Characterization of the protective surface films formed on molten magnesium in air/HFC-134a atmospheres

The surface films formed on molten magnesium in an air/HFC-134a gas mixture at 700 °C were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The results showed that there was a protective film on molten magnesium surface, which can prevent molten magnesium from oxidation and ignition. The surface film contained primarily four elements: magnesium, fluorine, oxygen and carbon, and was composed of MgF₂, MgO and C. The film properties depended on the HFC-134a concentration in the gaseous mixture and exposure time. The thickness of the film formed after exposure to air containing 0.5% HFC-134a for 10 min was about 1–2 μm.     

Wetting of aluminium oxide by molten aluminium and other metals

The sessile drop technique has been used to measure the contact angle of molten aluminium, aluminium-nickel and aluminium-copper alloys, copper and gold, with sapphire, ruby and recrystallised alumina. Measurements were madein vacuo, and as a function of time and temperature over the range 800 to 1500° C. Cinematography and time-lapse photography were used. At temperatures below 950° C, sessile drops of aluminium reached equilibrium only after a period of time which increased with decrease in temperature and could be in excess of one hour. A rapid increase in contact area occurred around 900° C. Above 1150° C drops of aluminium and of the aluminium alloys were observed to spread and contract repeatedly. Contractions were observed with both polycrystalline and single-crystal alumina, although they were much more pronounced with the latter, and were associated with the formation of a series of reaction rings on the plaque. Ruby and sapphire behaved similarly. The shape of the rings depended on the crystallographic orientation of the plaque: the reaction profile tended to terminate in certain low index directions. Neither contractions nor reaction was observed with copper or gold. The observations are discussed in terms of the combined effects of evaporation, chemical reactivity and interfacial tensions in the system.     

Interfacial effects between molten aluminium and sapphire

The sessile-drop technique has been used to examine the interfacial effects that occur between molten aluminium and single-crystal -Al₂O₃ when heated in vacuo. In the region of 1300° C the growth of crystallites, probably of -Al₂O₃, at the liquid-solid and liquid-vapour interfaces has been observed, together with the formation of etch pits on the sapphire plaque as the result of vapour attack. These effects are described and discussed.     

Reactivity of molten aluminium with polycrystalline ZnO substrate

The microstructure studies of the reaction product region (RPR) obtained due to the interaction between the liquid aluminium and polycrystalline zinc oxide substrate at 1273 K has been studied. The RPR extended over the oxide substrate and showed a typical C4 (co-continuous-ceramic-composites) structure composed of two interpenetrating phases. The scanning electron microscopy studies revealed that the large crystals of alumina were surrounded by an Al(Zn) metallic phase. Moreover, the transmission electron microscopy investigation showed the presence of a thin (~250 nm) layer next to the ZnO. The chemical analysis accompanied by the selected area electron diffraction patterns indicated in both cases the same stoichiometric aluminium oxide but of different crystallographic structure, i.e., large crystals had α-Al₂O₃ structure while the layer was identified as metastable δ-Al₂O₃. The results were compared to those reported for interaction between liquid aluminium and monocrystalline ZnO.     

Oxidation behavior of molten magnesium in air/HFC-134a atmospheres

The oxidation behavior of molten magnesium in the atmosphere of air containing HFC-134a has been investigated in the temperature range of 660–760 °C. The oxidation rates and kinetics have been measured by the weight gain method and oxidation products have been characterized by XRD, SEM, EDS, XPS and AES. The results show that the oxidation kinetics is complex which cannot be described by simple equations. The rate of oxidation of molten magnesium in air/HFC-134a covering gas mixtures varies with the concentration of HFC-134a and molten temperature. Increasing the concentration of HFC-134a and decreasing the temperature can slow down the oxidation rate of molten magnesium. The film formed +on the surface of molten magnesium is mainly composed of MgF₂, MgO and C. MgF₂ is predominant product at the top layer and decreased gradually with the depth while MgO and C remain almost constant with relatively low content. The mechanisms of the oxidation of molten magnesium in air containing HFC-134a have also been discussed based on the experimental results.     

Degradation of oxidized SiC-Si3N4 in molten aluminium

Silicon nitride-bonded silicon carbide ceramic tubes are used to provide indirect immersion heating in non-ferrous foundries. Samples of such tubes have been oxidized for varying lengths of time and then immersed in liquid aluminium. Long-term prior oxidation renders the ceramic susceptible to wetting by aluminium with subsequent degradation of mechanical properties. Samples not oxidized before immersion were stable in the liquid metal.     

