Prof. Dr.-Ing. Raabe (Max-Planck Institut für Eisenforschung, Düsseldorf)
Dr.-Ing. Zaefferer (Max-Planck Institut für Eisenforschung, Düsseldorf)
Short description TPC4, Zaefferer/Raabe: Advancing methods for high-resolution characterization of crystal lattice defects on bulk samples and application of these methods to the investigation of defect structure evolution during cyclic loading of high-Mn steels.
TP C4 aims to reach two goals at the same time: advancing scanning electron microscopy (SEM)-based diffraction methods, which allow high resolution investigation of defect structures in bulk samples, and using these methods to study the fatigue behaviour of high-Mn-steels. The methods under consideration are the electron channelling contrast imaging (ECCI) method, the EBSD cross-correlation method (XR-EBSD) and the newly developed Kikuchi bandlet method (KB-EBSD). The first method, ECCI, allows direct observation of individual crystal defects like dislocations, stacking faults or nano-twins. XR-EBSD enables quantification of residual stresses and very small lattice rotations. The KB-EBSD method, finally, serves to quantify high dislocation densities in highly deformed materials. All methods, but particularly the KB-EBSD method, have great development potential which shall be further explored in TP C4.
An ideal and at the same time important materials problem for application of the mentioned techniques is the investigation of the formation of defect structures during cyclic loading of high-Mn-steels all the way to damage. During this loading mode lattice rotations stay small, but areas of very different dislocation densities and residual stresses develop. Cyclic loading leads to fatigue (low and high cycle fatigue, LCF, HCF) which may, from a practical view point, be more important for materials applications than monotonic loading modes. Thus, following up the earlier project periods, in this period the evolution of defect structures, particularly during LCF loading of high-Mn steels will be studied. The characterization methods will optimized at the same time.
In detail the following investigations will be carried out: on the materials science side it will be investigated how the stacking fault energy, the dislocation friction forces caused by solute carbon and dislocation obstacles caused by κ-precipitates will affect the evolution of the fatigue structures. In particular, it will be investigated under which conditions lamella or cellular dislocation walls are formed and whether planar or wavy slip of dislocations is observed. Additionally, it will be investigated whether twin or ε-martensite boundaries have a different influence on dislocation evolution and crack initiation than conventional large angle grain boundaries. On the methodologic side the resolution limits of the ECCI method and the possibilities for in-situ observation of dislocation movements will be investigated. Furthermore, it will be studied whether mechanical serial sectioning can be applied to perform 3D ECCI. This would, for the first time, allow observing the 3-dimensional structure of persistent slip bands (PSB). The XR EBSD method will be used to study residual stresses in the neighbourhood of PSBs. It will be investigated whether also this technique can be applied on serial sections for 3D reconstruction. Finally, the KB EBSD technique will be developed further to enable quantitative dislocation density measurements inside of PSBs and fatigue cell walls.
Local and global texture and anisotropy in the Fe-Mn-C system
The basic aim of project C4 is to describe and understand the formation of the deformation and recrystallisation texture of Fe-Mn-C materials. The deformation texture is sensitively dependent on the deformation mechanisms proceeding in the material and can therefore be used to study the activity of deformation mechanisms in a statistical manner. As sheet rolling is technologically the most important deformation process and as rolling yields one of the strongest deformation textures the focus is on the texture evolution during rolling at different temperatures.
The measurement of crystallographic textures is traditionally a domain of x-ray diffraction (XRD). With increasing power of the electron backscatter diffraction (EBSD) technique, however, textures can be measured with higher information and accuracy with this technique. The reasons for that are, first, that EBSD directly measures the orientation distribution and does not require pole figure inversion which is prone to a number of experimental and calculation errors. Second, the EBSD-based measurement of textures allows the correlation of textures with the microstructure and the position in the material.
Finally, the EBSD technique – if applied with statistically representative sampling – allows measurement of very weak textures, which are actually difficult to be described by XRD. The latter is illustrated by pole figures in figure 1. Although the maximum pole density is only 1.23 mrd (!) the texture is clearly visible.
The combination of the described techniques (EBSD-based texture measurements and ECCI-observation of lattice defects) allows us to understand the strain hardeningbehaviour of TWIP material. This is illustrated as an example in figure 5. It shows astrain hardening curve of a TWIP steel during a tensile experiment. At two positions of the curve texture and microstructure are indicated. At the position of the first minimum of strain hardening rate (position a), the microstructure shows few twins but a high density of dislocations which form – surprisingly for a low stacking fault energy material – dislocation cells. The texture is very weak but shows clear peaks at the (111) and (100) || ND positions. At the position of the maximum of strain hardening (position b) the microstructure shows a high amount of primary twins but also – visible only in the ECCI images – an important amount of secondary twin systems. The texture is still weak but significantly stronger (about 3 times in terms of pole figure densities) than at position a. In particular the (110) || ND position is now completely empty.