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Pressureless-sintering of non-stoichiometric barium titanate (BaTiO3) powder of TiO2-excess compositions has been investigated. Crystalline phases were analysed by X-ray diffractometry. Attention has been paid to the analysis of the corresponding sintered microstructure by adopting scanning and transmission electron microscopy. Large plate-like grains grown abnormally containing {111} double twin lamellae are commonly found in low-temperature (1332oC) sintered BaTiO3, by presence of a liquid phase, or by twinning, or as discussed in this experiments by partial dislocations. Samples were sintered in air at 1250oC for 1¡B4 and 50 h. In this respect, the emphasis is discussed about the role of the partial dislocations in the early stage of twin formation. Both analytic dislocations exist on the {111} double twin lamellae within abnormal grain and the interface between abnormal grain and normal grain. In the experimental process, dislocation type with the Burgers vector ‹110› and ‹112› has been found except for the ‹111›. For the Burgers vector ‹110› in the perovskite structure, it is possible to envisage that edge dislocations with [100] Burgers vector can be dissociated in their climb plane ([100] ¡÷ 1/2[101] + 1/2[10 ]). Moreover, the perovskite structure has been deformed form ‹110› dislocation (perfect dislocation). Oppositely, the [112] Burgers vector has been formed both partial dislocation type and perfect dislocation type. For sintering at 50 h, we can conjecture the grain growth mold by Shockley or Frank partial dislocation affect on the macroscopic step. Experimental results show that Shockley and Frank partial dislocations have not been found on the {111} double twin lamellae.

Dynamic abnormal grain growth (DAGG) in commercial-purity molybdenum sheets was investigated through a series of tensile tests at temperatures between 1450°C and 1800°C. DAGG is abnormal grain growth (AGG) which requires the presence of concurrent plastic strain. Most AGG phenomena previously documented in the literature can be categorized as static abnormal grain growth (SAGG) because they occur during static annealing, sometimes following plastic strain, but do not occur during plastic deformation. The DAGG boundary migration rate is much faster than the SAGG boundary migration rate, and DAGG may be utilized to obtain large single crystals in the solid state. Dynamic abnormal grains were found to exhibit a crystallographic orientation preference with respect to the specimen geometry, generally described as derivative from a <101> fiber texture. DAGG was found to prefer growth on the surface of the specimen rather than the interior. The growth of dynamic abnormal grains, which initiated and grew during plastic straining, generally ceased when the application of plastic strain was removed. The DAGG boundary migration rate was found to be a direct function of plastic strain accumulation, regardless of the strain-rate. Therefore, it is hypothesized that the rapid boundary migration rate during DAGG results from an enhanced mobility of certain boundaries. A model is proposed based on the rate of boundary unpinning, as mediated by the emission of dislocations from pinning sites. / text

Dynamic abnormal grain growth (DAGG) is a phenomenon that produces abnormal grain growth at elevated temperatures during plastic deformation. It is distinct from classically studied static abnormal grain growth phenomena in that it only occurs during plastic deformation. Previous investigations of DAGG in a Mo sheet material produced using powder metallurgy techniques observed DAGG grains to grow more rapidly near the sheet surface than near the sheet center. This phenomenon is explored in the present study. A Mo sheet material produced using arc melting techniques is also studied to determine the morphology of DAGG grains. A preference for growth near the sheet center is observed in this material. The through-thickness variations in texture and grain size for both the arc-melted and powder-metallurgy Mo sheet materials are investigated. The preference for growth near the surface in the powder-metallurgy material is due to a through-thickness variation in grain size, with smaller grains near the surface and larger grains near the center. The preference for DAGG grain growth at the center of the arc-melted sheet material is because of very large grains that grow near the sheet surface. These large grains may be the product of multiple abnormal grains occurring near the sheet surface because of texture variation through the sheet thickness. Regardless, the DAGG grain cannot consume these large grains and leaves them as island grains decorating the region near the sheet surface. These results suggest that DAGG is driven primarily by grain boundary curvature. Microstructures that include DAGG grains are investigated with electron backscatter diffraction (EBSD). A new method to evaluate geometrically necessary dislocation densities using EBSD data is derived. DAGG grains are relatively undeformed compared to the polycrystalline microstructure. DAGG grains are not oriented either favorably or unfavorably for slip. Results of the analysis of the grain boundaries between DAGG grains and normal grains do not indicate any special character preference for these grain boundaries. / text

