Papers by Keyword: Segregation

Abstract: This paper focuses on the role played by the liquid metal management on the solidification microstructure in industrial solidification processes. In particular attention is paid to the elimination of solidification defects by governing the microstructure evolution through fluid-dynamics and heat and mass transport in the liquid. The formation of hot tearing and gas porosities as well as columnar and equiaxed microstructures and micro and macro segregation are analyzed to explain how the liquid management is used to avoid defects. Examples on continuous casting and welding are also included.A very powerful tool for dealing with the complex phenomena associated with the solidification process is numerical modeling. Its increasingly growing use contemplates fluid-dynamics of the liquid phase, mass transport of solutes and solid-liquid interface evolution. Models using phase field and volume-averaging techniques, as well as models integrating multi-physics and multi-scale phenomena, are described as their use is taking on increasing importance in the design of solidification processes.

Abstract: The effect of steel strip-feeding ratio and superheat degree of molten steel on density and segregation of casting ingot is investigated by an original position analysis. It indicates that under the same degree of superheat condition, with the increase of feeding ratio, the average density of the casting ingot increases and the central segregation of the ingot decreases. Under the same feeding ratio conditions, the density and the central segregation increase with the increase of the degree of superheat.

Abstract: The purpose of this work is to consider the basic concepts on the present state of understanding of photocatalytic energy conversion using oxide semiconductors. This work also considers the approaches in derivation of theoretical models that allow explanation of the effect of properties on the performance of oxide-based photocatalysts in photocatalytic water oxidation. In this work we show that the performance of photocatalytic systems must be considered in terms of a range of the key performance-related properties (KPPs) that, in addition to the band gap, include the concentration of surface active sites, charge transport and Fermi level. Taking into account that all these KPPs are related to defect disorder, defect engineering may be applied in processing oxide semiconductors with optimal properties that are required to exhibit maximised performance in solar-to-chemical energy conversion.

Abstract: Low-pressure metal injection molding (LP-MIM) is an advanced manufacturing technology where a wax-based feedstock is injected into a complex shape before densification heat treatments. Feedstock is generally designed to minimize segregation, maximize flowability, maximize the strength of the molded component, maximize the solid loading potential and ease of debinding. In this study, the emphasis is placed on the evaluation of the effect of segregation on different wax-based Inconel 718 superalloy feedstocks used in LP-MIM. In powder metallurgy, particle or phase segregation generates a fluctuation of the particle distribution in powder-binder mixtures from point to point. Such demixing generally occurs before or during the injection process, and can lead to the formation of defects such as cracks, distortions or heterogeneous shrinkage of the sintered parts. Different wax-based feedstocks were poured in cylindrical hot molds (95°C), maintained in molten state for 1 minutes or for 60 minutes, and rapidly cooled to room temperature. The specimens were then extracted from the top and bottom regions of each cylindrical part. A thermogravimetric analysis technique was used to measure the volume fraction of powder at these two locations in order to quantify the degree of segregation in green parts. The best candidate feedstocks minimizing segregation are the mixtures containing only paraffin wax, or those containing paraffin wax and ethylene vinyl acetate combined. An increase in the time spent in the molten state and the use of beeswax or stearic acid promote the powder-binder separation of feedstocks.

Abstract: Chemisorbed atomic oxygen inducing Co segregation in CoNi (111) alloy is studied using periodic self-consistent density functional theory (DFT) calculations. In particular, the coverage dependence and possible adsorption-induced segregation phenomena are addressed by investigating segregation energies (the difference in calculated total free energy between surface sites and bulk-like sites) of isolated Co in CoNi (111) alloy. In agreement with previous experimental and theoretical investigations, segregation of Co is found to be oxygen-coverage dependent. While for ‘clean’ CoNi (111), Co prefers to be in the bulk. In the presence of more than 2/9 ML of oxygen, Co segregates to the surface. The analysis of oxygen adsorption trends and surface electronic structures explains the change in the local atomic arrangement which is expected to occur on the surface of alloys under reaction conditions. Our predictions for the high oxygen coverage cases are particularly relevant in underlining the importance of segregation phenomena to the hydrogen evolution performance of CoNi alloy hydrogen evolution electrode.

