Papers by Author: Dierk Raabe

Abstract: A Co-20at%Ni polycrystal produced by electrodeposition was studied in its detail using orientation microscopy. By analyzing the local crystallographic texture, grain morphology and the grain boundary character on three distinct sections of the film, we have obtained a complete understanding of its microstructure. The microstructure and grain morphology is very complex, consisting of grains elongated in deposit direction, clusters of coarse and fine grains and further very fine structures. The deposition parameters generate a strong (1120)//ND texture and the columnar grain reveals inner orientation gradients along the growth direction

Abstract: In this study we present experimental investigations on the microscopic structure, constituent phases, and crystallographic textures of the exoskeleton of three types of decapod crustaceans, namely, lobster, crab, and horseshoe crab. The carapace of such animals is a biological multiphase nano-composite consisting of an organic matrix (crystalline chitin and non-crystalline proteins) and biominerals (calcite, phosphate). The synchrotron measurements of the crystalline chitin and of the biominerals which are embedded in the chitin-protein matrix (in case of lobster and crab) reveal strong textures. The horseshoe crab does not seem to contain notable amounts of crystalline minerals. The Debye-Scherrer images of the lobster specimen suggest that the biominerals form clusters of crystals with similar crystallographic orientation. TEM images support this suggestion. The crystallographic texture of the chitin is arranged with its longest cell axis parallel to the normal of the surface of the exoskeleton.

Abstract: We present a numerical study on the influence of crystallographic texture on the earing behavior of a low carbon steel during cup drawing. The simulations are conducted by using the texture component crystal plasticity finite element method which accounts for the full elastic-plastic anisotropy of the material and for the explicit incorporation of texture including texture update. Several important texture components that typically occur in commercial steel sheets were selected for the study. By assigning different spherical scatter widths to them the resulting ear profiles were calculated under consideration of texture evolution. The study reveals that 8, 6, or 4 ears can evolve during cup drawing depending on the starting texture. An increasing number of ears reduces the absolute ear height. The effect of the orientation scatter width (texture sharpness) on the sharpness of the ear profiles was also studied. It was observed that an increase in the orientation scatter of certain texture components entails a drop in ear sharpness while for others the effect is opposite.

Abstract: Crystallographic slip, i.e. movement of dislocations on distinct slip planes, is the main source of plastic deformation of most metals. Therefore, it was an obvious idea to build a constitutive model based on dislocation densities as internal state variables in the crystal plasticity. In this paper the dislocation model recently proposed by Ma and Roters (Ma A. and Roters F., Acta Materialia, 52, 3603-3612, 2004) has been extended to a nonlocal model through separating the statistically stored dislocation and geometrically necessary dislocation densities. A nonlocal integration algorithm is proposed, which can be more easily used in conjunction with commercial software such as MARC and ABAQUS than the model proposed in the work of Evers(Evers L.P., Brekelmans W.A.M., Geers M.G.D., Journal of the Mechanics and Physics of Solids, 52, 2379-2401, 2004).

Abstract: Crystal plasticity FEM simulations of plane strain compression were performed. The Texture Component Crystal Plasticity-FEM was used for the texture mapping. Two different starting textures (random and hot rolling texture) were studied using four different FE meshes and two different sets of boundary conditions. While for the random starting texture the evolution of the texture with deformation was found to be rather similar in all cases studied, the simulations using an experimental hot rolling texture as staring texture are much more sensitive to the boundary conditions and probably also to changes in the mesh geometry.

Abstract: We report the results of the microstructural characterization of a Ti-bearing IF-steel deformed at high strain rates (» 6.104 s-1) in a split Hopkinson bar. The shock-loading tests were performed in hat-shaped specimens to induce the formation of adiabatic shear bands (ASB). The samples were deformed at 223 K and 298 K. High-resolution electron backscatter diffraction (EBSD) reveals the development of an ultrafine-grained structure within the ASB. A closer inspection reveals the presence of deformation twins in grains adjacent to the shear band. These twins bend towards the ASB suggesting that mechanical twinning occurs before the flow associated to shear banding. The results of microtexture have indicated the presence of a sharp <111> g-fiber texture in the ASB for both temperatures.

Abstract: This article introduces a 3D cellular automaton model for the prediction of spherulite growth phenomena in polymers at the mesoscopic scale. The automaton is discrete in time, real space, and orientation space. The kinetics is formulated according to the Hoffman-Lauritzen secondary surface nucleation and growth theory for spherulite expansion. It is used to calculate the switching probability of each grid point as a function of its previous state and the state of the neighboring grid points. The actual switching decision is made by evaluating the local switching probability using a Monte Carlo step. The growth rule is scaled by the ratio of the local and the maximum interface energies, the local and maximum occurring Gibbs enthalpy of transformation, the local and maximum occurring temperature, and by the spacing of the grid points. The use of experimental input data provides a real time and space scale.

Abstract: This is a study on texture, decrystallization, and recrystallization in rolled and heat treated semi–crystalline polymers. Experiments were conducted using wide angle Debye-Scherrer-type X– ray diffraction. Changes in crystallinity were quantitatively monitored as a function of strain and annealing time. It was observed that crystallinity drastically drops during deformation in PET. We suggest that amorphization (decrystallization) is a deformation mechanism which acts as an alternative to crystallographic slip depending on the orientation of the nanocrystalline lamellae. Heat treatment leads to the recrystallization of amorphous PET material and to an enhancement of the original texture of the deformed crystals observed before the heat treatment. We explain this phenomenon in terms of oriented nucleation where amorphous material crystallizes alongside existing crystalline lamellae.

Abstract: The recrystallization behavior of coarse-grained niobium depends on the nature of its deformation microstructure. In this regard, a longitudinal section of a high-purity coarse-grained niobium ingot was cold rolled to a thickness reduction of 96% followed by annealing in vacuum at 800°C for 1 h. Metallographic inspection in cold-rolled and annealed specimens was carried out in a field emission gun scanning electron microscope (FEG-SEM). Microtexture was determined by electron-backscattered diffraction (EBSD) coupled to the FEG-SEM. The use this technique has evidenced details of the boundary character and subgrain structure found in partially recrystallized regions. The early stages of primary recrystallization are associated to the presence of high-angle lamellar boundaries found in the cold-worked state. Abnormal subgrain growth has been evidenced as a viable mechanism for nucleation of recrystallization.

Abstract: The ferritic rolling strategy allows for the production of two different hot strip grades, a "soft" and a "hard" hot strip. The "soft" hot strip is rolled in the upper ferrite region and a sufficiently high coiling temperature ensures direct recrystallisation in the coil. The "hard" hot strip is rolled at relatively lower temperatures in the ferrite temperature region and exhibits a strained microstructure with a desirable rolling texture. Furthermore, these ferritic rolled hot strips can be used as initial strip for subsequent cold rolling. The current investigation focuses on the development of the recrystallisation texture of cold rolled and annealed ferritic rolled hot strip for different cold reductions. For this purpose "soft" and "hard" hot strips were produced on a laboratory hot rolling mill. These strips were cold rolled with a total reduction of 40 to 80% to a final thickness of 0.5mm. Subsequently the strips were subjected to simulated continuous annealing, using a salt bath furnace. The macro texture of both types of specimens was measured and correlated to the mechanical properties, including the Lankford values. A very different development of the recrystallisation texture and hence mechanical properties has been observed. However, both grades yielded improved deep-drawing properties.