Secondary abnormal grain growth (SAGG) during sintering of barium titanate has been explained in terms of twin plane re-entrant edge (TPRE) growth mechanism by {111} double twin lamellae. But during sintering of Ti-excess barium titanate, {111} double twins lamellae are observed with out SAGG. In our group, Lin founded that when combine two different Ba/Ti ratio of powder to sintering above the eutectic temperature, the SAGG is observed in the interface between two different powders. Therefore, this thesis consists of three major researches: (a) {111} double twin, (b) Ba/Ti ratio, (c) liquid phase. In the experiment, we follow Lin¡¦s experiment to sinter the specimen contain with SAGG. And in this specimen, it can observe the specimen divided into three type of growth grain: (a) top surface with normal grain growth (NGG), (b) intermediate layer with abnormal grain growth (AGG), (c) bottom layer with secondary abnormal grain growth. It can all observed {111} double twin in these three different type of layer. This result confirmed that SAGG are not induced by TPRE growth mechanism. Then we used SEM/EDS to analysis the Ba/Ti ratio in the different type of grain growth layer. The Ba/Ti ratio in this analysis is not differing in NGG, AGG and SAGG. Therefore, we used OM, SEM, TEM to observe the grain boundary and triple grain junction in NGG, AGG and SAGG. It can observe that only the grain boundary and triple grain junction in SAGG are complete wetting. The experimental results shows that the grain growth behavior controlled by the liquid phase wetting degree.

Dynamic abnormal grain growth (DAGG) is a phenomenon by which single crystals up to centimeters in length are produced at elevated temperature during the application of strain. DAGG was previously demonstrated in commercial-purity molybdenum (Mo) materials. This is the first investigation to confirm DAGG in another material, tantalum (Ta). Previous experiments initiated and propagated DAGG using constant true-strain rate tensile tests, but this study demonstrates that DAGG can also occur under constant true-stress tensile conditions. A Mo material was tested under constant true stress, and two Ta materials were tested under constant true-strain rate. The effects of temperature, stress, strain rate, initial microstructure and texture on tensile test data and the resulting microstructures are examined. The microstructures of the Ta materials are analyzed using electron backscatter diffraction (EBSD) data to quantify the orientation, deformation, grain boundary character, and slip properties of the DAGG grains and unconsumed microstructure. The DAGG grains were found to be relatively undeformed compared to the unconsumed microstructure following DAGG and to not be oriented favorably, or unfavorably, for slip. The grain boundaries between DAGG grains in one Ta material were found to commonly have [sigma]3 character. This was likely due to a strong initial <111>-fiber texture. Previous investigations of DAGG in Mo indicated that DAGG grains commonly grow along the surface of the specimen, but this was not observed with significant frequency in Ta. Results suggest that the distance the DAGG grain boundary travels is proportional to the accumulated strain during DAGG, and the velocity of the DAGG grain boundary is proportional to the applied strain rate but is not related to the orientation of the DAGG grain or its slip properties. / text

No description available.

Static abnormal grain growth (SAGG) was studied in Al-Mg alloy AA5182 sheet by varying four processing parameters: deformation temperature, strain rate, annealing temperature, and annealing time. SAGG is a secondary recrystallization process related to geometric dynamic recrystallization (GDRX) and requires both deformation at elevated temperature and subsequent static annealing. A minimum temperature is required for both SAGG and GDRX. Recrystallized grains only develop at strains larger than the critical strain for SAGG, [epsilon]SAGG. The size of the recrystallized grains is inversely related to and controlled by the density of SAGG nuclei, which increases as local strain increases. The results of this study suggest that SAGG is controlled by two thermally-activated mechanisms, dynamic recovery and recrystallization. During deformation, dynamic recovery increases as deformation temperature increases or strain rate decreases, increasing the critical strain for SAGG. SAGG is subject to an incubation time that decreases as annealing temperature increases. SAGG can produce grains large enough to reduce yield strength by 20 to 50 percent. The results of this study suggest strategies for avoiding SAGG during hot-metal forming operations by varying processing conditions to increase [epsilon]SAGG. / text

