Dr. Hajo Dieringa

Compared to aluminium alloys magnesium alloys still suffer under lower mechanical strength and creep resistance especially at elevated temperatures. The goal of substitution of aluminium alloys by magnesium alloys will only be achieved with comparable mechanical properties. A reinforcement of fibres and/or particles in the micrometer scale has already been in the focus of research since some decades but never came into industrial application. In recent years low cost nano-particles are available which promise to be a reinforcement for magnesium alloys in a way that they promote Orowan-strengthening much better than micrometer particles. It is expected that strength and creep resistance in a temperature range up to 250°C shows significant improvement. Development of processes for introduction of nano-particles into the magnesium alloy melt and resulting high strength and creep resistant materials would strongly support the leading position of magnesium based activities at Helmholtz-Zentrum Geesthacht.
One of the challenges will be the processing route via melt stirring. It is important that nano-particles are distributed randomly in the matrix and no agglomeration of particles occurs. From experience with micro-particle reinforced magnesium alloys it can be expected that additional support during stirring has to be conducted. Power ultrasound can help avoiding agglomeration as well as supporting fine disperse distribution. This is effected by micro sized bubbles which form during intense sonification and than implode instantaneously. The second important challenge is the finding of ideal combinations of ceramic particles and magnesium melt. Depending on alloying elements and ceramic composition chemical reactions may occur. A thin layer promotes a good bonding between both materials but when contact is extended reinforcement might simply dissolute. Knowledge of chemical fundamentals as well as process development has to be built up.
Additional to microstructure analysis via SEM/TEM studies and X-ray diffraction mechanical properties and creep response at room and elevated temperatures has to be evaluated. Thermal expansion and wear behaviour will be investigated in a second step.

Dr. Norbert Hort,
Prof. Dr.-Ing. Rainer Schmid-Fetzer

The viscosity of unary, binary and multi-component liquid alloys is of great importance in the theory of liquid metal behavior, but also for fluid flow behavior in metallurgical processes, such as casting, welding, and solidification. The goal of this project is to elaborate a model for the viscosity of liquid alloys and to combine it with dedicated experimentation on liquid magnesium alloys. The predictive capability of that model to calculate the concentration and temperature dependence of viscosity over a wide range of multicomponent alloys shall be enhanced by the implementation of fully consistent Calphad-type thermodynamics of these alloys. The model will be validated against a wide range of metallic liquids, such as solder, aluminum and magnesium alloys and shall be useful for implementation in integrated software for calculation of thermodynamic and thermophysical properties but also for solidification modeling at large. Moreover, new and systematic experimental data for both the viscosity and density of liquid magnesium alloys will be generated and combined with the analytical model incorporating thermodynamic data. That will enable a reliable calculation of liquid viscosity even for rapidly changing values of (residual) liquid composition and temperature, relevant for solidification processing and modeling of magnesium alloys.

Prof. Regine Willumeit,
Dr. Norbert Hort