Papers by Author: Dierk Raabe

Abstract: The exoskeleton of the crustacean Homarus americanus, the American lobster, is a biological multiphase composite consisting of a crystalline organic matrix (chitin), crystalline biominerals (calcite), amorphous calcium carbonate and proteins. One special structural aspect is the occurrence of pronounced crystallographic orientations and resulting directional anisotropic mechanical properties. The crystallographic textures of chitin and calcite have been measured by wide-angle Bragg diffraction, calculating the Orientation Distribution Function (ODF) from pole figures by using the series expansion method according to Bunge. A general strong relationship can be established between the crystallographic and the resulting mechanical and physical properties.

Abstract: Using SEM/EBSD the substructure and texture evolution in dual phase steels in the first steps of the process chain, i.e. hot rolling, cold rolling, and following annealing were characterized. In order to obtain dual phase steels with high ductility and high tensile strength an industrial process was reproduced by cold rolling of industrially hot rolled steel sheets of a thickness of 3.75 mm with ferrite and pearlite morphology down to a thickness of 1.75 mm and finally annealing at different temperatures. Such technique allows a compilation of ferrite and martensite morphology typical for dual phase steels. Due to the competition between recovery, recrystallization and phase trans-formation during annealing a variety of ferrite martensite morphologies was produced by promoting one of the mechanisms through the variation of technological parameters such as heating rate, intercritical annealing temperature, annealing time, cooling rate and the final annealing temperature. Annealing induced changes of the mechanical properties were determined by hardness measurements and are discussed on the basis of the results of the substructure investigations.

Abstract: Polycrystalline iron was deformed by eight ECAE passes using the route Bc to a total strain of 9.2. After deformation the material was annealed at temperatures up to 800oC. Scanning electron microscopy (SEM) and high-resolution electron backscatter diffraction (EBSD) were used to characterize both deformed and annealed structures. In the as-deformed state, the mean grain size is 650 nm and the volume fraction of high angle boundaries (VHAB) is 56%. Upon annealing there is a pronounced softening above 300oC. At the beginning of recrystallization, at about 400oC, the VHAB increases to 71%. The results indicate that discontinuous recrystallization is the main softening mechanism in severely deformed iron.

Abstract: Obtaining knowledge on the grain boundary topology in three dimensions is of great importance as it controls the mechanical properties of polycrystalline materials. In this study, the three dimensional texture and grain topology of as-deformed ultra fine grained Cu-0.17wt%Zr have been investigated using three-dimensional orientation microscopy (3D electron backscattering diffraction, EBSD) measurements in ultra fine grained Cu-0.17wt%Zr. Equal channel angular pressing was used to produce the ultra fine grained structure. The experiments were conducted using a dual-beam system for 3D-EBSD. The approach is realized by a combination of a focused ion beam (FIB) unit for serial sectioning with high-resolution field emission scanning electron microscopy equipped with EBSD. The work demonstrates that the new 3D EBSD-FIB technique provides a new level of microstructure information that cannot be achieved by conventional 2D-EBSD analysis.

Abstract: The study presents an analytical model for predicting crystallographic textures and the final grain size during primary static recrystallization of metals using texture components. The kinetics is formulated as a tensorial variant of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The tensor form is required since the kinetic and crystallographic evolution of the microstructure is described in terms of a limited set of growing (recrystallizing) and swept (deformed) texture components. The number of components required defines the order of the tensor since the kinetic coupling occurs between all recrystallizing and all deformed components. The new method is particularly developed for the fast and physically-based process simulation of recrystallization textures with respect to processing. The present paper introduces the method and applies it to the primary recrystallization of low carbon steels.

