Showing papers on "Aluminium published in 2001"

Microstructure and interface characteristics of B4C, SiC and Al2O3 reinforced Al matrix composites: a comparative study

Abstract: Three aluminium metal matrix composites containing reinforcing particles of B4C, SiC and Al2O3 (0-20 vol. %) were processed. The stir-casting manufacturing route followed by hot extrusion was utilized, being one of the cost-effective industrial methods. In this study, the feasibility of processing B4C reinforced Al composite was investigated and a comparison was made with the other two composites. The microstructural distribution of reinforcing particles in all three composites was studied by means of optical and scanning electron microscopy (SEM). The distribution and chemical composition of the phases formed at matrix/particulate interface of the processed composites were also investigated by SEM and energy dispersive x-ray spectroscopy (EDX). A clear interfacial reaction product/layer was found at Al/SiC interface for composites held for a relatively long processing time (> 30 minutes). No reaction product was observed at Al/B4C and Al/Al2O3 interfaces at the resolution limit of the SEM used. On the other hand, two secondary phases (alumina and another phase containing aluminium, boron and carbon) were found in the aluminum matrix away from the interface in Al-B4C composites. From the fracture surface analysis, B4C reinforced Al composite seemed to exhibit a better interfacial bonding compared to the other two composites.

The Developments of Cold-rolled TRIP-assisted Multiphase Steels. Al-alloyed TRIP-assisted Multiphase Steels

Abstract: The influence of heat-treating conditions on the retention of carbon-enriched austenite of TRIP-assisted multiphase steel grades containing different amounts of silicon and/or aluminium is investigated. The ensuing mechanical properties resulting from the TRIP effect are also scrutinised. The bainite transformation kinetics was followed by dilatometry whereas a detailed characterisation of the microstructures led to the construction of transformation maps giving the volume fractions of the different phases and the carbon content of austenite. The role of silicon and aluminium additions (i) on the retention of austenite by partial bainite transformation and (ii) on the mechanical properties is enlightened. A strong influence of the solid-solution strengthening effect of silicon is highlighted. Aluminium seems to be an effective alloying element for the retention of austenite in TRIP-aided steels even if lower strength levels can be attained. A mixed Al-Si TRIP-aided steel seems to be a very good compromise between the processing needs, the required mechanical properties and the industrial constraints.

Electrodes and charge injection in low-density polyethylene using the pulsed electroacoustic technique

Abstract: The effects of electrode materials on space charge formation in low-density polyethylene (LDPE) have been investigated experimentally using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either the laboratory or power cable industry were selected, i.e. aluminum, gold and carbon loaded crosslinked polyethylene (XLPE), and space charge accumulation after the application of high electric stress was monitored. Experimental results demonstrated that charge injection processes take place in all cases once the applied stress has exceeded a threshold. However the amount of charge, and the polarity of the dominant injected charges showed a significant dependence on the electrode materials under the same applied electric stress.

Surface processes and diffusion mechanisms of ion nitriding of stainless steel and aluminium

Abstract: Model experiments with low-energy ions under controlled vacuum conditions have been performed in order to identify the surface processes and diffusion mechanism associated with nitrogen transport during ion nitriding of stainless steel 316 and pure aluminium. A necessary condition for efficient nitriding is the transmission of the implanted ions through a surface oxide layer, which results from a balance of ion sputtering and re-oxidation from the residual gas. For ion energies of approximately 1 keV and ion current densities of approximately 0.2 mA/cm 2 , oxygen partial pressures of less than approximately 3×10 −6 and 3×10 −7 mbar are required for stainless steel and aluminium, respectively. Diffusion under the influence of traps controls the transport of nitrogen in stainless steel, with dynamic trapping at the Cr atoms of the alloy. In contrast, stoichiometric AlN grows on aluminium, due to Al diffusion from the underlying bulk. From the surface and diffusion mechanisms, limitations of plasma nitriding and plasma immersion ion nitriding are derived, and the implications for industrial applications are discussed.

Mesoporous materials prepared using coal fly ash as the silicon and aluminium source

Abstract: Coal fly ash was converted into two types of porous materials, MCM-41 and SBA-15 (both of hexagonal structure), using the supernatant of the fly ash. It was found that most of the Si and Al components in the fly ash could be effectively transformed into mesoporous materials, depending on the hydrothermal conditions, and that fusion is essential. Investigation by 29Si and 27Al MAS NMR demonstrated that fusion plays an important role in enhancing the hydrothermal conditions for synthesis of these materials. A high concentration of Na ions in the supernatant of the fused fly ash was not found to be critical in the formation of Al-MCM-41 when prepared under controlled pH conditions. Pyridine adsorption experiments on Al-MCM-41 prepared from coal fly ash revealed the presence of Bronsted and Lewis acid sites. It was also found that the catalytic activity in the cumene cracking reaction is linked only to the accessible aluminium, and not to the total incorporated aluminium present in the Al-MCM-41.

