Papers by Author: Dierk Raabe

Abstract: The activated slip systems were analyzed in the cold rolling of a Ni₃Al single crystal with an initial orientation of ~[-112](512), which showed an irregular rolling deformation, i.e. widening, bending, and shear deformation. A phenomenological crystal plasticity model was applied using a spectral method. The boundary condition was optimized to reproduce the actual rolling deformation, as follows. That is, the orthogonal components of the deformation gradient were given from the measured widening and reduction, and the shear components were iteratively optimized as to that the final orientation was as close to the experimental one as possible. The calculated result showed that three slip systems, a3, b1, and d1 in the Bishop-Hill notation, were mainly activated in the irregular rolling deformation, which result was consistent to the previous observation of the slip traces [Kishida et al., Philos. Mag. 83 (2003) 3029]. The three activated systems were identical to those activated in the plane-strain condition. However, the quantitative comparison revealed that the activity of b1 was significantly reduced in the irregular rolling deformation, while the activity of d1 was enhanced instead. The less activity of b1 and the enhancement of d1 can be understood assuming a strong interaction between a3 and b1. The reaction of this pair has been reported to form the superlattice intrinsic stacking fault (SISF) in Ni₃Al [Chiba and Hanada, Philos. Mag. A. 69 (1994) 751]. It is likely that the formation of the SISF, which are considered immobile in Ni₃Al, restrained the activation of b1, leading to the irregular rolling deformation.

Abstract: We have investigated the strain-hardening mechanisms across the relevant scales in a Fe-22Mn-0.6C (wt.%) twinning induced plasticity steel by multi-scale microstructure characterization. The approach makes use of electron microscopy techniques such as electron channeling contrast imaging (ECCI) to characterize microstructure features at the micro/nanoscale, and atomic-scale investigations of partitioning behavior across interfaces and solid solution/clustering effects by atom probe tomography (APT). The contribution of most relevant microstructure features to strain hardening is analyzed.

Abstract: We have investigated the formation of dislocation substructures in high-Mn steels by electron channeling contrast imaging in the SEM. The coupling of electron channeling contrast imaging (ECCI) with electron backscatter diffraction (EBSD) provides an efficient and fast approach to characterize dislocation substructures under controlled diffraction conditions with enhanced contrast. The dislocation substructure of high-Mn steels at intermediate strain levels is characterized by cells and cell blocks with strong crystallographic orientation dependence. We observe a significant effect of strain path on dislocation patterning. Microband formation is enabled under shearing conditions. We explain this effect on terms of Schmid’s law.

Abstract: Abstract. Ferritic-martensitic steels like Eurofer-97 are candidate structural materials for future fusion reactors. In the tempered state, this steel contains fine particles dispersed in the ferritic matrix. The aim of this work is to investigate abnormal grain growth in Eurofer-97 steel. The microstructural evolution was followed by isothermal annealing between 200 and 800°C (ferritic phase field) after cold rolling to 70, 80, and 90% reductions. Representative samples were characterized by scanning electron microscopy in the backscattered electron mode. Microtexture was evaluated by electron backscattered diffraction. We propose a mechanism based on the size advantage acquired by nuclei with misorientation angles above 45º relative to their nearest neighbors to explain abnormal grain growth. Abnormal grain growth textures have components belonging to the α- and γ-fibers with predominance of {111}, {111}, and {100}.

Abstract: Duplex stainless steels (DSSs) are based on the Fe-Cr-Ni system and formed by ferrite (30-70%) and austenite. They have high tensile strength, good toughness and weldability and excellent corrosion resistance including stress-corrosion cracking and resistance to localized corrosion. The increase of the raw materials of the last years, there has been a motivation to develop new stainless steels with lower contents of nickel and molybdenum. Lean duplex stainless steels (LDSSs) are almost Mo free and nickel content lower than 4%. The lean duplex grades are expected to substitute not only 304/316 grades but also other duplex stainless steel grades. LDSSs are used for structural applications and for the less corrosion conditions such as liquor tanks and suction rolls. The aim of the present work was to study the kinetics of recovery and recrystallization of the lean duplex stainless steel 1.4362 during annealing treatment. The material was subjected to hot rolling and cold rolling of 70%, annealing treatment for different times at temperatures from 1000 to 1100°C and subsequently water quenched. Optical microscopy and electron back scattering diffraction (EBSD) were employed to study the evolution of the microstructure during the annealing treatment. After cold rolling austenite exhibited more strain hardening than ferrite. Consequently, the driving force of the austenite for recrystallization is higher. During annealing, recovery took place in ferrite, while the austenite remained nearly unrecovered until beginning of recrystallization. The layered grain morphology produced during cold rolling remained after the annealing treatment. The volume fraction of the phases did not show significant changes with the annealing time. Nevertheless, the volume fraction of austenite decline with an increasing of annealing temperature. After 60 seconds at 1100°C, annealing primary recrystallization had progressed in both phases, which show a bamboo-structure where the grain boundaries ran perpendicular to the phase boundaries. Grain growth progressed for longer annealing times. After 600 seconds, the bamboo-structure started to change for a more globular grain structure, pearl-structure. It continued and became more pronounced at longer annealing times. At lower temperatures, the recrystallization behavior is similar; however, the structure was refined. Recovery is favored by the high stacking fault energy of ferrite and the layered grain morphology. The lean duplex stainless steel 1.4362 shows a similar recrystallization kinetics compare with standard duplex stainless steels.