Abstract: PM 1000 is a nickel-based oxide dispersion strengthened (ODS) superalloy used for high-temperature applications. The primary recrystallization of a <100>-fiber textured coarsegrained oxide dispersion strengthened nickel-based superalloy (PM 1000) has been investigated. The annealing behavior of this alloy is quite complex. Even when annealing is performed at high homologous temperatures (e.g. 0.9 Tm, Tm is the melting point), recrystallization is partial. In order to understand such a behavior, the microstructure of specimens in both the as-received, deformed, and annealed conditions has been imaged in detail using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron backscatter diffraction (EBSD). In the annealed state we observe a significant volume fraction of tiny crystals in the interior of the recovered grains. These tiny grains are elongated and grow mostly along the existing low angle dislocation boundaries (anisotropic growth). In the present paper we propose a twinningassisted nucleation mechanism to clarify their origin during recrystallization.

Abstract: High-purity niobium single crystals were deformed by equal-channel angular extrusion (ECAE) at room temperature to an equivalent Von Mises strain of about 1.15. Deformed samples were annealed in vacuum from 500 to 800oC for 1 hour to investigate their microstructure evolution. The microstructure of deformed and annealed samples was characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Vickers microhardness testing. The deformed structure after one ECAE pass is rather inhomogenous and consists of parallel sets of coarse shear bands whose spacing varies from one region to another in the cylindrical billet. the microstructure within the shear bands consists of elongated subgrains with sizes below 3 μm and lamellar boundaries. The remaining non-sheared regions display a coarser subgrain structure. Recrystallization is virtually absent in samples annealed at 500oC for 1 hour. Nucleation begins mostly within shear bands. The new grains with sizes ranging from 10 to 50 μm are arranged in clusters rather than being homogenously distributed. The recrystallized volume fraction also varies from one region to another indicating an inhomogenous distribution of stored energy. At 700oC, recrystallization is complete after annealing for 1 hour resulting in a structure with a mean grain size of about 100 μm.

Abstract: Recrystallisation behavior was studied in two Fe3Al-based alloys containing both large and fine particles with a different fine particle dispersion level using high-resolution SEM and EBSD. High misorientation of 15-30° was created around large particles after a hot rolling process in the two alloys. The kinetics of recrystallisation were, however, considerably retarded in the alloy containing dense fine particles. It was observed that the growth of subgrains created around the large particles was inhibited by the presence of the fine particles. This result clearly suggests that when the particle density (Ns) is high relative to local stored energy (E) around large particles, nucleation can be completely hindered. As the Ns/E level decreases, nucleation may occur and the kinetics of recrystallisation might be determined by both the nucleation rate and the growth of nuclei into a matrix with fine particles.

Abstract: This work studies the rotations of a (111) Cu single crystal due to the application of a conical nanoindent. With the aid of a joint high-resolution field emission SEM-EBSD set-up coupled with serial sectioning in a focused ion beam (FIB) system in the form of a cross-beam 3D crystal orientation microscope (3D EBSD) a 3D rotation map underneath the indent could be extracted. When analyzing the rotation directions in the cross section planes (11-2) perpendicular to the (111) surface plane below the indenter tip we observe multiple transition regimes with steep orientation gradients and changes in rotation direction. A phenomenological and a physically-based 3D elastic-viscoplastic crystal plasticity model are implemented in two finite element simulations adopting the geometry and boundary conditions of the experiment. While the phenomenological model predicts the general rotation trend it fails to describe the fine details of the rotation patterning with the frequent changes in sign observed in the experiment. The physically-based model, which is a dislocation density based constitutive model, succeeded to precisely predict the crystal rotation map compared with the experiment. Both simulations over-emphasize the magnitude of the rotation field near the indenter relative to that measured directly below the indenter tip. However, out of the two models the physically-based model reveals better crystal rotation angles

Abstract: In this work we present deformation experiments of polymer-coated polycrystalline aluminium sheets. We observe that the straining is accompanied by the development of microstructural defects at the sample surface as well as in the interface between the metal and the different polymers. These defects are due to a variety of dynamical mechanisms which are essentially induced by bulk plasticity of the metal substrate. They micromechanically interact with the polymer coating and transfer some of the metallic roughness to the coating and to the surface.