Jan T. Bonarski

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.

Crystallographic relationships in the crossed lamellar microstructure of the shell of the gastropod Conus marmoreus.

The crossed lamellar microstructure of mollusk shells shows a very complex hierarchical architecture constituted of long rod-shaped aragonite crystals stacked parallel to each other inside each first order lamella, which are almost perpendicular to the ones contained in parallel neighboring lamellae. To better understand the construction and properties of the crossed lamellar microstructure we have performed a detailed study to determine the crystallographic characteristics and their evolution during shell growth using scanning electron microscopy, transmission electron microscopy and X-ray diffraction texture analysis. The arrangement of crystals is rationalized by a set of twin law relationships between aragonite crystals. Specifically, the aragonite rods, or third order lamellae within each first order lamella, internally consist of polysynthetic twins bounded by {110} mirror planes. In turn, the polysynthetically twinned aragonite crystals also show a constant crystallographic orientation with respect to aragonite crystals in adjacent first order lamellae. It can be seen as another twin law in which crystals from adjacent lamellae are bounded by (110) planes but with their c-axes rotated within this plane by 30°. Thus there are two sets of twin laws that relate crystal units at lower (third order lamellae) and higher (first order lamellae) length scales. These hierarchical relationships play a crucial role in the construction, organization and properties of this complex microstructure. The later orientational relationships have never been described in geological aragonite and are only found in biogenic materials with a crossed lamellar microstructure. Their occurrence is probably determined by the presence of shell organic components which regulate crystal growth and may favor unusual crystallographic relationships.

Crystallographic control on the substructure of nacre tablets

Nacre tablets of mollusks develop two kinds of features when either the calcium carbonate or the organic portions are removed: (1) parallel lineations (vermiculations) formed by elongated carbonate rods, and (2) hourglass patterns, which appear in high relief when etched or in low relief if bleached. In untreated tablets, SEM and AFM data show that vermiculations correspond to aligned and fused aragonite nanogloblules, which are partly surrounded by thin organic pellicles. EBSD mapping of the surfaces of tablets indicates that the vermiculations are invariably parallel to the crystallographic a-axis of aragonite and that the triangles are aligned with the b-axis and correspond to the advance of the {010} faces during the growth of the tablet. According to our interpretation, the vermiculations appear because organic molecules during growth are expelled from the a-axis, where the Ca-CO3 bonds are the shortest. In this way, the subunits forming nacre merge uninterruptedly, forming chains parallel to the a-axis, whereas the organic molecules are expelled to the sides of these chains. Hourglass patterns would be produced by preferential adsorption of organic molecules along the {010}, as compared to the {100} faces. A model is presented for the nanostructure of nacre tablets. SEM and EBSD data also show the existence within the tablets of nanocrystalline units, which are twinned on {110} with the rest of the tablet. Our study shows that the growth dynamics of nacre tablets (and bioaragonite in general) results from the interaction at two different and mutually related levels: tablets and nanogranules.

Effects of thermal modification on wood ultrastructure analyzed with crystallographic texture

Abstract The changes in ultrastructure of thermally modified beech and poplar wood have been studied by means of the crystallographic texture analysis. The experimental pole figures have been registered based on the X-ray diffraction (XRD). The 3D texture function, i.e., the orientation distribution function (ODF) was calculated with the use of the procedure implemented for materials with the monoclinic lattice symmetry. The inverse pole figures were recalculated from the obtained ODF. Areas with specific crystallographic organization in wood were identified and gradual changes in intensities of moderately oriented areas were observed. Simultaneously, additional reorganization of crystalline cellulose was observed as a result of heat treatment. The integrated skeleton lines confirmed the gradual decomposition of hemicelluloses as well as the temporary increase of cellulose self-organization.

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.

