CD-Laboratory for Early Stages of Precipitation

Technically relevant structural high-performance materials, such as tool steels, Nickel-base alloys and refractory alloys, owe their superior mechanical properties to specific microstructural features such as small or extremely small grain sizes, the existence of extremely narrow spaced interfaces within grains or a fine dispersion of second phase particles such as precipitates. Hardening by particles is considered as one of the most important strengthening mechanisms. Therefore, improvement of conventional materials and development of new materials require a deep understanding of precipitation reactions and their influence on mechanical properties.

Precipitation reactions in multi-component materials are characterized by complex interactions between the various alloying elements. To gain sufficient insight into these processes, extensive analysis of the kinetics of precipitation is essential. It is necessary to analyse the spatial extension and the amplitude of compositional fluctuations of incipient second-phase particles as well as morphology, number density, size, and chemical composition of individual precipitates at various stages of the reaction. For this purpose, microanalytical tools are required that are capable of resolving very small typically of a few nm solute clusters and which allow an analysis of their chemical composition. The most prominent tools which meet these requirements are transmission electron microscopy and atom probe field ion microscopy (direct imaging techniques) as well as small-angle scattering and differential-scanning-calorimetry (indirect imaging techniques). In the research performed in this laboratory, all these experimental techniques are applied in a complementary way, since none of these techniques alone can provide the required information and the complete picture of the precipitation process.

The state-of-the-art approach in experimental analysis of precipitation processes is combined with advanced computer simulation tools to verify experiment on simulation and - vice versa - simulation on experiment. In addition to traditional thermodynamic equilibrium tools ('Computational Thermodynamics'), a novel approach for simulation of the precipitation kinetics in multi-component multi-phase multi-particle systems (Software MatCalc) is applied to study the evolution of the precipitate microstructure on the researchers desktop. With the simulated precipitation kinetics, predictions on the strengthening effect of precipitates are made and compared to experimental data on the evolution of the mechanical properties in the course of thermal and thermo-mechanical treatment. Only the combination of both, theoretical and experimental approach, allow a complete and comprehensive characterization of microstructural transformations and the prediction of mechanical properties from the results of computer simulations.