Jan Pospiech

Crystallographic orientation inhomogeneity and crystal splitting in biogenic calcite

The calcitic prismatic units forming the outer shell of the bivalve Pinctada margaritifera have been analysed using scanning electron microscopy–electron back-scatter diffraction, transmission electron microscopy and atomic force microscopy. In the initial stages of growth, the individual prismatic units are single crystals. Their crystalline orientation is not consistent but rather changes gradually during growth. The gradients in crystallographic orientation occur mainly in a direction parallel to the long axis of the prism, i.e. perpendicular to the shell surface and do not show preferential tilting along any of the calcite lattice axes. At a certain growth stage, gradients begin to spread and diverge, implying that the prismatic units split into several crystalline domains. In this way, a branched crystal, in which the ends of the branches are independent crystalline domains, is formed. At the nanometre scale, the material is composed of slightly misoriented domains, which are separated by planes approximately perpendicular to the c-axis. Orientational gradients and splitting processes are described in biocrystals for the first time and are undoubtedly related to the high content of intracrystalline organic molecules, although the way in which these act to induce the observed crystalline patterns is a matter of future research.

Orientation and misorientation characteristics of annealed, rolled and recrystallized copper

The texture and microstructure of annealed, rolled and recrystallized copper (99.99%) were investigated using conventional X-ray diffraction and single-crystal electron backscattering diffraction (EBSP) in a scanning electron microscope. From the EBSP data, misorientation distribution functions for next neighbor orientation relationships were derived, which show that the undeformed (annealed) starting material and the recrystallized sample have primary and secondary twin correlations, whereas in the deformed sample no preferred misorientations are observed. The microtexture of a single grain with twin lamellae in the deformed sample was analyzed in detail using automated analysis of electron backscattering diffraction patterns. The twin relationship is transformed continuously into a high angle grain boundary during deformation. Orientation gradients within one grain increase steadily up to 20°.

Microstructure and texture of Mg-based AZ alloys after heavy deformation under cyclic strain path change conditions

The effects associated with the change of the deformation path - such as the replacement of homogeneous multi-slip by heterogeneous deformation and a decrease of global strain hardening - have been utilised in the metal forming operation termed KOBO technology. In the case of extrusion it consists in reversible, cyclic twisting of a billet under the extrusion force. The technology enables extrusion of metals with very large deformation in one operation at low temperature. A complex scheme of straining, large cumulated deformation and low temperature of the process results in a fine grained microstructure of the extruded material (product). The new technology requires detailed studies of the mechanism of the plastic deformation with the specific geometry of the zone of metal flow during extrusion. Essential in these studies is the information on the texture and microstructure in the deformation zone. The aim of this work is therefore to disclose the deformation mechanisms on the basis of the observations of microstructure and texture evolution in the zone of plastic flow of the extrudate. Coarse grained polycrystalline billets of magnesium alloys AZ31 were extruded by KOBO at room temperature and also by a conventional method at about 400°C. Methods of texture topography as well as optical observations reveal the specific microstructure and texture in mezzo and micro scale of heavily deformed material after extrusion. It is worth mentioning that the KOBO process leads to compact and rather homogeneous extrudates even in the case of AZ alloys. These hexagonal metals cannot be cold-formed to a high reduction with conventional techniques.

Microstructure and Texture in Copper Sheets after Reverse and Cross Rolling

The objectives of this investigation are structural effects in electrolytic copper sheets which are caused by the change of the direction of rolling. Unidirectional, reverse as well as cross-rolling at 90° respectively at 45° to the precedent rolling direction have been applied down to final deformations as low as 80% reduction in thickness. Texture has been determined by ACOM (Automated Crystal Orientation Measurement, “Automated EBSD”) in the SEM and by X-ray pole figure measurement. The main benefits of ACOM are a high spatial resolution which enables the investigation of texture gradients from the mid plane to the surface of the sheet, and the visualization of the microstructure by crystal orientation mapping. In addition to local texture, statistical distributions of misorientations across grain boundaries and of S grain boundaries have been derived from the individual grain orientation data. The change of the path of plastic deformation induces a destabilization of the substructure which is formed during the primary step of unidirectional rolling. A distinct change of texture is found depending on the deformation process. In cross rolling, the b fiber changes into the unstable b90 fiber which almost disappears with progressive deformation along the new rolling direction.

