Materials Science Forum Vols. 495-497

Abstract: It is often assumed that the texture formation during solid state transformations in low carbon steels critically depends on the local crystallographic misorientation at the interface between transformed and not yet transformed material volume. In some cases, a theoretical crystallographic orientation relation can be presumed as a necessary prerequisite for the transformation to occur. Classical examples of such misorientation conditions in steel metallurgy are the orientation relations between parent and product grains of the allotropic phase transformation from austenite to ferrite (or martensite) or the hypothetical <110>26.5º misorientation between growing nuclei and disappearing grains in a recrystallization process. One way to verify the validity of such misorientation conditions is to carry out an experiment in which the transformation is partially completed and then observe locally, at the transformation interface, whether or not the presumed crystallographic condition is complied with. Such an experiment will produce a large set of misorientation data. As each observed misorientation Dg is represented by a single point in the Rodrigues-Frank (RF) space, a distribution of discrete misorientation points is obtained. This distribution is compared with the reference misorientation Dgr, corresponding to a specific physical condition, by determining the number fraction dn of misorientations that are confined within a narrow misorientation volume element dw around the given reference misorientation Dgr. In order to evaluate whether or not the proposed misorientation condition is obeyed, the number fraction dn of the experimentally measured distribution must be compared with the number fractions dr obtained for a random misorientation distribution. The ratio dn/dr can be interpreted as the number intensity fi of the given reference misorientation Dgr. This method was applied on the observed local misorientations between the recrystallizing grains growing into the single crystal matrix of a Fe-2.8%Si alloy. It was found that the number intensity of the <110>26.5º misorientation increased with a factor 10 when the misorientation distribution was evaluated before and after the growth stage. In another example the method was applied to the misorientations measured at the local interface between parent austenite and product martensite grains of a partially transformed Fe-28%Ni alloy. It could be established that the Nishiyama- Wasserman relations ({111}g//{110}a <112>g//<110>a) prevail over the Kurdjumov-Sachs relations ({111}g//{110}a and <110>g//<111>a) although a considerable scatter was observed around either of the theoretical correspondences. A full parametric misorientation description was also applied to evaluate the relative grain boundary energies associated with a set of crystallographic misorientations observed near triple junctions in Fe-2%Si. In this instance it was found that the boundaries carrying a misorientation of the type <110>w carry a lower interfacial energy than the <100> or <111> type boundaries.

Abstract: In this paper, the ‘orange peel’ defect in the surface range of the st14 steel sheet has been investigated using the electron backscatter diffraction (EBSD) technique. It has been found that the ‘orange peel’ defect in the st14 steel sheet was resulted from the local coarse grains which were produced during hot-rolling due to the critical deformation in dual-phase zone; During deep drawing, the coarse grains with {100}<001> microtexture can slip on the {112}<111> slip system to form bulging and yields orange peel defects, while the coarse grains with {112}<110> orientation do not form the defect as the Schmid factor of {112}<111> slip system in it equals zero.

Abstract: In the present work the characterisation of both the microstructure and the microtexture of two ferritic stainless steel sheets - AISI 430 (Fe17% Cr) and AISI 434 (17% Cr, 1% Mo) - known to display different ridging behaviours, was carried out by optical metallography, EBSD and OIM techniques. It was concluded that severe ridging is a consequence of: (a) presence of {111} and {001} colonies of grains alternating throughout the sheet plane; (b) through thickness texture inhomogeneity with layers of {001}<110> oriented large grains of low Rvalues sandwiched between layers of higher R-value textured material.

Abstract: The determination of an ODF, C-coefficients, property tensors and portions of texture components from EBSD orientation measurements is afflicted with statistical errors introduced by incomplete sampling of the grains. Since the measurements are highly spatially correlated and stochastically dependent, classical sampling theory does not apply. A general statistical method for error estimation in the presence of stochastically dependent observations has been developed and applied to the most important quantities of texture analysis. The method is based on the assumption of a finite range of dependence between different measurements and on the estimation of the covariance in the observed set of orientation. The methods allows the computation of standard measurement errors and confidence limits for the mentioned texture quantities. It can be used for an objective decision whether two textures are statistically equal or not, based on the comparison of estimated ODFs. Further we can decide statistically whether the ODF obeys certain types of symmetry (e.g. whether it is a girdle textures or whether it is symmetric about the shear plane observed in the field).

Abstract: Historically, twinning classification has been obtained by optical microscopy, bulk x-ray and neutron diffraction, and transmission electron microscopy (TEM). Recent research has shown that automated electron backscatter diffraction (EBSD) can be used to quantify twin content and thereby greatly improve the reliability of twinning statistics. An automated twin identification technique for use with EBSD has facilitated a greater understanding of deformation twinning in materials. The key features of this automated framework are the use of the crystallographic definition of twin relationships, and the correct identification of the parent orientation in a parent/twin pair. The complex nature of the parent/twin interactions required the use of a voting scheme to correctly identify parent orientations. In those few cases where the voting scheme was unable to determine parent orientation (< 2%) the algorithm allows for manual selection. Twin area fractions are categorized by operative twin systems along with secondary and tertiary twinning. These statistics are reported for deformation and annealing twin populations in deformed a-zirconium and asannealed 316L stainless steel, respectively. These improved twin statistics can help provide insight into the effect of deformation processes on microstructural evolution, as well as provide validation of plasticity models for materials that exhibit deformation twinning.

Abstract: An automatic crystallographic orientation indexing procedure is developed for transmission electron microscopes. The numerical identification is performed by mapping the spot diffraction patterns with pre-calculated templates. The diffraction patterns are acquired thanks to an external CCD camera that points to the fluorescent screen through the TEM window. Orientation maps with spatial resolution better than 10 nm were obtained with this low cost equipment.

Abstract: The mechanical processing of metals results often in development of preferred orientation of grains or texture. On the other hand, the orientations of grains have a significant effect on the friction and wear behaviour of materials in contact. The objective of this investigation was to determine the crystallographic anisotropy of the coefficient of friction. Medium carbon steel AISI 1045 were selected as the substrate material. Automated orientation imaging analysis system (OIM) was used to identify orientations of grains in polycrystalline specimens. Micro-friction tests were carried out using a Tribocope, a combination of nano-mechanical probe and atomic force microscope with an electrochemical cell. The micro friction tests were performed along different crystallographic directions inside different grains.The obtained results have shown that the coefficient of friction for individual grains depends on crystallographic orientation, and that the coefficient of friction for higher atomic density planes is lower than that for lower atomic density planes.

Abstract: Reflected light optical analysis and Electron Backscatter Diffraction (EBSD) analysis have been used to m easure grain sizes in 2D Al foil samples, annealed for different times. There are significant differences in the results of the two techniques. It is shown that in Al it is possible to detect boundaries in optical images down to a misorientation angle of 7-8º. Nevertheless, in most samples the critical angle of easy etching lies above 10º. The observed differences in grain size measurements between optical analysis and EBSD analysis can be largely attributed to three phenomena: (1) individual samples may behave slighty differently due to differences in the effectiveness of etching (2) the grain size is heterogeneous over large areas and (3) the effect of etching is not only a function of misorientation angle but also grain boundary plane. Despite these uncertainties, optical analysis seems to be reliable for analysis of processes in which mainly grain boundaries with misorientation angle of > 10º are involved i.e. grain growth.