Reactor BER II of the HZB in Berlin.

Diffusion in Metallic Melts

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).

Interdiffusion and Thermodynamic Forces in Binary Liquid Alloys

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.

In Situ Measurements of Hydrogen Diffusion in Duplex Stainless Steels by Neutron Radiography

Hydrogen embrittlement (HE) is a widely known phenomenon and under investigation already for more than a century. This phenomenon, though thoroughly studied, is not yet completely understood, and so far, there are several suggested mechanisms that try to explain the occurrence of HE. One important factor of understanding the HE phenomenon and predicting hydrogen-assisted failure is the descent knowledge about the hydrogen transport behaviour in the material. Neutron radiography is a proven method for tracking hydrogen diffusion and it was applied successfully in various research studies. In the presented study, we examined the hydrogen effusion behaviour in duplex stainless steel by means of neutron radiography and calculated the effective diffusion coefficient from the obtained transmission images.

MSL compatible isothermal furnace insert for high temperature shear-cell diffusion experiments

For long-time diffusion experiments shear-cell techniques offer more favourable terms than the traditional long capillary techniques. Here, we present a further developed shear-cell that enables the measurement of diffusion coefficients up to temperatures of 1600 °C. Hence, diffusion experiments can be carried out at temperatures not accessible until now by conventional capillary or shear-cell techniques. The modified shear-cell, which can contain up to six samples of a total length of 90mm and a diameter of 1.5 mm, is built of 30 shear discs of 3mm thickness each. It is operated in an isothermal furnace insert which can be accommodated in the Materials Science Laboratory of the International Space Station. This provides the opportunity that the shear-cell can be applied to microgravity and to ground-based experiments, respectively. The heater insert with an overall length of 518mm and a diameter of 210mm consists of four heating zones with a total power of 3.5 kW. Temperature homogeneity along the graphite sample compartment is better than 2K at 1600°C. Details of the new design are discussed and results of first successfully performed heating and shearing cycles are presented.