Stefan Zaefferer

Micromechanical and macromechanical effects in grain scale polycrystal plasticity experimentation and simulation

A polycrystalline aluminum sample with a quasi-2D single layer of coarse grains is plastically deformed in a channel die plane strain set-up at ambient temperature and low strain rate. The microtexture of the specimen is determined by analysis of electron back scattering patterns obtained in a scanning electron microscope. The spatial distribution of the plastic microstrains at the sample surface is determined by measurement of the 3D plastic displacement field using a photogrametric pixel-based pattern recognition algorithm. The initial microtexture is mapped onto a finite element mesh. Continuum and crystal plasticity finite element simulations are conducted using boundary conditions which approximate those of the channel die experiments. The experimental and simulation data are analyzed with respect to macromechanical and micromechanical effects on grain-scale plastic heterogeneity. The most important contributions among these are the macroscopic strain profile (friction), the kinematic hardness of the crystals (individual orientation factors), the interaction with neighbor grain, and grain boundary effects. Crystallographic analysis of the data reveals two important points. First, the macroscopic plastic strain path is not completely altered by the crystallographic texture, but modulated following soft crystals and avoiding hard crystals. Second, grain- scale mechanisms are strongly superimposed by effects arising from the macroscopic profile of strain. The identification of genuine interaction mechanisms at this scale therefore requires procedures to filter out macro- scopically induced strain gradients. As an analysis tool, the paper introduces a micromechanicalTaylor factor, which differs from the macromechanical Taylor factor by the fact that crystal shear is normalized by the local rather than the global von Mises strain.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

On the influence of the grain boundary misorientation on the plastic deformation of aluminum bicrystals

Abstract Aluminum bicrystals with symmetric 〈1 1 2〉 tilt boundaries and misorientations of 8.7° (small angle), 15.4° (transition), and 31.5° (large angle) were deformed in a channel die experiment in order to study the influence of misorientation on the deformation at grain boundaries. Samples were characterized by strain measurements and microtexture mappings. The experiments were compared to crystal plasticity finite element simulations. We studied strain heterogeneity at the macroscopic and at the microscopic level. Even macroscopically homogeneous areas showed microscopic heterogeneity in the form of bands of different sets of glide systems. We observed clear effects of the grain boundary misorientation on the deformation kinematics close to the boundaries. The 8.7° grain boundary did not show any orientation change which was interpreted in terms of free dislocation penetration. In contrast, the 15.4° and 31.5° bicrystals showed orientation changes which were attributed to dislocation pile-ups.

EBSD as a tool to identify and quantify bainite and ferrite in low‐alloyed Al‐TRIP steels

Bainite is thought to play an important role for the chemical and mechanical stabilization of metastable austenite in low‐alloyed TRIP steels. Therefore, in order to understand and improve the material properties, it is important to locate and quantify the bainitic phase. To this aim, electron backscatter diffraction–based orientation microscopy has been employed. The main difficulty herewith is to distinguish bainitic ferrite from ferrite because both have bcc crystal structure. The most important difference between them is the occurrence of transformation induced geometrically necessary dislocations in the bainitic phase. To determine the areas with larger geometrically necessary dislocation density, the following orientation microscopy maps were explored: pattern quality maps, grain reference orientation deviation maps and kernel average misorientation maps. We show that only the latter allow a reliable separation of the bainitic and ferritic phase. The kernel average misorientation threshold value that separates both constituents is determined by an algorithm that searches for the smoothness of the boundaries between them.

Effects of topology on abnormal grain growth in silicon steel

This work addresses the role of grain topology on abnormal grain growth in silicon steel. The question was investigated whether the abnormal grain growth of Goss grains during secondary recrystallization can be interpreted in terms of an initial size advantage that these grains inherit from rolling and primary recrystallization. For this purpose the correlation between crystallographic orientation, size and number of next neighbors of large grains in the subsurface layer of a primary recrystallized silicon steel sheet was investigated. It was found that most of the large grains have an orientation on the h-fiber ( 001 axis parallel to the rolling direction) but are not particularly close to the Goss orientation. Also, no tendency of grains to be larger the closer they are to the Goss orientation was visible. Rather it was found that the scatter of the angular deviation to the Goss orientation is similar over a large range of grain sizes and this was found to be true too if the number of next neighbors of a grain rather than its grain size was checked. One single grain, however, was found that was close to the Goss orientation and had a high number of next neighbors and might therefore act as a nucleus for secondary recrystallization. Nevertheless, grains with a similarly high number of neighbors and a large deviation to the Goss orientation were found, too. Thus, a topological reason for the Goss texture evolution could not yet be proved. However, it might be that the extreme rareness of Goss nuclei (1 out of 10 6 grains) has prevented, up to now, from observing a true nucleus.

Characterization of phases of aluminized nickel base superalloys

Aluminum rich coatings, built up by a diffusion zone and a NiAl-cover layer, can protect the surface of turbine blades against oxidation. Within the single crystalline substrate and the adjacent layer, phases in the range of several tens of nanometers up to a few micrometers develop during production and operation of the turbine blade, were characterized. Investigations with transmission electron microscopy, nanoindentation and local crystal orientation mapping with a scanning electron microscope have been carried out in order to determine composition, morphology and distribution of the different phases. The diffusion zone has in general a defined orientation relative to the superalloy substrate and is built up by at least three phases embedded in a softer matrix, with significant differences in nanohardness. Local internal stress states in the diffusion zone are estimated. The NiAl-cover layer is a coarse columnar grained, non-textured B2 ordered intermetallic NiAl-phase.

