Antonio Javier Criado

Growth of eutectic silicon from primary silicon crystals in aluminium-silicon alloys

Technological interest in aluminium-silicon alloys derives, fundamentally, from their excellent moulding properties provided by the presence of silicon. In addition, they possess high thermal and electrical conductivities and good corrosion resistance. The possibility of modification of their cast structure by adding small quantities of alkaline and alkaline-earth elements makes their mechanical characteristics very competitive. Metallographic observations of the structures of primary and eutectic silicon crystals, and their possible synergistic influence, provide useful data on the critical stages of formation and growth of eutectic silicon phase. The nucleation and growth of eutectic silicon, removed from the cooperative precipitation zone, induce refinement in crystal size and, therefore, an improvement of the mechanical properties of the moulded structure. The aim of this investigation was to observe the influence of primary silicon crystals on the nucleation and growth of eutectic silicon.

Preparation of aluminium boride by powder technology

The problem with using boron in a sintering process is the formation of a surface layer of oxide [Combust., Explosion Shockwaves 10 (1974) 539] which hampers the process. The incorporation of reducing elements into the amorphous boron is done by powder technology. Suitable working conditions were designed to provide sintered boride free of oxide. The reducing elements served the double purpose of forming borides and of avoiding the formation of boric oxide in the first stage of sintering. Hermetically sealed crucibles were designed for the sintering by thermal shock. The source materials were amorphous boron and atomized aluminium.

Colour Metallography of Cast Aluminium Alloys

Abstract For better observation of the microstructure of cast aluminium alloys under an optical microscope, the contrast obtained for the different micro constituents was enhanced by making use of a copper phthalocyanine based colorant. The proposed method allows distinguishing structural details which would remain hidden or poorly resolved when studying the conventionally etched sample. The method produces high relief for silicon precipitates with the tint further accentuating the outlines, allowing detection of morphology change of the aluminium-silicon eutectic due to chemical modification with strontium. Magnesium micro segregations in the α-aluminium phase, caused by incorrect thermal treatment during ageing, can also be clearly distinguished. The coloured images obtained by optical microscopy are very sharp and provide similar or better information than the corresponding Scanning Electron Microscopy images. The method proved to be suitable for Quality Control in the fabrication of cast aluminium alloy compounds, sourcing down possible errors to different stages of the process.

Selective etching procedure for observation of primary silicon morphologies in V-AlSi20 master alloy

Abstract Aluminium castings with silicon content above the eutectic are becoming an increasingly important product, meeting a wide range of high end engineering specifications, particularly in automotive and aerospace industries. Its outstanding properties highly depend on their cast microstructures; conventional casting will produce large primary silicon particles in a variety of morphologies. This causes drastically shortened tool life when machining this material, reason for which melt refinement is an ongoing field of investigation. An ultrasound assisted chemical deep etching procedure could be proven to reveal primary and eutectic silicon crystals in all detail while preserving the spatial distribution they had when precipitating from the eutectic during solidification. Different polyhedral growth morphologies, mostly coarser platelets, star-like and blocky structures, were observed in an as cast alloy using Scanning Electron Microscopy. As this metallurgic sample treatment provides to a certain extend spatially resolved information about morphology and distribution of silicon crystals, it is suitable to assess the grade of success of refinement processes aiming to achieve fine and well dispersed silicon particles in the eutectic phase.

Effect of Sintering Temperature on the Formation of Intermetallics in Al-Fe-B Nanocomposite

Mixing and sintering aluminium and 20% mechanically alloyed Fe/B nanoparticles provokes the formation of intermetallics in the aluminium matrix when following a powder metallurgy route. Materials were sintered in a wide range of temperatures (from 600 to 1100 oC). Previous studies have shown that these materials present neither important dimensional changes during sintering nor significant differences in mechanical properties. However, sintering temperature strongly affects corrosion resistance and hot rolling capability. Low sintering temperature provides nanocomposites with lower corrosion properties and hot rolling capability. In this work the nanocomposites obtained at different temperatures were characterized by X-ray diffraction. This technique allows following the formation of different intermetallics at each temperature, since being non-equilibrium processes, the use of ternary phase diagram of these elements is not possible.

An Archaeological Analogue for a Composite Material of Carbon Steel, Copper and Magnetite

Abstract The study of archaeological analogues is a helpful tool to asses long-term corrosion behaviour for a wide range of materials. In this work, a celtiberic belt-buckle is studied as analogue for a composite material of carbon steel, bronze and a final coating of magnetite, also providing a hypothesis about the ancient fabrication of these coatings. The paper goes through a metallographic examination of the sample and compares the corrosion phenomena suffered with those of two other metallic objects, recovered from the same archaeological site. It includes geochemical analysis of the soil from which the reference objects were recovered. The belt-buckle, after being cremated and buried over two millennia into a rather aggressive environment, showed remarkably high resistance against corrosion.

Untersuchung von Bronzeeinlagerungen in einem Krug aus Graupastenkeramik aus der Tartesierzeit (VII Jahrhundert. V. CHR.)

Kurzfassung Mit Hilfe der Rasterelektronenmikroskopie sowie EDX-Analyse wurden Bronzeeinlagerungen in einem Krug aus Graupastenkeramik untersucht. Dieser stammt von einer Fundstelle in der Nähe von Toledo, Spanien und wird auf das VII. Jahrhundert vor Christus datiert. Sowohl die ermittelte Zusammensetzung der binären Kupfer-Zinn-Bronze als auch die Interpreation der Kristallfeinstruktur erhärten die Hypothese, dass es sich bei dem Krug um ein Artefakt der tartesischen Kultur handelt. Dies deutet seinerseits darauf hin, dass der Einflussbereich dieser frühiberischen Kultur wesentlich ausgedehnter war als bisher angenommen.