Effect of compositional variations on characteristics of coarse intermetallic particles in overaged 7000 aluminium alloys

Abstract: To provide an understanding of how compositional variations affect the characteristics of coarse intermetallic particles in 7000 aluminium alloys, three Al-Zn-Mg-Cu-Zr alloy plates with different zinc, magnesium, and copper contents have been studied by optical microscopy based image analysis, scanning electron microscopy SEM, energy dispersive X-ray spectroscopy EDS, and differential scanning calorimetry DSC. These coarse intermetallic particles are detrimental, especially to the toughness of the alloy. Experimental observations have been interpreted successfully on the basis of the phase diagram and the temperature dependent S phase solvus, derived in turn on the basis of the regular solution model. The temperature dependent S phase solvus indicates that some compositions in the composition windows of 7050 and 7x75 type alloys will give rise to the detrimental S phase, which cannot be dissolved during solution treatment. Also, the T phase has been analysed and conditions for its formation and di...

Production of aluminium matrix composite components using conventional PM technology

Abstract: The machining difficulties and processing costs related to particle reinforced aluminium matrix composites have limited the application range of these advanced materials. The present investigation deals with the production of net shape SiC particle reinforced AA6061 aluminium alloy components by a conventional powder metallurgical processing technique, involving the uniaxial cold pressing and sintering of composite powders. Three aluminium alloy powder sizes were combined with three SiC particle sizes in various volume fractions of up to 20% SiC, and processed to produce net shape tensile test samples. The tensile test results and the morphological analysis of the fracture surfaces show that this process may be used to produce well sintered, small-to-medium sized net shape aluminium composite components exhibiting ultimate tensile strength (UTS) of up to 324 MPa. The successful production of these aluminium powder based components is affected strongly by the aluminium powder size, SiC volume fraction and the SiC particle size. It is also linked to the penetration of the aluminium powder oxide layer by the hard SiC particles, which occurs during cold compaction. Based on the present investigations, a number of processing improvements are suggested, which should further advance the usefulness of this technically uncomplicated net shape processing method for the production of aluminium matrix composites.

Effects of Si on the aging behaviour and formability of aluminium alloys based on AA6016

Abstract: The heat treatable 6xxx series (Al–Mg–Si–(Cu)) aluminium alloys are finding increasing use in automotive skin panel applications where relatively high formability and in-service strength for dent resistance are major requirements In Europe, the alloy of choice for such applications is currently the low Cu-containing alloy AA6016, which typically contains approximately 04 wt% Mg and 10 wt% Si, and which derives its strength from the precipitation hardening phase, Mg2Si The volume fraction of Mg2Si is, in turn, affected primarily through the level of Mg within the alloy, although the Si content is also important The level of Si within the alloy influences the solution heat treated (T4) strength and the subsequent aging response of the 6xxx series alloys, again predominantly through its effect on the volume fraction of Mg2Si In this paper, the effects of Si content on the aging behaviour and mechanical properties (formability) of alloys based on the AA6016 composition are described

Bi-continuous metal matrix composites

Abstract: Bi-continuous alumina:aluminium composites were made by infiltrating an alumina preform which had the structure of a reticulated ceramic foam. The low density preforms were prepared from a polyurethane suspension of alumina powder which was pyrolysed and sintered after foaming. Higher density preforms consisted of ceramic foams with open cells. All these preforms were infiltrated with 6061 aluminium alloy using a modified squeeze caster fitted with a vacuum system and fine control of speed and pressure. The microstructure of the preform fitted an established relationship between the ratio of window diameter to cell diameter (k) and void volume fraction (Vp). Low k foams were infiltrated fully but on cooling below the solidus, interfacial debonding took place due to differential thermal contraction. This was overcome by modifying the processing conditions. High k foams which had high fractional porosity, retained sound interfacial bonding. The composites possess higher elastic modulus than conventional MMCs with a homogeneous reinforcement distribution at a given volume fraction. The loss of electrical conductivity is negligible in the lower volume fraction range because of the three dimensionally continuous aluminium phase. The experimental results are compared with a number of theoretical predictions. © 2001 Elsevier Science B.V. All rights reserved.

Structural properties of molten silicates from ab initio molecular-dynamics simulations: Comparison between C a O − A l 2 O 3 − S i O 2 and SiO 2

Abstract: We present the results of first-principles molecular-dynamics simulations of molten silicates, based on the density functional formalism. In particular, the structural properties of a calcium aluminosilicate $[\mathrm{CaO}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\ensuremath{-}{\mathrm{SiO}}_{2}]$ melt are compared to those of a silica melt. The local structures of the two melts are in good agreement with the experimental understanding of these systems. In the calcium aluminosilicate melt, the number of nonbridging oxygens found is in excess of the number obtained from a simple stoichiometric prediction. In addition, the aluminum avoidance principle, which states that links between ${\mathrm{AlO}}_{4}$ tetrahedra are absent or rare, is found to be violated. Defects such as two-fold rings and five-fold coordinated silicon atoms are found in comparable proportions in both liquids. However, in the calcium aluminosilicate melt, a larger proportion of oxygen atoms are three-fold coordinated. In addition, fivefold coordinated aluminum atoms are observed. Finally evidence of creation and anihilation of nonbridging oxygens is observed, with these oxygens being mostly connected to Si tetrahedra.