Interdigitating biocalcite dendrites form a 3-D jigsaw structure in brachiopod shells

We report a newly discovered dense microstructure of dendrite-like biocalcite that is formed by marine organisms. High spatial resolution electron backscatter diffraction (EBSD) was carried out under specific analytical conditions (15 and 10 kV) on the primary layer of the modern brachiopod Gryphus vitreus. The primary layer of modern brachiopods, previously termed nanocrystalline, is formed by an array of concave/convex calcite grains with interdigitated recesses and protrusions of abutting crystals without any cavities in or between the dendrites. The interface topology of this structure ranges from a few tens of nanometres to tens of micrometres, giving a nanoscale structure to the material fabric. The dendritic grains show a spread of crystallographic orientation of several degrees and can thus be referred to as mesocrystals. Individual dendritic mesocrystals reach sizes in one dimension larger than 20 μm. The preferred crystallographic orientation is similar in the primary and adjacent fibrous shell layers, even though these two layers show completely different crystal morphologies and grain boundary topologies. This observation indicates that two separate control mechanisms are active when the primary and the fibrous shell layers are formed. We propose a growth model for the interdigitated dendritic calcite grain structure based on a precursor of vesicles filled with amorphous calcium carbonate (ACC).

Computer-aided crystallographic analysis in the TEM

Publisher Summary This chapter presents the techniques for computer-aided crystallography in analytical transmission electron microscopy (TEM). The techniques include different methods for the semiautomated and completely automated determination of crystal orientations from different forms of electron diffraction patterns. The chapter discusses the analysis of different lattice defects like grain boundaries and dislocations. It presents the methods for the determination of crystal lattice parameters and for the calibration of microscope parameters. The concepts for the determination of crystal orientations are introduced and compared especially under the aspect of reaching the highest spatial resolution and orientation accuracy. Although the use of Kikuchi patterns is the most precise method, the use of spot patterns can be very interesting and becomes even indispensable in the case of orientation measurements in highly deformed metals. The accuracy of the spot pattern orientation determination can be much improved by taking into account the positions and intensities of the diffraction spots.

Advances in TEM orientation microscopy by combination of dark-field conical scanning and improved image matching

A new approach to automatic TEM-based orientation microscopy is presented, which is based on a combination of the techniques of dark-field conical scanning and improved image matching, and a diffraction pattern simulation method. For indexing, a full experimental diffraction pattern is compared to all possible pre-calculated diffraction patterns for the given structure by image matching. In order to speed up this relatively calculation-intensive algorithm, polar transformation and, most important, circular projection that increase the speed of pattern indexing by a factor of about 50 are proposed. A microstructure of submicron scale and crystallographic orientations in nanocrystalline materials are measured successfully. It is proposed that the taken approach of dark-field conical scanning and improved image matching may be, in principle, better suited for TEM-based orientation microscopy than serial orientation mapping.

Characterization of order domains in γ-TiAl by orientation microscopy based on electron backscatter diffraction

A new approach to resolve the slight tetragonality of L10-ordered γ-TiAl by electron backscatter diffraction (EBSD) is presented. The phase has a c/a ratio of only about 2% larger than unity. The corresponding EBSD patterns therefore exhibit cubic pseudosymmetry. As a consequence, different order variants cannot be easily distinguished on the basis of their EBSD patterns. Automated orientation mapping results in frequent misindexing. In the past, either this problem was overcome by identifying order domains by relatively laborious transmission electron microscopy, or the order domain structure was ignored altogether by using a generic face-centered cubic structure to solve for the crystal orientations, accepting a significant loss of microstructural information. The presented approach is based on the detection of the minor tetragonal distortion of the diffraction patterns by an accurate measurement of backscatter Kikuchi band positions. To this end an accurate pattern center calibration together with high-accuracy parameters for pattern acquisition and indexing are required. Together with a modified indexing algorithm, the order domains in a lamellar microstructure of Ti–45.9Al–8Nb (at%) could be reliably identified. The occurrence of superlattice reflections in the Kikuchi patterns was used to validate the technique. The developed method was successfully applied to create a crystal orientation map of Ti–45.9Al–8Nb (at%) with a fully resolved domain microstructure.

Three-dimensional EBSD study on the relationship between triple junctions and columnar grains in electrodeposited Co–Ni films

Electrodeposited nanocrystalline materials are expected to have a homogeneous grain size and a narrow grain size distribution. In Co–Ni electrodeposited films, however, under certain conditions an undesired columnar grain structure is formed. Fully automated three‐dimensional (3D) orientation microscopy, consisting of a combination of precise material removal by focussed ion beam and subsequent electron backscatter diffraction (EBSD) analysis, was applied to fully characterize the grain boundaries of these columnar grains in order to gain further understanding on their formation mechanisms. Two‐dimensional orientation microscopy on these films indicated that the development of columnar grains could be related to the formation of low‐energy triple junctions. 3D EBSD allowed us to verify this suggestion and to determine the boundary planes of these triples. The triplets are formed by grain boundaries of different quality, a coherent twin on the {} plane, an incoherent twin and a large‐angle grain boundary. These three boundaries are related to each other by a rotation about the 〈〉 direction. A second particularity of the columnar grains is the occurrence of characteristic orientation gradients created by regular defects in the grain. Transmission electron microscopy was applied to investigate the character of the defects. For this purpose, a sample was prepared with the focussed ion beam from the last slice of the 3D EBSD investigation. From the TEM and 3D EBSD observations, a growth mechanism of the columnar grains is proposed.