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Abstract

Eutectoid growth, as the important reaction mechanism of the carbon steel heat treatment, is the basis to control the microstructure and performance. At present, most studies have focused on lamellar growth, and did not consider the nucleation process. Mainly due to the nucleation theory is inconclusive, a lot of research can support their own theory in a certain range. Based on the existing nucleation theory, this paper proposes a cooperative nucleation model to simulate the nucleation process of eutectoid growth. In order to ensure that the nucleation process is more suitable to the theoretical results, different correction methods were used to amend the model respectively. The results of numerical simulation show that when the model is unmodified, the lateral growth of single phase is faster than that of longitudinal growth, so the morphology is oval. Then, the effects of diffusion correction, mobility correction and ledges nucleation mechanism correction on the morphology of nucleation and the nucleation rate were studied respectively. It was found that the introduction of boundary diffusion and the nucleation mechanism of the ledges could lead to a more realistic pearlite.
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Abstract

Cast irons are good examples of materials which are more sensitive to chemical composition and production conditions. In this research to improve casting quality, solidification and nucleation process in grey cast iron was investigate. In particular, attempts have been made to rationalize variation in eutectic cells with nucleation sites and eutectic solidification undercooling. Four castings with different diameter and similar chemical composition and pouring temperature and different inoculant percentage was casted. The cooling curve and maximum and minimum undercooling for each castings was measured. Also optical metallography and image analyzer has been used to determine the average eutectic cells diameter, and linear and surface densities, and volume density was calculated. The results of this research show a competitive behavior between nucleation sites and eutectic undercooling. Higher nucleation sites and higher eutectic undercooling cause higher eutectic cell density. But increasing nucleation sites by introducing inoculants to molten metal, is accompanied with reduction in eutectic undercooling. It means that inoculation and undercooling have opposite effect on each other. So, to achieve maximum cell density, it is necessary to create an optimization between these parameters.
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Abstract

The orientations of recrystallization nuclei and their adjacent as-deformed regions have been characterised in deformed single crystals of different metals (Ag, Cu, Cu-2%wt.Al and Cu-8%wt.Al) in which twinning and/or shear banding occur. {112}<111> oriented crystals of these metals have been compressed to different strains, then lightly annealed, and the crystallographic aspects of the recrystallization process along shear bands examined by local orientation measurement in TEM and SEM. The results clearly show the existence of a well-defined crystallographic relation between the local deformation substructure and the first recrystallized areas of uniform orientation. The first-formed nuclei always exhibit near 25–400(<111>–<112>) type misorientations, in the direction of highest growth, with respect to one of the two main groups of the deformation texture components. The rotation axes can be correlated with the slip plane normal of highest activity. As recrystallization proceeds, recrystallization twinning develops strongly and facilitates rapid growth; the first and higher generations of twins then tend to obscure the initial primary crystallographic relation between the shear bands and recrystallization nuclei .
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Abstract

Reduction of three industrial nickel oxides (Goro, Tokyo and Sinter 75) with a hydrogen bearing gas was revisited in the temperature interval from 523 to 673 K (250 to 400°C). A pronounced incubation period is observed in the temperature interval tested. This period decreases as the reduction temperature increases. Thermogravimetric data of these oxides were fitted using the Avrami-Erofeyev kinetic model. The reduction of these oxides is controlled by a nucleation and growth mechanism of metallic nickel over the oxides structure. Rate kinetic constants were re-evaluated and the activation energy for the reduction of these oxides was re-calculated.
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