Local and Global Effects in Texture and Microstructure Observed after Channel-Die Compression of Copper Single Crystals and after Cross-Rolling of Copper Sheets

The change of the deformation path leads to destabilization of the substructure and affects the texture of the deformed metal. The observed changes of texture and microstructure are, as a rule, significant and their characteristics depend on the geometry of the deformation process. Previous investigations on copper (and copper alloy) samples after deformation by rolling and channel-die compression were based on X-ray pole figure measurements and on observations in the light microscope. Hereby only global texture and structural characteristics have been obtained. The present study is mainly based on measurements of individual crystal orientations performed by ACOM (Automated Crystal Orientation Measurement, “Automated EBSD”) in the SEM which enables a precise local analysis of the investigated phenomena. For the channel-die experiments, (1 1 2)[1 1 -1] and (1 1 2)[1 -1 0] oriented copper single crystals have been used. After pre-deformation, a second deformation step has been carried out in transverse direction. The {1 1 2}<1 1 0> orientations are destabilized by channel-die compression, and clusters of layers develop which are composed of complementary {1 1 0}<1 1 2> components. The deformation process in polycrystalline sheets after rotating the rolling direction leads again to a distinct disintegration of the microstructure and destabilization of the b fiber. This process of microstructure reorganization after pre-deformation is fast and of high dynamics.

The Effect of Shear Bands on the Evolution of Rolling and Recrystallization Texture in Cold-Rolled Direct Chill Cast Strips of Brass

The texture of cold rolled and annealed cast strips of 70/30 brass was investigated. A delayed development of the main {110} component of brass type rolling texture was attributed to the intense deformation twinning which is associated with appearance of the strong {211} texture component at 50% reduction. The predominace of the {011} texture component in the range of 70%-90% reductions was related to the extensive shear banding. As for the rolling textures, the delay in development of the {236} component of brass type recrystallization texture was found.

Angular Extrusion of Double-Metal Ingot

Relatively high mechanical strength and simultaneously good plasticity of a crystalline material are determined by the state of its internal structure, preferably nano- or ultra-fine grained one. To achieve the above combination of properties, various manners of plastic deformation and heat treatment are applied in practice. One of the most effective processes in this field is severely plastic deformation, e.g. by the method of equal angular channel pressing (ECAP). During the ECAP, favourable effects of grain fragmentation and the formation of specific orientation relations can be attenuated by the process of structure recovery, especially, when the real temperature of angular extrusion is elevated for physical or technological reasons. An attempt to modify the ECAP technology was considered, to avoid the unfavourable temperature effects and to increase at the same time the efficiency of manufacturing the ultra-fine structure of material. Extrusion of dual-material (AZ31 + Al) ingot was performed at room temperature. As it seems, the well known difficulties with plastic deformation of materials with hexagonal lattice symmetry, like AZ31 alloy, have been decreased. Both experimental and methodological aspects of the angular extrusion of the dual-material ingot and chosen microstructure characteristics (texture, stress, morphology) are presented. On the basis of the suggested modification, the text discusses an explanation of physical origins of the microstructure evolution in the investigated material revealed by experiments.

Strain Induced Fracture in Textured Al-Li Alloys

Metallurgical as well as geometrical factors may be responsible for severe in-plane anisotropy of mechanical properties and abnormal fracture behaviour of aluminium-lithium alloys sheets. The crystallographical texture is considered to explain the observed anisotopy of tensile strength and the coefficient anisotropy (r-value). Morphology of the grain is additionally taken into account when discussing the fracture behaviour.