Papers by Keyword: Abnormal Grain Growth

Abstract: Simulation of mobility-driven abnormal grain growth in the presence of particles in a 3D Potts Monte Carlo model has been investigated, and even though the driving force in this case is identical to normal grain growth, Zener pinning does not occur. Instead the particles seem merely to have a small inhibiting effect on the number of abnormal grains, and this effect only has a noticeable influence for volume fractions of particles above 5 vol%.

Abstract: The pancake-shaped growth of Goss grains in Fe-3%Si steel is approached by solid-state wetting mechanism. The fraction of grains with Σ9 relation with Goss grains in the 0.3 mm thick specimens after primary recrystallization is higher near the surface than at the center. This result indicates that the probability of solid-state wetting is higher near the surface than at the center, leading to the pancake-shaped growth.

Abstract: The controversy about the start up of the abnormal growth as heterogeneously engendered in the microstructure or produced by peculiar continuous incubation process has still not been solved. In this work the statistical theory of grain growth has been applied to treat the case of grain growth in the presence of an homogeneously unstable Zener drag. By the simulations presented it will be shown as abnormal grain growth is a result out of a continuous and homogeneous process without requiring any heterogeneity in the microstructure presumed to give local advantages to some grains. The mechanism by which during an incubation period the preconditions for the unstable growth are built up in the microstructure is clarified and discussed. Moreover the peculiar shape of Grain Size Distribution (GSD) approaching the “structural instability” will be also analytically defined and compared with experimental results obtained in a grain oriented Silicon Iron just before the onset of abnormal grain growth.

Abstract: The possible mechanisms concerning abnormal growth of Goss grains in grain oriented electrical steels were investigated. The density of inhibitor particles near sheet surface, where the Goss grains located, was lower than that in center layer before secondary recrystallization, and the grains near surface could grow more easily because of reduced pinning effect. Few Goss grains could survive the growth competition and reach the sheet surface, after which the inhibitor particles inside the Goss grains coarsened slower. The phenomenon resulted in easy growth of the Goss grains at the expense of smaller neighboring grains while they could hardly be consumed by larger neighboring grains during the high temperature secondary recrystallization. Very large final size of the Goss grains was then obtained. The mechanisms were discussed based on the hot rolling characteristics and the elastic anisotropy of the ferrite matrix.

Abstract: In order to verify the origin of Goss nuclei for secondary recrystallization in Fe-3%Si steel, the effect of cold-rolling direction on the secondary recrystallization was examined in this study. The cold-rolling direction was rotated through 0 ~ 90 degrees about the hot-rolling direction on normal direction axis of hot-rolled sheet. In spite of the different initial texture before cold rolling, the 88% cold rolled texture was formed by similar α and γ fiber regardless of the rotation of cold rolling direction. Likewise, regardless of the cold rolling direction, the primary recrystallized sheets had a similar texture. In particular, the area fraction of Goss component (tolerance angle within 15º) in the primary recrystallized sheets was increased in the cold rolling condition of rotating through 60, 90 degrees from the hot rolling direction. After high temperature annealing at 1200°C, the secondary recrystallized grain was fully evolved in the all conditions. The sharpness of Goss texture in secondary recrystallized sheet was decreased as increasing the rotation angle of cold rolling direction.

Abstract: Low energy grain boundaries were considered to be important in abnormal grain growth by theoretical deduction. The disorientation angles and coincidence site lattice grain boundaries distribution of more than 20 Goss grains and their neighboring matrix grains in primary recrystallized Fe-3%Si alloy were investigated using an electron backscatter diffraction method. It was found that the frequency of low energy grain boundaries of Goss grains which are more likely to abnormally grow are higher than their neighboring matrix grains, which indicated that low energy grain boundaries play a dominant role in the abnormal grain growth of Fe-3%Si alloy. The result meets well with the abnormal grain growth theory.

Abstract: Friction Stir Welding process introduces a degree of deformation in the material that is related to process parameters. They directly regulate the heat input of welding process and then the morphology and the microstructure characteristics. In the present work, an investigation was carried out on 6082T6 butt joints obtained by Friction Stir Welding process. The microstructure and mechanical characterizations were performed before and after a post-welding treatment at 535°C for 2 hours and cold deformation. The thermal stability of fine recrystallized grains in the nugget zone depends on process parameters and post-welding heat treatment and it has unusual consequences on mechanical properties. In fact abnormal grain growth, that occurs in the nugget zone, increases ductility of the joints and homogenizes the hardness profile of the whole joint.

Abstract: Strontium Bismuth Niobium (SBN) ceramics was prepared successfully using the powders synthesized from molten salt method. The evolution of microstructure with temperature increasing was studied and the influences of abnormal grain growth on dielectric properties were also presented in this paper. The results showed that pure strontium bismuth niobium ceramics without abnormal grain growth could be formed at 1150°C while the density was almost 95% of the theoretical density. Further investigations on dielectric properties indicated although abnormal grain growth did not shift the Curie point obviously, they lowered the dielectric constant corresponding to the Curie point. The dielectric loss was found to change with the frequency remarkably.

Abstract: Abnormal grain growth was simulated by phase field model in order to find ways of producing scattered a few enormous grains in a nano-structural single phase AZ31 alloy to improve its ductility. It is shown that the abnormal grain growth is controlled by the three keys factors of interface energy, strain restored energy and interface mobility. Therefore, the microstructure with scattered a few enormous grains in the nano-structural matrix can be achieved after an annealing treatment if there is a small group of specially orientated nano-size grains in the original nao-structure with local low grain boundary energy or local high strain energy or local high interface mobility. The morphology of abnormal grains is also examined as function of annealing time to optimize the microstructure.

Abstract: The distribution of the precipitates on the grain boundaries in Fe-3%Si steel during secondary recrystallization annealing were studied using high-angle annular dark filed (HAADF) scanning transmission electron microscopy (STEM). Because HAADF image can show both grain boundaries and precipitates clearly, the change of precipitate distribution on grain boundaries can be quantitatively analyzed. It was observed that the total area of the precipitates on grain boundaries increased in the order of non-annealed, 600°C, and 900°C sample and the total area of precipitates on grain boundaries in the 1000 °C sample was much lower than that in the 900 °C sample. The compositions of the precipitates were also analyzed using X-ray energy-dispersive spectrometer (XEDS). The most precipitates were multi-phase ones, mainly composed of AlN and MnS. Our analysis results suggest that such a precipitate behavior is responsible for the abnormal grain growth of Fe-3%Si steel occurring under the temperature above 900 °C.