Papers by Keyword: Abnormal Grain Growth

Abstract: Abnormal grain growth (AGG) takes place in many metallic systems especially after recrystallization of deformed polycrystals. A famous example of AGG in metallic system is the Goss texture in Fe-3%Si steel. During high temperature annealing of Fe-3%Si sheet, a few near Goss {110} <001> grains grow exclusively fast and consume the matrix grains. Therefore, the grains which have near Goss orientation have special advantage over other grains. As a new approach to the growth advantage of AGG, we suggested the solid-state wetting mechanism, where a grain wets or penetrates the grain boundary or the triple junction of its neighboring grains. The solid-state wetting mechanism for the evolution of the Goss texture in Fe-3%Si steel was studied experimentally and by phase-field model (PFM) simulation.

Abstract: The scope of this work was to study the physical metallurgical behaviour of the microstructure and the texture of ultra low carbon (ULC) steel during cold rolling and subsequent thermally activated phenomena. It was the intention to contribute to the scientific search for the answer to many open questions raised in recent literature. The powerful tool of quantitative texture analysis, together with modern measurement equipment was used for this purpose. At first, a ULC steel was cold rolled to two different rolling reductions and the local strain heterogeneities after the cold rolling were studied. Secondly, crystallographic orientation selection during primary recrystallization was considered both for cold rolled ULC steel and for a Fe-2.8%Si single crystal. The latter was a re-evaluation of the historic growth selection experiment by Ibe and Lücke. Finally, secondary recrystallization in ULC steels was evaluated in terms of oriented nucleation and selective growth.

Abstract: The effect of annealing temperature on grain growth, texture development and magnetic properties of Al-free and Al-1% added non-oriented electrical steel were investigated. Normal grain growth occurred in Al-free steel. On the other hand, abnormal grain growth occurred in Al-added steel which was annealed at 800°C for 24h. Precipitates in these two steels were different. TiN precipitated in Alfree steel, but in the case of Al-added steel, AlN and TiC precipitated. The TiC in Al-added steel was so fine that it inhibited the normal grain growth and finally caused the abnormal grain growth. Main textures of both steels were near {111}<112>, but the intensity of near {111}<112> in the abnormal grain growth was higher than that in the normal grain growth. Magnetic flux density (B50/Bs) was decreased by the grain growth. Especially B50/Bs in the abnormal grain growth was lower than that in normal grain growth. B50/Bs in these steels can be estimated by their three-dimensional textures in vector method.

Abstract: The present study aims to investigate the mechanism of the development of abnormal grain sizes in the through-thickness direction of hot rolled steel strips. For this purpose, industrially prepared steel strips were further hot rolled in a laboratory hot rolling mill, setting a variety of rolling parameters. As found, the deformation rate in the hot rolling practice exerts an important role in explaining the mechanism of abnormal grain growth, especially in the close vicinity of the strip surface. Furthermore, the influence of the cooling penetration depth, induced by the roll contact was examined closely, as this phenomenon might support abnormal grain growth mechanisms. Additional information was found in performing a texture analysis in the throughthickness direction of the steel strips, in accordance with the optical metallurgical survey of the microstructures. It will be shown that, the combination of particular hot rolling parameters provokes the occurrence of abnormal grain growth in the through-thickness direction of the ELC steel strips. These particular conditions were considered to be related to the finish hot rolling temperature and thus the roll cooling penetration depth imposed on the steel strip, the finishing reduction degree and especially the strain rate conditions. Moreover, the observed abnormal grain growth is sensitive to the coiling temperature applied. From the experiments, it can be concluded that the mechanism of the formation of a large grained ferrite band below the strip surface is strongly influenced by the development of a fine-grain ferrite layer at some distance below the strip surface. The existence of this layer of very small ferrite grains can be explained on the basis of texture analysis and calculations based on literature data. In this way, it was considered that dynamic recrystallisation of austenite at some depth below the steel strip surface is of most significance in supporting the development of abnormally large ferrite grains. In this paper, further considerations on the mechanism of the abnormal grain growth phenomenon will be dealt with.

Abstract: In order to investigate the effects of Nb carbo-nitride precipitation conditions on abnormal grain growth behavior during high temperature carburizing, size of Nb carbo-nitride precipitates was controlled by precipitation treatment at 1173-1273K for 0.6-54ks, and the specimens were quasi-carburized at 1323K. Abnormal grain growth was enhanced when the size of Nb precipitates was fine or coarse, so there is a suitable size range in Nb precipitates to suppress abnormal grain growth. The reason why abnormal grain growth was enhanced is the lack of pinning force as the conventional theory proposed by Hillert or Gladman; however, it cannot be explained by this theory that small precipitates promote abnormal grain growth. It is considered that Ostwald ripening rate of precipitates is also an important factor in controlling abnormal grain growth in addition to the amount and size of precipitates and austenite grain size, which were parameters in the Gladman‘s theory on abnormal grain growth behavior.

Abstract: The present paper deals with the control of microstructure of friction stir processed aluminum alloys focusing on grain refinement, thermal stability at elevated temperature and texture development in some aluminum alloys such as 5083, 6061 and 7075 commercial aluminum alloys. 3mm thickness plates of 5083, 6061 and 7075 Al alloys were friction stir processed/welded with several rotation speeds and travelling speeds. Optical microscopy revealed the grain refinement in the stirred zone of each alloy and the average grain size decreased with decreasing rotation speed under various travelling speeds. Annealing of the joints brought about abnormal grain growth at temperatures higher than 773K for 5083 alloy. Critical temperature of the abnormal grain growth tended to decrease as the rotation speed decreased for the fixed travelling speed. Dissimilar joining of 5083 Al alloy to 6061 Al alloy also showed abnormal grain growth when annealed at 773K. A peculiar texture development of 7075 Al joint showing (111)//ND-oriented grains existing throughout the nugget was revealed by EBSP analysis.

Abstract: One common point amongst extant theories of abnormal grain growth (AGG) is that they treat this phenomenon in terms of the relative grain size, or grain radius, of the abnormal grains. Topological and metrical quantities of abnormal grains, such as the number of their faces, or their grain boundary curvature, are taken into account only indirectly through the grain size itself. This paper, by contrast, treats AGG in terms of concepts, that include both the boundary curvature and the number of faces of the abnormal grain. Two cases are examined: 1) AGG, in which the matrix grains are fully pinned, so normal grain growth cannot occur; 2) AGG in which the matrix grains are free to evolve, so that normal grain growth ensues simultaneously in the matrix.

Abstract: Growth behavior and kinetics of grains in a liquid matrix has been studied by computer simulation for various physical and processing conditions. The kinetics of growing and dissolving grains is considered to follow that of single crystals in a matrix. Depending on the crystal shape, i.e. rounded or faceted, different kinetic equations were adopted for growing grains and a single equation was assumed for dissolving grains. Effects of critical parameters such as step free energy, temperature, and liquid volume fraction were evaluated.

Abstract: Calcium-hexaluminate phase(CA6) phase is known to be effective for the crack shielding due to the spinel block crystal structure. In this study, we focused to control the CA6 morphology for the good damage tolerance behavior in alumina and zirconia/calcium-hexaluminate(CA6) composites. calcium-hexaluminate(CA6) composites were prepared from zirconia, alumina and calcium carbornate powders. Calcium-hexaluminate(CA6) phase was obtained by the solid reaction through the formation of intermediate phase(CA2). CA6 phase showed column type abnormal grain grown behavior composed of small blocks. Due to the typical microstructure of CA6, alumina and zirconia/calcium-hexaluminate composites provide well controlled crack propagation behavior.