Dislocation Activity in AZ31B Magnesium Deformed at Moderately Elevated Temperatures via EBSD
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This chapter details strategies for determining structures based on intensity data suffering from twinning. Progress can often be made in the early stages of structure determination without taking the twinning into account, whereas proper refinement of structures must account for the effects of twinning. Complications that can occur when handling anomalous-scattering information are discussed. Opportunities are identified for more formal treatments of the problem of phasing twinned data. The facility with which twinning can be analysed and discriminated from local noncrystallographic symmetry during the final stages of analysis is emphasized.
Keywords:
twinning;
merohedral twinning;
structure determination
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Basal plane
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The temperature effect on slip and deformation twinning has been investigated in Cd single-crystals in tension in the temperature range from room temperature to 500 K. The observations of traces on the surfaces of the crystals show that there are twinning and slip at room temperature. Above 430 K slip takes place on the limited slip plane. On the other hand, as the temperature increases deformation twinning occurs on some new twinning plane after considerable slip, although deformation twinning is a common low temperature mode of plastic deformation. It is shown that the multiplicity of twinning systems aids deformation by twin rather than by slip in Cd single-crystals at high temperatures.
Atmospheric temperature range
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Electron Backscattered Diffraction(EBSD) technique in the scanning electron microscopy was used to analyze the twinning of AZ31 magnesium alloy compressed at room temperature and 150℃,with the compression axis parallel to ND and TD.In samples deformed at 150℃ extensive twinning {10-12} only occurs in samples where the basal plane normals are initially aligned approximately perpendicular to the compressive stress axis.We discuss the particular advantage of EBSD for research on the relationship between grain orientation and twinning,and the difficulty of preparing magnesium sample for EBSD studies.
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Reports of Type II twins are quite rare for most crystal structures. When they do occur, they are usually one of a number of possible twinning modes observed in a particular material. However, for the triclinic phase devitrite, Na2Ca3Si6O16, which nucleates from commercial soda−lime−silica float glass subjected to suitable heat treatments, the only reported twinning mode to date is a Type II twinning mode. In this study, this Type II twinning mode is first examined by molecular dynamics simulation to determine the lowest energy configuration of perfect twin boundaries for the twin mode. This is then compared with the lowest energy configurations of perfect twin boundaries found for six possible Type I twinning modes for devitrite for which the formal deformation twinning shear is less than 0.6. The most favourable twin plane configuration for the Type II twinning crystallography is shown to produce reasonably low twin boundary energies and sensible predictions for the optimum locations of the twin plane, K 1, and the [1 0 0] rotation axis, η 1, about which the 180° Type II twinning operation takes place. By comparison, all the Type I twinning modes were found to have very energetically unstable atomic configurations, and for each of these twinning modes, the lowest energy configurations found all led to high effective K 1 twin boundary energies relative to perfect crystal. These results therefore provide a rationale for the experimental observation of the particular Type II twinning mode seen in devitrite.
Triclinic crystal system
Crystal (programming language)
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In the present work, we summarized two calculation methods to determine some specific crystallographic elements based on electron diffraction orientation measurements by SEM/EBSD or TEM. The first one is to determine the twin type and twinning elements of crystal twins based on the minimum shear criterion, using the experimentally determined twinning plane for Type I twin and compound twin or twinning direction for Type II twin as initial input. The method is valid for any crystal structure. The second one is one to determine the plane indices of the faceted interfaces where the orientation relationships (ORs) between the adjacent crystals are reproducible. The method requires one prepared sample surface instead of two perpendicular surfaces. These methods are expected to facilitate the related microstructural characterizations.
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Single crystals of copper were deformed at 4.2°K and 77.3°K. At 4.2°K, after a large strain produced by normal slip, jerky flow (discontinuous slip) occurs. From the study of reactor irradiated crystals, it was deduced that a packet of 30 slip lines, each containing 104 dislocations, was released to form each jerk of the discontinuous flow. For samples of certain orientation the region of discontinuous flow was followed by a region of deformation twinning. The fact that deformation twinning was taking place was verified by x-ray methods. The twinning elements were determined to be the (111) plane and the [112] direction. At 77.3°K, discontinuous slip was not observed. In a limited range of orientations, twinning has been observed at 77.3°K. The effects of reactor irradiations on discontinuous slip and on twinning were also studied and it was determined that the occurrence of twinning was unaffected by the irradiations.
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