Papers by Author: Adam Morawiec

Abstract: Kossel microdiffraction is one of a few experimental methods of investigating heterogeneities of elastic stresses within crystallites. With digitally recorded back-reflection Kossel patterns, one can determine absolute lattice parameters, and hence lattice strains and stresses, based on geometry of Kossel lines, but the strain resolution of this approach is limited by finite widths of the lines. A new method is proposed which considerably improves the resolution in cases when the patterns originate from areas with similar lattice orientations. The method is based on determination of differences between pattern geometries: lattice strains are calculated from mutual shifts of intensity profiles of Kossel lines. The strain accuracy of this profile-based approach was estimated. It is demonstrated that the limit of strain resolution reaches a few parts per hundred thousand, i.e., it is nearly one order of magnitude better than that of the conventional Kossel-based lattice parameter refinement. This improvement concerns the critical range of lattice strain, and it constitutes a qualitative leap in resolution. The paper describes main aspects of the new approach and strain resolution tests.

Abstract: The Kossel microdiffraction in a scanning electron microscope allows for local stress determination. This technique has been applied to monitor stress evolution within grains of austenite in the course of martensitic transformation in a shape memory alloy. Kossel diffraction patterns were recorded during in situ tensile straining of Cu-Al-Be alloy. These innovative measurements show large stress heterogeneities between grains, with the stress ratio exceeding two. As martensite variants are stress-induced, shear stress components appear in individual grains of austenite.

Abstract: Current issues concerning the characterization of grain boundary networks via five-dimensional (5D) grain boundary distributions are considered. A quantitative measure of reliability of such distributions is adapted from conventional texture analysis. Application of the measure shows that with the currently available size of experimental data sets of boundary parameters, only strong components of the boundary distributions can be reliably evaluated. Improvements of the computational part of the analysis are possible if the the binning based on Euler and polar/azimuth angles is replaced by searching the data sets based on a suitably defined distance. Moreover, it is indicated that for textured materials the stereological approach has limited reliability. Finally, it is suggested that coherent twins can be used for estimating experimental errors, and that the distributions cannot be a basis for conclusions about tilt boundaries unless additional restrictions are applied. The approach used in the paper is theoretical with support by computer simulations.

Abstract: This paper describes nucleation and grain coarsening at very early stages of recrystallization in AA3104 aluminum alloy containing complex structure of second phase particles. Measurements of individual sub-cell orientations in ECAP-processed and slightly annealed structures were possible by using TEM-based orientation mapping in combination with recrystallization carried out in the microscope. The results were compared with those obtained by SEM/EBSD system. The investigation shows that the 40^o<111>-type relation occurs rarely, and the rotation axes of misorientations between deformed and recrystallized areas were strongly scattered. Grouping of the misorientation axes near the <012>, <221>, <112> and <110> crystallographic directions was noticeable.

Abstract: There are a number of classifications of homophase grain boundaries. It is quite common to divide them into twist, tilt and general boundaries. As in the case of the classification into coincident lattice (CSL) boundaries and non-CSL boundaries, one may ask about the possible frequencies of incidence of tilt and twist boundaries in a set of “random” boundaries. The proba¬bilities of occurrence of these particular boundary types are clearly defined if small deviations from pure twist and tilt conditions are allowed. We estimated the probabilities numerically for the cases of cubic and hexagonal holohedries. For a given randomly generated boundary, a computer program searched for the nearest pure-tilt and pure-twist boundaries. All symmetrically equivalent representations of the random boundary were processed, and the smallest distance was taken as the result. The distance was based on both the difference in misorientations and the deviation between boundary inclinations. The findings concerning tilt boundaries turned out to be striking. For instance, if the allowed deviation from pure-tilt conditions is only 1°, then as many as 39.0% and 21.2% of random boundaries have near-tilt character for the cubic and hexagonal cases, respectively. If the limiting deviation is raised to 5°, the frequencies of near-tilt boundaries reach 98.6% and 77.0%, respectively.

Abstract: The nucleation of new grains during annealing in high purity single crystals of Cu-8%wt. Al alloy with initial (112)[111] orientation has been characterized by detailed transmission electron microscopy (TEM) orientation measurements. The samples were channel-die deformed and then recrystallized to form a substructure composed of a mixture of deformation twins, shear bands (SBs) and the recrystallized grains. Substantial progress in understanding the mechanism of texture transformations at the early stages of recrystallization was possible thanks to orientation mapping using TEM. Microtexture analysis of partly recrystallized samples indicates a simple 25-40o (<111> or <112>) relation, most frequently observed during the early stages of recrystallization between single nuclei and one of the two as-deformed groups of components (twins or matrix). As recrystallization proceeds, recrystallization twinning develops strongly and facilitates rapid growth of the recrystallized phase. The first and higher generations of twins systematically tend to obscure the initial crystallographic relation between the deformed state and the recrystallization nuclei.

Abstract: The ambiguity in determination of complete elastic strain tensor by convergent beam electron diffraction can be overcome by simultaneous use of multiple diffraction patterns. Numerical tests of strain determining procedure based on multiple patterns have been carried out. Patterns were simulated using both kinematic and dynamic approaches, and then they were used as input in the tested procedure. The tests indicate that, in practice, at least three patterns are needed in order to determine a complete strain tensor with reasonable accuracy. The strain resolution of two parts per ten thousand was achieved with five diffraction patterns. Moreover, the impact of errors in voltage and camera length is considered. It is shown that within the kinematic description, the deviations from the correct voltage are equivalent to errors in the isotropic part of strain.

Abstract: The local crystallography within shear bands (SB) has been examined in a single crystal of {112}<111> orientation of pure copper deformed at 77K by channel-die compression to strains of about 1. Setting up a system for making high-resolution orientation maps using transmission electron microscopy (TEM) has opened new advantageous circumstances for the analysis of orientation changes within SB. This method with spatial resolution higher than 10nm allows the examination of microstructure images composed of nanoscale subcells forming SB. It has been found that for well-developed shear bands, a crystal lattice rotation about <112> direction tends to dominate and this process is usually accompanied by activation of new slip systems. The present work shows that despite the plane strain deformation mode, the mechanism of lattice rotation within emerging SBs may lead to Goss and Brass texture components.

Abstract: An original approach based on semi-automatic treatment of Kikuchi patterns collected by Transmission Electron Microscopy is used to analyse the a2 an g variant distributions in a quaternary TiAl-based alloy (Ti-48at.%Al-2%Cr-2%Nb), with two lamellar microstructures of different fineness. Statistic data analysis shows that neither g variant orientations nor interface misorientations are randomly distributed within a given grain. The largest deviation from randomness is observed for the coarser microstructure. We also pointed out the presence of g|g subboundaries between same but slightly misoriented variants, as well as the existence of some semicoherent g|g twins accommodated by dislocation lattices. The analysis of the misorientation distributions of g|g and g|a2|g interfaces suggests that a variant selection occurs during growth and coarsening stages of the a ® a2 + g transformation process.