Recozimentos realizados nos aços elétricos semiprocessados pelos fabricantes de motores sem um controle adequado da descarbonetação pode resultar em regiões das läminas com pequeno tamanho de grão e consequentemente aumento das perdas magnéticas com queda no rendimento dos motores elétricos. A maioria dos fabricantes de motores nacionais utilizam este tipo de aço e fazem internamente o recozimento final em fornos contínuos com atmosfera à base de nitrogênio com cerca de 5 a 10% de H2 e uma pequena fração de vapor dágua suficiente para gerar uma atmosfera que reaja com o carbono do aço, removendo-o. Caso esta descarbonetação não seja realizada com êxito há uma perda significativa no rendimento do motor pela aumento das perdas magnéticas. Amostras comerciais de diferentes aços com variados teores de carbono foram selecionadas para avaliar a influência do teor de carbono, do grau de redução, do tempo e da atmosfera do forno no aumento do tamanho de grão e nas perdas magnéticas. Foram realizados recozimentos a vácuo e em atmosfera descarbonetante. Medições de perdas magnéticas e do tamanho de grão além da avaliação da microestrutura foram efetuados para determinar esta influência. Foi verificado que no recozimento a vácuo, mesmo em variados graus de deformação, amostras com o teor de carbono inicial maior que 300 ppm, tiveram um significativo atraso no aumento do tamanho de grão. Nas situações em que não ocorreu significativo aumento do TG as perdas magnéticas apresentaram valores altos e 9 indesejáveis. Por outro lado amostras isentas de deformação não apresentaram aumento significativo do tamanho de grão mesmo com descarbonetação durante o recozimento. Foi verificado ainda que a formação de grãos colunares ocorre em faixas de deformação específicas, em recozimentos com descarbonetação para aços com mais de 300 ppm de carbono e onde haja um rápido aquecimento da amostra até a temperatura de recozimento. / Annealing applied by the motors plants that are made on the semi processed electrical steels without an exact and precisious control of the decarburization on the lamination can brings undesirable results on the grain size coarsening and as a result an increasing on the core losses with a significant depreciation on the efficiency of the electrical motors. Most of the domestic motors fabrics make use of this kind of electrical steel and they do on their process the final annealing on continuous furnaces with atmospheres based on nitrogenium with around 5 until 10% of hidrogeniun and a little fraction of water vapor enough to generate an atmosphere that reacts to the carbon from the steel, eliminating it. If this decarburization is not been made sucessfully there will be an effective loss on the motor efficiency due the incresing on the core losses. Commercial samples from different steels with several carbon contents were choosen to evaluate the influence of carbon content, strain grade, time and furnace atmosphere on the grain size coarsening and core losses. It was effetived vacumn and decarburizing annealing. Core losses and grain size measurements were made in addition to the evaluation of the microstructure to determine this influence. It was verified that on the vacumn annealing, even on differents strain reductions grades, the samples with initial carbon content bigger than 300 ppm, had a meaninfull late on the grain size increase. On the situation that do not occurred a significative increase on the grain size the core losses presented undesirable high values. On the other hand samples without strain did not presented a significative increase on the grain size even with decarburization during the annealing. It was still verified that the columnar grain formation occur on specific strain grades, on decarburizing annealing and to more than 300 ppm carbon content steels. In addition where there is a fast heating rate until the annealing temperature.

Recozimentos realizados nos aços elétricos semiprocessados pelos fabricantes de motores sem um controle adequado da descarbonetação pode resultar em regiões das läminas com pequeno tamanho de grão e consequentemente aumento das perdas magnéticas com queda no rendimento dos motores elétricos. A maioria dos fabricantes de motores nacionais utilizam este tipo de aço e fazem internamente o recozimento final em fornos contínuos com atmosfera à base de nitrogênio com cerca de 5 a 10% de H2 e uma pequena fração de vapor dágua suficiente para gerar uma atmosfera que reaja com o carbono do aço, removendo-o. Caso esta descarbonetação não seja realizada com êxito há uma perda significativa no rendimento do motor pela aumento das perdas magnéticas. Amostras comerciais de diferentes aços com variados teores de carbono foram selecionadas para avaliar a influência do teor de carbono, do grau de redução, do tempo e da atmosfera do forno no aumento do tamanho de grão e nas perdas magnéticas. Foram realizados recozimentos a vácuo e em atmosfera descarbonetante. Medições de perdas magnéticas e do tamanho de grão além da avaliação da microestrutura foram efetuados para determinar esta influência. Foi verificado que no recozimento a vácuo, mesmo em variados graus de deformação, amostras com o teor de carbono inicial maior que 300 ppm, tiveram um significativo atraso no aumento do tamanho de grão. Nas situações em que não ocorreu significativo aumento do TG as perdas magnéticas apresentaram valores altos e 9 indesejáveis. Por outro lado amostras isentas de deformação não apresentaram aumento significativo do tamanho de grão mesmo com descarbonetação durante o recozimento. Foi verificado ainda que a formação de grãos colunares ocorre em faixas de deformação específicas, em recozimentos com descarbonetação para aços com mais de 300 ppm de carbono e onde haja um rápido aquecimento da amostra até a temperatura de recozimento. / Annealing applied by the motors plants that are made on the semi processed electrical steels without an exact and precisious control of the decarburization on the lamination can brings undesirable results on the grain size coarsening and as a result an increasing on the core losses with a significant depreciation on the efficiency of the electrical motors. Most of the domestic motors fabrics make use of this kind of electrical steel and they do on their process the final annealing on continuous furnaces with atmospheres based on nitrogenium with around 5 until 10% of hidrogeniun and a little fraction of water vapor enough to generate an atmosphere that reacts to the carbon from the steel, eliminating it. If this decarburization is not been made sucessfully there will be an effective loss on the motor efficiency due the incresing on the core losses. Commercial samples from different steels with several carbon contents were choosen to evaluate the influence of carbon content, strain grade, time and furnace atmosphere on the grain size coarsening and core losses. It was effetived vacumn and decarburizing annealing. Core losses and grain size measurements were made in addition to the evaluation of the microstructure to determine this influence. It was verified that on the vacumn annealing, even on differents strain reductions grades, the samples with initial carbon content bigger than 300 ppm, had a meaninfull late on the grain size increase. On the situation that do not occurred a significative increase on the grain size the core losses presented undesirable high values. On the other hand samples without strain did not presented a significative increase on the grain size even with decarburization during the annealing. It was still verified that the columnar grain formation occur on specific strain grades, on decarburizing annealing and to more than 300 ppm carbon content steels. In addition where there is a fast heating rate until the annealing temperature.