Electron microscopic analysis of melting processes and solidification structures
The formation of a molten phase during manufacturing processes triggers complex thermomechanical processes that have a long-lasting effect on the microstructure, the resulting properties and the component geometry. This sub-project aims at clarifying how the physical processes involved in melting and solidification are related, and how they could ultimately be employed to improve the microstructure and the resulting properties. An understanding of the structure-property correlation requires a scale-bridging characterization of the affected material areas and their microstructural changes, both in situ and post mortem. Moreover, fundamental knowledge of the thermodynamically and/or kinetically controlled processes can be obtained from the characterization results, with the aim of making conclusions about better conditions for producing the alloys and also exploring the improvement of properties under applied conditions. The present sub-project thus makes an important contribution to the holistic view of materials and processes aimed at in the Collaborative Research Center. The multiscale cross-correlative materials characterization results by electron microscopy provide information about the following physical sub-processes:
The interface phenomena at the solid/liquid phase boundary, which are to be investigated both post mortem and in situ (including heating, high temperature bending and cooling)
The phase/structure transformation in the solidified melt and in the heat affected zone
Mass transport and precipitation processes in the thermally stressed areas down to atomic scale
Volumetric changes (contraction or expansion) often results in the development of internal strain due to variation in the cooling rates.
In addition, we aim to gain a deeper understanding of the mechanisms which take place during the formation and solidification of the melt by means of electron microscopic in situ experiments. In the large-chamber scanning electron microscope installed at the GFE, the processes involved in soldering and laser welding are closely monitored and the processes taking place are mapped directly with the electron beam. The aim of the sub-project is a fundamental understanding and a further quantitative description of the thermodynamic and thermomechanical properties, as well as the relationships between the structures formed during solidification and the geometric and mechanical properties of the macroscopic materials examined. Additionally, using the same tool, high temperature bending tests are performed which shed light on the tensile strength of brittle materials that are generally difficult to test in uniaxial tension due to cracking in the grips. Moreover, the in situ bending experiments help to determine the elastic modulus and flexural stress/strain (failure stress/strain) of a material.
In the planned work, the microstructure is also investigated using advanced electron microscopy techniques in a correlative approach. The microstructure in the area of the solidified molten phase and in the surrounding heat-affected zones is analyzed at different length scales to gain better understanding of the stability of phases as well as the influence of chemical kinetics. The desired understanding of the processes includes both the coarsening of existing phases, the transformation into new phases and the formation of diffusion zones and residual stress zones.