Papers by Keyword: Microband

Abstract: Compression tests were performed on Fe-3%Si specimens with few grains. The deformation microstructure and microtexture were investigated by electron backscatter diffraction (EBSD) and related to the initial crystal orientation and grain boundary characteristics. Groups of microbands were found that are characterised by a periodic change in crystal orientation, shear at the grain boundary, and the formation of new grains. It is supposed that these microband groups represent an early stage of microshear band development.

Abstract: The effects of strain path reversal, using forward and reverse torsion, on the microstructure evolution in the aluminium alloy AA5052 have been studied using high resolution electron backscatter diffraction. Deformation was carried using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300°C and strain rate of 1s-1 to a total equivalent tensile strain of 0.5. Sections of the as-deformed gauge lengths of both test specimens were then annealed at 400°C for 1 hour in a salt bath in order to investigate their subsequent recrystallisation response. In both strain path histories the deformation substructure in the grains analysed consisted of microband arrays within an equiaxed dislocation cell structure. The material subjected to forward/forward deformation did, however, have a slightly greater number of low angle boundaries, i.e. boundaries < 15° misorientation, whilst the forward/reverse material had some grains containing little evidence of substructure. On annealing both materials had significantly reduced levels of low angle boundaries but only the forward/forward material had an increased number of high angle boundaries and a reduced grain size, indicating recrystallisation had only occurred in this material. This would suggest that the deformation microstructure within the forward/forward condition was sufficient to initiate and maintain recrystallisation whilst the microstructure produced by the forward/reverse test contained insufficient nuclei or internal energy to produce a recrystallised material within 1 hour. Further work is now required at different annealing times in order to determine if the major effect of strain path is on retarding nucleation, growth or both.

Abstract: The paper surveys various types of dislocation substructure that are created by plastic deformation in metals. Special reference is made to those substructures that accommodate sharp misorientations as these are of fundamental importance to the nucleation of recrystallisation. Several different mechanisms can give rise to high misorientations; these are discussed in terms of the factors that control them and their relationships to orientation and texture. Different mechanisms for nucleation of recrystallisation may occur depending on the type of substructure, allowing some practical control over the final recrystallised texture.

Abstract: A Goss-oriented single crystal was cold rolled up to 89 % thickness reduction, and subsequently annealed at 550°C or 850°C. During deformation most of the initially Goss-oriented material rotated into the two symmetrical {111}<112> orientations. In addition, Goss regions were observed related to microbands or microshear bands. Goss regions in microshear bands formed during straining, whereas Goss regions between microbands were retained from the initial Goss orientation. The recrystallisation texture for annealing temperatures of both 550°C and 850°C is characterised by a Goss texture. However, the origin of the Goss recrystallisation nuclei appeared to be different for the different annealing conditions. In the material annealed at 550°C, the Goss texture originated from the Goss regions in the microshear bands. In contrast, for an annealing temperature of 850°C, the Goss grains between the microbands are likely to form recrystallisation nuclei.

Abstract: The effects of strain path reversal on the microstructure in AA5052 have been studied using high resolution EBSD. Deformation was carried out using two equal steps of forward/forward (F/F) or forward/reverse (F/R) torsion at a temperature of 300°C and strain rate of 1s-1 to a total strain of 0.5. In both cases the deformation microstructure in the majority of grains analysed consisted of microband arrays clustering at specific angles to the macroscopic deformation axes. For the F/F condition microbands clustered around -20° and +45° to the maximum principle stress direction, whilst for the F/R condition significantly more spread in microband angle was observed. This suggests that the microbands formed in the forward deformation have or are dissolving and any new microbands formed are related to the deformation conditions of the final strain path. This leads to the conclusion that instantaneous deformation mode determines the orientation of new microbands formed whilst a non-linear strain path history influences the range of misorientation angle in the material through the dissociation of previously formed microbands and the formation of new microbands at the new straining condition, leading to a lower level of misorientation angle. Analysis of material subjected to static annealing at 400°C for 1 hour appears to correspond with these observations as the F/F material was completely recrystallised with a fine grain structure whilst the F/R material had no major signs of recrystallisation.

Abstract: The aim of this study is to acquire more quantitative data on the cold-deformed state, in order to determine the key parameters for nucleation (like orientation gradient, dislocation density, degree of fragmentation…) and to correlate these to parameters which can be predicted by a mean field deformation model. Experimentally, a Ti-IF hot band has been deformed by cold rolling up to 51%. The deformed microstructures are finely characterised using EBSD inside the SEM. It appears then that the heterogeneity of deformation inside the grains begins very early. Some grains undergo fragmentation whereas some others deform very homogeneously. Misorientation and degree of fragmentation are found to be orientation and rolling level dependent. These deformation parameters are plotted against the Taylor factor, calculated in each grain.

Abstract: 2519A aluminum plate was shot obliquely by an incendiary projectile with the diameter of 7.62mm at impact velocity of 818m/s. The penetration inhomogeneities of the crater of 2519A aluminum target plate were investigated with optical microscopy (OM), scanning electron microscopy (SEM). Melted substance, a mixture of the target material and the projectile was found at the interface of target and projectile. Different microstructural characteristics were observed along the crater depth. Grains were elongated near the entrance area, while severely kinked grains and adiabatic shear bands were present in the middle region, and microbands, as well as abnormal grown grains, appeared at the crater bottom. The failure form of target is mainly ductile expanding.

Abstract: A silicon steel single crystal with initial Goss orientation, i.e. the {110}<001> orientation, was cold rolled up to 89 % thickness reduction. Most of the crystal volume rotates into the two symmetrical equivalent {111}<112> orientations. However, a weak Goss component is still present after high strain, although the Goss orientation is mechanically instable under plane strain loading. Two types of Goss-oriented crystal volumes are found in the highly deformed material. We suggest that their origin is different. The Goss-oriented regions that are observed within shear bands form during the cold rolling process. In contrast, those Goss-oriented crystal volumes that are found inside of microbands survive the cold rolling.