of the laboratory
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.
The image shows the 3D-reconstruction of NiAl-precipitates (blue particles) in a "PH 13-8 Mo"
maraging-steel, measured with the 3D atom probe (LEAP 3000X HR). The red spots are 5% of all
single fe-atoms, which represent the shape of the sample. The axis are scaled in nm.