Materials Science Forum Vols. 587-588

Abstract: Vanadium oxides are a class of materials with outstanding physical and chemical properties. They find a wide field of technological applications such as optical and electrical switching devices, light detectors, temperature sensors, micro batteries, etc. There are several studies regarding the production of vanadium oxide films by radio-frequency (RF) magnetron sputtering, and with increasing interest on the thermochromic VO2 phase. However, literature with focus on vanadium oxide films deposited by direct current (DC) magnetron sputtering is very limited. In this work, we have successfully deposited vanadium oxide thin films by reactive DC magnetron sputtering under several processing conditions. The effect of substrate type, temperature, and O2/Ar flow ratio on phase formation has been studied. Structural analysis and phase determination have been carried out by X-ray diffractometry (XRD). Some single phase samples were also analysed with respect to surface morphology by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The thermochromic behaviour of single phase VO2(M) films has been evaluated by optical spectrophotometry.

Abstract: Thin film transistors (TFTs) have been produced by rf magnetron sputtering at room temperature, using non conventional oxide materials like amorphous indium-zinc-oxide (IZO) semiconductor, for the channel as well as for the drain and source regions. The obtained TFTs operate in the enhancement mode with threshold voltages of 2.4 V, saturation mobility of 22.7 cm2/Vs, gate voltage swing of 0.44 V/dec and an ON/OFF current ratio of 7×107. The high performances presented by these TFTs associated to a high electron mobility, at least two orders of magnitude higher than that of conventional amorphous silicon TFTs and a low threshold voltage, opens new doors for applications in flexible, wearable, disposable portable electronics as well as battery-powered applications.

Abstract: To address the increasing demand for high performance and high quality die castings, Zn-Al casting alloys have been developed, in particular for applications in the automotive industry. This family of alloys has good physical, mechanical and tribological properties and is commonly used as foundry alloys in a variety of applications. Particularly, the Zn-22 wt% Al eutectoid alloy is a classic commercial material and a fine-grained structure can be obtained by using a suitable heat treatment which can induce a superplastic behavior. On the other hand, this heat treatment can affect the corrosion behavior of such alloy. The present work focuses on the influence of as-cast and quenched microstructures of the Zn-22wt%Al eutectoid alloy on its electrochemical corrosion behavior. The typical microstructural patterns were examined by using optical microscopy techniques. In order to evaluate the corrosion behavior of such alloy, corrosion tests were performed in a 0.5 M NaCl solution at 25°C by using an electrochemical impedance spectroscopy (EIS) technique and potentiodynamic polarization curves. An equivalent circuit by using the ZView software was also used to provide quantitative support for the discussions and understanding of the electrochemical corrosion behavior. It was found that the heat-treated samples are more susceptible to the corrosion action than the as-cast alloy sample.

Abstract: This work studies the behaviour of welding joints under abrasive wear in A355 Gr. P11 steel pipes carrying liquids at high temperatures. Various welding processes were tested on different joints in an effort to choose the one that gave the best wear response. For this purpose, a pin-on-disk tribometer was used at 200°C to simulate the service conditions of the pipes. The tests were carried out by pressing a hard chromium-plated steel pin, with a diameter of 4 mm and hardness of about 775 HV, on steel specimens in order to measure their mass loss and friction coefficient in the three welding zones: base metal (BM), heat affected zone (HAZ) and weld bead (WB). The tests conditions were 10 N of load, 200 rpm of rotation velocity and a radius of 3 mm. In addition, metallographic studies and hardness measurements of the three welding zones were done to complete this work.

Abstract: In the present paper, a selection of micrographs showing some typical microstructures and corrosion layers developed in copper and bronze alloys from different archaeological contexts will be discussed. Metallurgic interpretation of the microstructures observed, based in the proper binary equilibrium phase diagrams is presented. Micro-EDXRF and SEM-EDS analysis were carried out to assess the alloy composition and to contribute to the understanding of the corrosion processes occurred during the long periods of burial of the archaeological metals. Examples of surface decuprification, strong intergranular corrosion, cuprite under green corrosion layers and copper redeposition indicate particular corrosion processes. The presence of copper oxides, sulphides inclusions, lead globules and porosities is also documented.