Abstract: Investigations of the microstructure of materials processed via severe plastic deformation methods such as high pressure torsion (HPT) and their recrystallization behaviour is of great interest as they are capable of producing ultra fine grained material (UFD) with good mechanical properties.

Abstract: The paper introduces first investigations on how low angle grain boundaries can influence the recrystallisation behaviour of crystalline metallic materials. For this purpose a three-dimensional cellular automaton model was used. The approach in this study is to allow even low angle grain boundaries to move during recrystallisation. The effect of this non-zero mobility of low angle grain boundaries will be analysed for the recrystallisation of deformed Al single crystals with Cube orientation. It will be shown that low angle grain boundaries indeed influence the kinetics as well as the texture evolution of metallic materials during recrystallisation.

Abstract: The microstructure and texture of rolled and annealed dual-phase steels with 0.147 wt. % C, 1.868 wt. % Mn, and 0.403 wt. % Si were analyzed using SEM, EDX, and EBSD. Hot rolled sheets showed a ferritic-pearlitic microstructure with a pearlite volume fraction of about 40 % and ferrite grain size of about 6 µm. Ferrite and pearlite were heterogeneously distributed at the surface and distributed in bands at the center of the sheets. The hot rolled sheets revealed a through-thickness texture inhomogeneity with a plane-strain texture with strong α-fiber and γ-fiber at the center and a shear texture at the surface. After cold rolling, the ferrite grains showed elongated morphology and larger orientation gradients, the period of the ferrite-pearlite band structure at the center of the sheets was decreased, and the plane-strain texture components were strengthened in the entire sheet. Recrystallization, phase transformation, and the competition of both processes were of particular interest with respect to the annealing experiments. For this purpose, various annealing techniques were applied, i.e., annealing in salt bath, conductive annealing, and industrial hot-dip coating. The sheets were annealed in the ferritic, intercritical, and austenitic temperature regimes in a wide annealing time range including variation of heating and cooling rates.

Abstract: We study the dislocation and twin substructures in a high manganese twinning-induced-plasticity steel (TWIP) by means of electron channeling contrast imaging. At low strain (true strain below 0.1) the dislocation substructure shows strong orientation dependence. It consists of dislocation cells and planar dislocation arrangements. This dislocation substructure is replaced by a complex dislocation/twin substructure at high strain (true strain of 0.3-0.4). The twin substructure also shows strong orientation dependence. We identify three types of dislocation/twin substructures. Two of these substructures, those which are highly favorable or unfavorable oriented for twinning, exhibit a Schmid behavior. The other twin substructure does not fulfill Schmid’s law.

Abstract: The formation of deformation-induced shear bands plays an important role for the room temperature deformation of both, Mg and Mg-Y alloys, but the formation and structure of shear bands is distinctively different in the two materials. Due to limited deformation modes in pure Mg, the strain is localized in few shear bands leading to an early failure of the material during cold deformation. Contrarily, Mg-RE (RE: rare earth) alloys exhibit a high density of homogeneously distributed local shear bands during deformation at room temperature. A study of the microstructure of the shear bands by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) at different strains was performed. These investigations give insight into the formation of shear bands and their effects on the mechanical behaviour of pure Mg and Mg-3Y. Since in pure Mg mainly extension twinning and basal <a> dislocation slip are active, high stress fields at grain resp. twin boundaries in shear bands effect fast growth of the shear bands. In Mg-RE alloys additionally contraction and secondary twinning and pyramidal <c+a> dislocation slip are active leading to the formation of microscopic shear bands which are limited to the boundary between two grains. The effects of shear bands on the mechanical behaviour of pure Mg and Mg-RE alloys are discussed with respect to their formation and growth.