Wetting and spreading of molten aluminium against AlN surfaces

Wetting and spreading of molten aluminium against AlN substrates were investigated between 1100 and 1290°C. The contact angles decreased linearly with time under isothermal conditions between 1100 and 1200°C. The isothermal rate of spreading of molten aluminium against AlN substrates was constant between 1220 and 1290°C and the rate increased exponentially with increasing temperature. Crystals of Al4C3 nucleated and grew on the substrate surface beneath the liquid. However, the formation of Al4C3 may not be solely responsible for the changes in contact angle and spreading. It is postulated that carbon contamination from the substrate and/or experimental equipment coupled with the low oxygen partial pressure of the chamber in the presence of graphite, were primarily responsible for the observed contact angle and spreading phenomena. The activation energy for the spreading process was 448 kJ mol-1, suggesting the presence of some chemical reaction at the interface. Carbon-rich aluminium may be initiating a continuous surface reaction with the AlN substrates by reducing the native oxide layer on the substrate surface.     

Synthesis of boride and nitride ceramics in molten aluminium by reactive infiltration

The infiltration of solid powder mixtures with molten aluminium has been investigated as a potential route for the synthesis of ceramic/metal composites. Either titanium or tantalum powder was mixed with boron nitride flakes for the reaction powder mixture. The infiltration occurred spontaneously at 1473K for both [Ti+BN] and [Ta+BN] powder mixtures. Owing to reactions between the starting materials, both boride and nitride ceramics were produced in molten aluminium. TiB2 and AlN were produced from the [Ti+BN] powder mixture, and TaB2 and AlN were produced from the [Ta+BN] powder mixture. When the [Ti+BN] powder mixture was used, a reaction producing Al3Ti took place immediately after the infiltration of the molten aluminium, and a subsequent reaction producing TiB2 and AlN proceeded gradually. The time required to convert BN flakes to TiB2 and AlN particles at 1473K was in the range of 1800–3600 s. On the other hand, when the [Ta+BN] powder mixture was used, there was an initial incubation period to allow the tantalum and molten aluminium to react with each other. The reaction between tantalum, BN and aluminium took place after this incubation period. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal formation of corundum from aluminium hydroxides prepared from various aluminium salts

Aluminium hydroxides have been precipitated from various aluminium salts and the differences in their thermal behaviour have been investigated. Pseudoboehmite derived from the nitrate, sulfate and chloride all form γ-Al₂O₃ at ∼ 400°C but the formation of α-A1₂O₃ at 1200°C occurs more readily in the material derived from the sulfate. This contains a higher concentration of anionic impurities related to differences in the solubility of the original aluminium salts. The sulfate is retained in the gel to higher temperatures at which its eventual decomposition may lead to the formation of a reactive pore structure which facilitates the nucleation of α-A1₂O₃.     

Effect of annealing in various atmospheres on the photoluminescence of porous silicon

The effect of isochronous annealing in various atmospheres on the photoluminescence of porous p-type silicon samples obtained by anodization under identical conditions was studied. Comparative experimental data for the samples annealed in air, nitrogen, and vacuum are presented.     

High temperature compression creep of aluminium nitride after immersion in molten stainless steel

The effect of immersion in molten steel on subsequent creep behaviour has been investigated in two AlN ceramics. Steady-state creep was not significantly affected but an enhanced initial creep rate was observed, especially when the surface layer had not been removed after immersion. In immersed samples, SEM revealed some open porosity and microcracks near the surface and particles containing steel constituents were found by TEM deep inside samples, in grain-boundary secondary phases. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of pressure on bonding of alumina with molten aluminium as insert

Abstract

Alumina cylinders of 91% purity were bonded at 1073 K with aluminium foil of different thicknesses (5 and 50 μm) under different pressures (0–2 MPa) to examine the effect of pressure on the reaction of molten aluminium with silica contained in alumina as a binding agent. The reaction process at the bonding interface was also examined. The thicknesses of the interlayer and the reaction zone decreased with decreasing foil thickness. The interlayer was composed of silicon crystals and hypereutectic Al–Si alloy melt. Cavities were also observed. The constitution of the interlayer changed with time following the migration of silicon from the alumina: the amount of Al–Si alloy decreased with time, and the amounts of crystallised silicon and the total cavity volume increased with time. The bonding pressure reduced the thicknesses of the interlayer and the reaction zone. The amount of silicon contained in the interlayer was also reduced by pressure.

MST/3035     

Ultrasonic processing of molten and solidifying aluminium alloys: overview and outlook

Ultrasonic melt processing attracts since the 1930 a lot of interest both from academic researchers and industry. In the last 10 years the interest to ultrasonic melt processing grew with regard to understanding the underlying mechanisms of previously established effects, developing numerical models of ultrasonic cavitation and the development of nanocomposite technology. This review paper summarises the mechanisms involved in the ultrasonic melt processing, including cavitation, flows, nucleation, activation, fragmentation and their consequences for degassing, structure refinement and particle dispersion. Some typical mistakes made by researchers in performing experiments and in interpretation of the results are discussed. New advanced methods of studying ultrasonic treatment and phenomena are considered. The paper also gives an outlook to future developments and challenges.

This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.