Abstract: The aim of this research is the development of activating fluxes to improve weld bead geometry and increase weld penetration depth in austenitic stainless steels. The effect on bead geometry of two home-made fluxes, composed of titanium and aluminium oxides, was studied, in combination with two shielding gases, respectively Argon and an Argon/Helium mixture. A significant increase in penetration was obtained in welds done with the Ti based activating flux across the whole range of welding currents for both shielding gases, which was not the case for welds performed with the Al based flux. A decrease in δ-ferrite content in the weld metal with increasing current was observed only in welds done with the Ti based flux.

Abstract: Al-base and Fe-base powders have been amorphized by a high energy milling process in an Attritor miller. Microstructural evolution in powder particles has been analyzed by XRD, DSC, SEM and TEM. The conventional route of cold pressing and sintering applied to these powders does not result adequate to preserve their amorphous or nanometric character. An additional disadvantage of this route appears during the cold pressing stage, as a consequence of the insufficient green strength of the compacts, due to the high hardness of the milled powders. In order to avoid these difficulties a new consolidation technique, electrical resistance sintering (ERS), has been successfully employed. ERS consolidated compacts have been microstructurally characterized by optical microscopy and XRD, showing that compacts preserve their amorphous and/or nanometric character.

Abstract: Mechanically alloyed aluminium powder was prepared by attrition-milling for 10 hours in the presence of a wax. Milled powders were annealed in vacuum at different temperatures (500, 575, 600, 625 and 650°C). Compacts were consolidated starting from unannealed and from 600°Cannealed powders. Studies by SEM microfractography and quantitative metallography, to investigate the influence of Fe-Al intermetallics on compacts fracture, have been carried out. It is concluded that fracture takes place at regions where the area occupied by the intermetallics is high and intermetallics particles are big. Intermetallic particle size can be controlled by an appropriated heat treatment.

Abstract: The present study aims to prepare feedstocks for MIM (Metal Injection Molding) where the metal powders are 316L stainless steel powders (SS). The master objective is to compare the performance of a biodegradable binder with a commercial one based on polyolefins. Different challenges must be overcome in SS injection molding, as follows: to decrease binder/carbon content in feedstocks; to decrease carbon contamination during debinding and sintering; to avoid the formation of chromium carbide and presence of precipitation-free zones; to avoid the grain growth during sintering and to reduce the feedstock price. The optimization of the feedstocks was performed using a torque rheometry technique. Feedstocks of coated and uncoated SS powders mixed with an agar-based binder were used to produce sound parts. A feedstock constituted by SS powders mixed with a high quality commercial binder was the standard. SS with agar-based on feedstocks can admit solids content similar to those based on the commercial binder (62 vol.%). For similar powder content, the sinters resulting from feedstocks with the agar-based binder shows a lower quantity of solid solution of carbon and chromium carbides, absence of precipitation-free zones than commercial feedstocks and good sinter soundness. Coating powders with nanocrystalline stainless steel contribute to control grain growth during debinding and sintering.

Abstract: Magnesium is one of the lightest metals and magnesium alloys have good strength to weight ratio making them very attractive for many particular applications [1]. The main drawback of magnesium alloys is their high corrosion susceptibility. Improving the corrosion protection by deposition of thin hybrid films can expand the areas of applications of relatively cheap magnesium alloys. This work aims at investigation of new anticorrosion coating systems for magnesium alloy AZ31B using hybrid sol-gel films. The sol-gels were prepared by copolymerization of 3- glycidoxypropyltrimethoxysilane (GPTMS), titanium alcoxides and special additives which provide corrosion protection of magnesium alloy. Different compositions of sol-gel systems show enhanced long-term corrosion protection of magnesium alloy. The sol-gel coatings exhibit excellent adhesion to the substrate and protect against the corrosion attack. Corrosion behavior of AZ31B substrates pre-treated with sol–gel derived hybrid coatings was tested by Electrochemical Impedance Spectroscopy (EIS). The morphology and the structure of sol-gel films under study were characterized with SEM/EDS techniques.