Texture function application for wood ultrastructure description. Part 2: Application

The practical application of the orientation distribution function (ODF), being the three-dimensional representation of the texture function, was presented. The investigations of the crystallograpically organized regions of wood were performed. The experimental methods of the X-ray diffraction were discussed in detail. The incomplete pole figures were measured in order to calculate the ODF. The dominant components of the crystallographical texture of wood were determined from the obtained ODF. The obtained texture function was also used to derive the complete pole figures, additional non-measurable figures as well as inverse pole figures. The figures were used to perform additional analysis of the crystallographical organization. The traditional concept of the microfibril angle was enhanced by defining two transformation parameters, i.e. a crystallographical axis and an angle of rotation.

Effects of cell wall ultrastructure on the transverseshrinkage anisotropy of Scots pine wood

Abstract A hypothesis for explaining the differential anisotropic shrinkage behavior of wood has been proposed, and it was based on the differences in the cell wall ultrastructure. The starting point of the consideration is that wood shrinkage is governed by its chemical composition, ultrastructure, and gross anatomy. It is also well known that the transverse shrinkage anisotropy of earlywood (EW) is more pronounced than that of the latewood (LW). In the paper, the cell wall ultrastructure and shrinkage anisotropy has been related to each other, and to this purpose, a set of crystallographic texture descriptors was applied. The descriptors are based on X-ray diffraction (XRD) experiments conducted on matched EW samples from different growth rings of Scots pine. The range of the microfibril angle (MFA) was identified. The ratio of the maxima of inverse pole figures (IPFs) of both the tangential (T) and radial (R) directions was determined. The ratios quantify the inhomogeneity of the spatial arrangement of the ordered areas. The results of the study clearly indicate that the transverse shrinkage of wood is governed mostly by a specific ultrastructural organization of moderately organized cell wall compounds.

Optimization of X-Ray Pole Figure Measurement

The traditionally applied registration method of the back-reflection pole figure is based on the equiangular measurement lattice. It determines also the equiangular character of presentation of measurement results in form of stereographic projection, termed as the pole figure. The mentioned registration mode is characterized by an unequal density of the measurement points on the pole figure. It is the evident disadvantage of the traditional registration mode. In order to eliminate the drawback, and to increase the efficiency of the measurement procedure, an optimization of the registration method was made. The optimization consists in dividing a reference sphere of the stereographical projection into uniform regions, so called equal solid angles (ESA). As a result of the applied optimization, over 40% reduction in number of the measurement points and in the registration time at preserved pole figure quality was obtained. For verification of the new solution, a set of experimental pole figures of cold rolled copper by the traditional mode as well as the introduced ESA one was recorded. Comparison of the results of texture analysis based on the orientation distribution function was carried out. The results of measurements performed by the ESA method confirm the efficiency of the introduced optimization.

Irregularities of crystallographic orientation and residual stresses in the crossed-lamellar shell as a natural functionally graded material

The microstructures of different groups of molluscs are characterized by preferential orientations of crystallites (texture), leading to a significant anisotropy of the physical properties of the shells. A complementary characteristic, usually neglected, is the distribution of the residual stresses existing within the shell wall. By means of X-ray diffraction, we study the distribution of stresses with thickness in the shell wall of the gastropod Conus marmoreus, which has a microstructure of the crossed-lamellar type. The results revealed an extraordinary texture inhomogeneity and the existence of tensional residual stresses along the shell thickness, the origins of which are unknown. Some of the observed changes in textural parameters and stresses coincide with the transitions between shell layers, although other features are of unknown origin. Our results provide insight into the microstructural regularities that govern the mesoscale construction of shells, such as that of C. marmoreus.

Texture function application for wood ultrastructure description. Part 1: theory

The three-dimensional orientation distribution function (ODF), being a probability of the density of crystallographic orientations, is widely used for describing internal arrangement of polycrystalline materials (especially metals and alloys). The application of the ODF was enhanced in the paper for the monoclinic crystal symmetry. The algorithm of the crystallographic texture analysis based on the arbitrarily defined cells method was presented together with the discussion of the input data required for analyzing the space organization of the ultrastructure of materials with the monoclinic lattice symmetry. The test analysis was performed for the model material of the same type of the crystallographic lattice as in wood cellulose. The obtained ODF was presented and discussed. The results of the analysis were supplemented both with reconstructed complete pole figures and with the inverse pole figures.