Abstract: Integrated continuous casting processes are an important element of the production process in modern industry. Requirements for quality and exploitation properties of products obtained using continuous casting processes necessitate a revision of the role of impurities distribution in the material. This paper concerns on the influence of the casting process on the distribution of alloy additions. Spectroscopy studies were performed for the most sensitive elements, from the segregation, (i.e. : iron and silicon) point of view. Next obtained research results were correlated with the parameters of chemical composition such as casting speed, the presence of a modifier and the processing of metal crystallization zone. Analysis of the results clearly shows the presence of iron and silicon normal macrosegregation. It was observed that the concentration of these elements is highest in the casting axis and, in the case of iron, is 50% higher in the axis than in the circumference of casts.

Abstract: Twin roll casting (TRC) technology is becoming increasingly popular because of the opportunity to receive aluminum strips which may serve as input for direct cold rolling processes. Furthermore, these strips have a favourable combination of mechanical and electric properties as well as dimensional tolerances. TRC line strips show segregation of alloying elements and structure providing a combination of casting structure and wrought alloy. As part of this study, the structure of the strips from the TRC line has been analyzed as well as the segregation of the alloying elements.

Abstract: Branched cracks are undesirable and dangerous, and under the oxidized surface of a continuously-cast slab they are mostly difficult to identify. They can cause the formation of defects in thick sheet or on thin strip. This work deals with the study of the background to the formation of branched cracks in a continuously-cast slab, at pulling rate 0.43 m.min^-1. The results of the work show that branched cracks extended to a depth of 10 mm below the slab surface and occurred mostly below oscillation marks. Cracks were mostly present in the edge parts of the slab, where they were also deeper in comparison with locations in the middle of the slab width. Microstructural analysis confirmed heterogeneity of ferrite grain sizes in the slab surface skin. The microstructure was formed predominantly of polyhedral ferritic-pearlitic grains, and on the ferritic grain boundaries the presence of tertiary cementite was observed. In the area around the cracks mainly non-equilibrium microstructure of acicular character was observed. A typical feature of these cracks after their opening is intercrystalline fracture. Wavelength-dispersive X-ray analysis of a sample with a branched crack showed chemical heterogeneity of harmful elements, mainly S and As. Large amounts of these elements point to their significant segregation. The formation of branched cracks is conditioned not only by segregation of impurities, but possibly also by the presence of undesirable brittle cementite networks. The presence of cementite was confirmed not only by microstructural analysis, but also diffraction of hard X-ray radiation. Precipitation of microalloying elements in connection with the cementite can lead to higher probability of surface crack formation, as also confirmed in this study.Keywords: slab, branched cracks, oscillation marks, segregation, precipitation

Abstract: Removal of the dilaterous effects of iron in silicon is critical for the performance of multicrystalline silicon (mc-Si) solar cells, with internal gettering at extended defects including stacking faults and grain boundaries being one possibility. We present the results of a density function study of the behavoiur of iron at the intrinsic stacking fault and (001)–Σ 5 twist grain boundary, which both represent examples of fully bonded systems. Our results show iron is bound more strongly to the grain-boundary than the stacking fault, which we ascribe to a combination of Si-Fe chemistry and strain relaxation. However, we find that the binding energy of a single Fe atom to these extended defects is modest, and less than 0.5 eV.

Abstract: Al-Cu-Mg-Ag alloy 2139 is a wrought alloy that is heat-treatable and used in aerospace constructions. This is mainly due to the addition of silver in the alloy. Hot-tearing is a problem for casting wrought alloys with conventional liquid casting techniques. The risk is reduced by using rheo-high pressure die casting (R-HPDC) to allow the alloy to be used for near-net shape forming of components. This study investigates the segregation characteristics of R-HPDC alloy 2139. The effects of segregation on the age-hardening response as well as the tensile properties are studied. The chemical composition differences across the casting were found to aid in correlation of results. It is found that segregation occurs in three dimensions, namely from the surface to the centre of the casting, as well as across the width and length of the casting.