Papers by Keyword: Segregation

Abstract: The influence of different cooling regimes on the surface microstructure of strip cast AA1XXX alloys was investigated. The cooling rate was varied over a wide range using a bench scale simulation apparatus. It was found that the type and morphology of the intermetallics varied over a wide range both at the cast surface and in the near surface regions. These observations were compared to the compounds formed in the bulk of the slab. It was found that, a surface layer containing a high concentration of alloying elements, an inverse segregation layer, forms under some conditions and can result in a high density of intermetallics. By varying casting conditions, the mechanism of formation for these structures was studied. The surface microstructure of the slabs was characterized as a function of the different local cooling regimes.

Abstract: High temperature heat treatment of aluminium alloys causes surface enrichment of the trace elements in Group IIIA - VA, specifically the low melting point elements Pb, Bi, In and Sn. The phenomenon has practical significance in promoting certain types of localised corrosion, such as galvanic and filiform corrosion, while mitigating other types, such as pitting corrosion of the bare surface. The purpose of this paper is to investigate the surface enrichment and microstructure of indium relative to the available data for Pb. Model binary AlIn alloys, containing 20-1000 ppm of In, were used after heat treatment at various temperatures. In addition to electrochemical investigations, the microstructures were characterised by field emission scanning electron microscopy (FEG SEM) and field emission transmission electron microscopy (FEG TEM). Heat treatment at temperatures as low as 300°C gave significant segregation of In as opposed to 600°C for Pb. As a result of this and yet unresolved oxide film breakdown mechanism on aluminium, In was significantly more effective than Pb in anodically activating aluminium. These results suggest the possibility that significant activation earlier observed on certain commercial alloys as a result of low temperature heat treatment may be due to the trace elements In.

Abstract: The influence of heat treatment on mechanical properties and textures of a meltextruded Al-4.5wt.%Cu alloy bar was studied. And its characteristics are compared with those of a hot-extruded Al-Cu alloy bar. The results show that the mechanical properties of the melt-extruded Al-Cu alloy bar were slightly higher than those of the hot-extruded bar, and the difference in their mechanical properties was narrowed after the T6 treatment. The recrystallization texture was found in the melt-extruded Al-Cu alloy bar, and the fiber texture of the hot-extruded Al-Cu alloy bar was found different from that of the melt-extruded Al-Cu alloy bar.

Abstract: In continuous casting, the severity of centerline macrosegregation and internal crack formation is linked to the cast structure, which can be minimized by increasing the equiaxed zone. Thus all the factors which favour an equiaxed structure are useful to quality. These are: low superheat, electro magnetic stirring (EMS) in the mould, large section size and especially in the case of internal cracks, uniform heat transfer between the strand and the mould. Microsegregation, which is the primary reason for macrosegregation, is due to the distribution coefficient and the growth rate of the solidification front. It is known that superheat has an influence on the solidification structure on both the micro- and macro- levels. Research has confirmed that superheat also affects crack formation in the solidifying front. These observations are usually reported on steels but little has been reported on low-alloyed coppers. In our experimental studies we examined the effects of superheat and uneven heat transfer on segregation behavior of phosphorus and iron in round, deoxidized, high phosphorus (DHP) copper billets. Furthermore crack formation and the solidification structure on both the micro- and macro- levels were studied. Results were verified by optical emission spectrometry (OES) and scanning electron microscopy (SEM).

Abstract: A numerical model aimed at simulating the segregations during the columnar solidification of a binary alloy is used to investigate the effects of a forced convection. Our objective is to study how the segregation characteristics in the mushy zone are influenced by laminar flows driven both by buoyancy and by AC fields of moderate intensity. Various types of magnetic fields have been tested, namely travelling, rotating magnetic field and slowly modulated electromagnetic forces. The calculations have been achieved on two types of alloys, namely tin-lead and aluminiumsilicon. It is shown that the flow configuration changes the segregation pattern. The change comes from the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. The pressure difference along the front drives a mush flow, which transports the solute in the mushy region. Another interesting type of travelling magnetic field has been tested. It consists of a slowly modulated travelling magnetic field. It is shown that in a certain range of values of the modulation period, the channels are almost suppressed. The normal macrosegregation remains, but the averaged segregation in the mushy zone is weaker than in the natural convection case. The optimal period depends on the electromagnetic force strength as well as the cooling rate. The latter phenomenon cannot occur in the case of rotating magnetic fields, since in that configuration the sign of the pressure gradient along the solidification front remains unchanged. Recent solidification experiments with electromagnetic stirring confirm the predicted macrosegregation patterns.

Abstract: A numerical approach for the segregation of atomic oxygen at Ag/MgO interfaces is presented. A general segregation kinetics is considered and the coupled system of partial differential equations is solved due to a one-dimensional finite difference scheme. Based on a model oxide distribution, the influence of the oxide distribution is numerically investigated and compared with the solution for equidistant arrangements. The numerical approach allows for the consideration of general boundary conditions, specimen sizes and time-dependent material and process parameters. Furthermore, a numerical procedure to convert two-dimensional microstructures into representative one-dimensional distributions is described.

Abstract: In this overview, we discuss the sum-rule relating phenomenological coefficients in randomly mixed systems and consider several applications to collective diffusion problems. These applications include intrinsic diffusion multicomponent alloys, chemical diffusion in strongly ionic mixed cation crystals and demixing of cations in randomly mixed quaternary transition metal oxides. In each case a substantial simplification is possible as a result of the sum-rule.

Abstract: Zinc oxide is widely used in semiconductors fabrication, pigments and mainly in varistors. The electrical properties of ZnO based varistors are related to the grain boundary characteristics, and it can be modified using additives. In this work, zinc oxide based powders were prepared by chemical route derived from the Pechini’s method, and small amounts of tin ions were added to the system. The XRD measurements showed that the tin oxide not forming solid solution in the bulk, and it was observed that the increasing of tin concentration causes an increasing of the surface area at isothermal conditions of calcination. Once the isoelectric point was dislocated to more acid values, the hypothesis of segregation onto surface was considered.