Papers by Keyword: Subgrains

Abstract: Micro-orientation data of a high purity Al rolled up to total thickness reduction of 80% at room temperature were determined using SEM-EBSD technique, conceptions of describing substructure information, such as subgrain misorientation (θcry), and average misorientation (θenv) of circumjacent subgrains for a special subgrain, etc., were suggested and corresponding GCDP-OI soft package was developed. It is found that the subgrains sizes increase rapidly from about 2 to 7 μm with increasing misorientations from 1° to 15°, and the total number frequency of which is more than 95%. However, taking into account local features of subgrais, whose sizes for Dcry/Denv > 1 are 2 times larger than that for Dcry/Denv < 1 on the same misorientation levels below 20°, and the relationships between misorientations and sizes are consistent with that if Dcry/Denv > 1, θcry/θenv > 1, vice versa.

Abstract: A self-organization model for repartition of dislocation cell structures and transition of subgrains on a three-stage hardening of single crystal are developed. Stress-effect coefficients models are proposed in order to introduce stress information into the reaction-diffusion equations. A FD simulation for dislocation patterning and a FE one for crystal deformation are simultaneously carried out for an FCC single crystal. It is numerically predicted that a cell structures are repartitioned and the generated dislocation pattern in stage III can be regarded as a subgrain.

Abstract: The composite model of plastic deformation is regarded as a realistic approximation of creep behaviour at elevated temperatures in a well-developed substructure consisting of dislocationdense subgrain boundaries (hard regions) and subgrain interiors (soft regions) with relatively low dislocation density. In the present contribution, the model is applied for an estimation of internal stresses that are experimentally measured by the dip-test technique. Two situations are considered within the model: (i) the density of moving dislocations is the same in both hard and soft regions and (ii) the density of moving dislocations is proportional to the local density in the respective region. The model enables to express the internal stress in terms of microstructural variables found by independent microscopic observations. It is shown that the magnitude of volume fraction of hard and soft region in the composite model has only a small effect on the value of internal stress.

Abstract: At present, the possibility of the formation of high-angle grain boundaries upon severe cold deformation, in particular, equal-channel angular (ECA) pressing is reliably proved. The structure formation upon multi-cycle ECA pressing substantially depends on the route determining the shear plane in the sample upon repeated passes. The route is defined by the rotation of the sample around its axis upon the multi-cycle ECA pressing. There are four main routes: route A, in which the sample is deformed by many passes without any rotations; route Ba, in which the sample is rotated by ± 90°; route Bc, in which the sample is sequentially rotated in the same direction by 90°and route C, in which the sample is rotated by 180° about its axis before each subsequent pass. By the methods of SEM, TEM and EBSD analysis it was shown that the fraction of high-angle boundaries in a-Fe upon cold ECA pressing with an angle of 90° between the channels and N=4 depends on the deformation route and increases according to the route sequence: Ba-C-Bc.

Abstract: The micron-size grain refinement of pure a-zirconium obtained with elevated temperature tensile deformation was investigated. The development of low-misorientation subboundaries caused the serration of the original grain boundaries at low strains. The final microstructure (e.g. strains > 3) was predominantly composed of fine, equiaxed “crystallites” with ⅔ of the boundaries being of very low misorientations (< 3°) and the remaining ⅓ being high angle boundaries (θ > 8°, and typically 25-35°). Discontinuous dynamic recrystallization was excluded as a possible mechanism due to the absence of newly formed grain nuclei. The bimodal distribution of the crystallite or (sub)grain boundary misorientations is inconsistent with the occurrence of continuous dynamic recrystallization and rotational recrystallization. The continual thinning of the original grains, the serration of the high angle boundaries, the bimodal misorientation distribution of misorientations, ⅔ of boundaries of very low misorientations at high strains all strongly suggest geometric dynamic recrystallization and dynamic recovery as the grain refinement and restoration mechanisms.

Abstract: NaCl is plastically anisotropic and forms a well developed substructure during deformation at 0.3-0.5Tm. EBSD was used to assess subgrain misorientations up to 0.5 true strain in dry NaCl. Equiaxed subgrains were ubiquitous but misorientations along segments of subgrain boundaries differed. Three types of subgrain boundary were identified: boundaries that surrounded equiaxed subgrains, boundaries that partly surrounded mantle subgrains, and extended subgrain boundaries, longer than the equiaxed subgrains. All of these subgrain features were recognised at low strains, <0.15. Misorientations of the majority of equiaxed subgrains were generally <2° at 0.5 strain, although segments could reach higher misorientations along kink-like boundaries. Mantle subgrains along grain boundaries tended to develop higher misorientations than in core subgrains. Long subgrain boundaries reached very high misorientations along segments of their length by 0.5 strain. Small new grains formed at triple points and more rarely within grains. Microstructures in NaCl are similar to those found in aluminium. Therefore, the dominant mechanism of high angle subgrain development at 0.5 strain and at 0.4Tm is probably an orientation splitting mechanism rather than equiaxed subgrain rotation.

Abstract: The recrystallization behavior of coarse-grained niobium depends on the nature of its deformation microstructure. In this regard, a longitudinal section of a high-purity coarse-grained niobium ingot was cold rolled to a thickness reduction of 96% followed by annealing in vacuum at 800°C for 1 h. Metallographic inspection in cold-rolled and annealed specimens was carried out in a field emission gun scanning electron microscope (FEG-SEM). Microtexture was determined by electron-backscattered diffraction (EBSD) coupled to the FEG-SEM. The use this technique has evidenced details of the boundary character and subgrain structure found in partially recrystallized regions. The early stages of primary recrystallization are associated to the presence of high-angle lamellar boundaries found in the cold-worked state. Abnormal subgrain growth has been evidenced as a viable mechanism for nucleation of recrystallization.