17,426 research outputs found
The effect of compositional variations on characteristics of coarse intermetallic particles in overaged 7000 aluminium alloys
No full textTo provide an understanding of how compositional variations affect the characteristics of coarse intermetallic particles in 7xxx alloys, three Al-Zn-Mg-Cu-Zr aluminium alloy plates with different Zn, Mg and Cu contents were studied by optical microscopy based image analysis, scanning electron microscopy (SEM), energy disperse X-ray spectroscopy (EDS) and differential scanning calorimetry (DSC). These coarse intermetallic particles are detrimental, especially to the toughness of the alloy. The experimental observations were interpreted successfully on the basis of the phase diagram and the temperature dependent S phase solvus which was derived 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 that can not be dissolved during solution treatment. Also the T phase is analysed and conditions for its formation and dissolution are discussed. It is shown that the present results can produce useful information on alloy design and thermo-mechanical processing via microstructural control
RETRACTED: Experimental and Theoretical Investigation of Aerosol Optical Properties
No full textThis article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy).This article has been retracted at the request of the Authors.It was prepared by Yan Li and Rui Xue, submitted by Yan Li, and published without the knowledge of the listed co-authors M.J. Ezell and B.J. Finlayson-Pitts. The experimental results are not interpretable as put forth in the published paper.The author Yan Li would like to apologize for the inconvenience caused to the co-authors and the readers
Identification and analysis of intermetallic phases in overaged Zr-containing and Cr-containing Al-Zn-Mg-Cu alloys
No full textThis paper investigates the effect of alloying elements on the characteristics of intermetallic phases in Zr-containing and Cr-containing 7xxx Al-Zn-Mg-Cu alloys at overaged conditions. Four Al-Zn-Mg-Cu alloy plates with different alloying element contents were studied by optical microscopy based image analysis, differential scanning calorimetry, scanning electron microscopy combined with energy disperse X-ray spectroscopy and transmission electron microscopy. The grain structures, recrystallisation, intermetallic phases and precipitates in the selected alloys have been analysed and the presence of coarse intermetallic phases has been interpreted using established phase diagrams. The different effects of Zr or Cr addition to the alloys have been compared. The experimental results showed that the recrystallised area fraction of Zr-containing alloys is less than that of Cr-containing alloys, being attributable to Zr reducing recrystallisation more effectively than Cr. The detected particles are mainly S phase, Al7Cu2Fe, as well as dispersoids of Al3Zr for Zr-containing alloys and Cr-rich E phase for Cr-containing alloys. These coarse particles, especially the S phase which can not be dissolved during solution treatment, are detrimental to the fracture toughness of the alloys
Vacuum Insulation Panels Applied in Building Constructions
No full textDue to sustainability and due to international treaties, it is desired and required to reduce greenhouse gas emissions drastically. One contributor to these emissions is the burning of fossil fuels for generating power and electricity to be used in and for buildings. Buildings and building-related processes are responsible for about 40% of the primary energy consumption in the European Union. More than half of this energy is applied for heating systems in dwellings and commercial buildings. The European Union therefore has laid down new energy performance requirements for buildings in the European Directive on the Energy Performance of Buildings. Moreover, a reduction of energy losses of buildings during their occupational phase is important for facilitating the implementation of sustainable energy sources in the built environment. Increasing the insulation value of the envelope of buildings may contribute to this reduction of primary energy use. Two strategies can be followed. The first strategy is to increase the thickness of the thermal insulation layer. Until recently, this strategy has primarily been adopted. If, however, German or Swiss Passivhaus standard is applied, the thickness of this insulation layer would increase to beyond 30 cm, resulting in very thick building enclosures. The second, more innovative, strategy for reducing energy losses through the building skin would be the application of more effective thermal insulators. One such more effective thermal insulator is a vacuum insulation panel, abbreviated as VIP. A VIP consists of an open-celled core material which is evacuated and then tightly sealed into a barrier envelope to maintain this vacuum. The vacuum inside the pores of the core material reduces the thermal conductivity of the product significantly, as a result of which the thickness of the insulation layer can be reduced to obtain a certain performance. This reduction of thickness is among the most promising features for large-scale application of VIPs in the building industry. However, integration of VIPs into buildings must be performed very meticulously for several reasons; first, due to its nature a VIP cannot be processed on site and needs careful planning in advance; second, it is very sensitive to mechanical damage thus requiring careful handling; third, thermal bridges along the panel’s edges reduce its performance; fourth, the composite system is highly subjected to aging. This dissertation therefore looks into many of these aspects, presents several calculation tools and shows how VIPs can be applied in façade panels, EPS insulation boards and as under-floor insulation. With the wide-spread proliferation of VIPs in buildings a more sustainable and healthy environment can then be achieved.Building TechnologyArchitectur
A model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu alloys
Get PDFA physically based model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu (7xxx series) alloys is presented. The model includes calculations of the ?- and the S-phase solvus (using a regular-solution model), taking account of the capillary effect and ? coarsening. It takes account of the conductivity of grains (incorporating dissolved alloying elements, undissolved particles, and precipitates) and solute-depleted areas at the grain boundaries. Data from optical microscopy, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDS), and transmission electron microscopy (TEM) are consistent with the model and its predictions. The model has been successfully used to fit and predict the conductivity data of a set of 7xxx alloys including both Zr-containing alloys and Cr-containing alloys under various aging conditions, achieving an accuracy of about 1 pct in predicting unseen conductivity data from this set of alloys
Assessment of mixed mode loading on macroscopic fatigue crack paths in thick section Al-Cu-Li alloy plate
Get PDFHigh strength, wrought 7xxx (Al-Zn-Mg) and Al-Li based alloys show a propensity for fatigue macroscopic crack deflections aligned along grain boundaries. The present work reports a study on a 3rd generation Al-Li based alloy in the form of a thick AA2297 (Al-Cu-Li alloy) plate, where it was found that although the lithium containing material may indeed be more susceptible to mixed mode grain boundary failure (and by implication crack deflection in conventional tests), the AA2297 alloy fatigue behaviour is mechanistically and functionally equivalent to 7xxx alloy behaviour. It is shown that crack paths are controlled by a combination of crack loading mixity (KII/KI ratio) and maximum strain energy release rates (expressed as Keq.max). Increasing KII/KI ratio is seen to favour sustained grain boundary failure. Crack growth rate behaviour is discussed in terms of extrinsic and intrinsic components of crack growth resistance. It is shown that the present approach can be successfully applied to predict crack deflection / crack paths for a range of sample geometries and orientations on a range of high strength orthotropic aluminium alloys, including 3rd generation Al-Li based alloys and well-established 7xxx alloys
DSC study on phase transitions and their correlation with properties of overaged Al-Zn-Mg-Cu alloys
No full textThis article investigates the phase transitions of complex quaternary Al-Zn-Mg-Cu alloys with Zr addition at overaged conditions. Differential scanning calorimetry (DSC) is employed to quantitatively analyze the phase transformation phenomena of a wide range of 7xxx series alloys through endothermic and exothermic reactions. The DSC observations detailing heat effect peaks and thermal parameters of ?? dissolution contain valuable information on the presence of equilibrium phases and the optimum alloying element contents. Based on DSC experimental data and phase diagrams, the balance of critical properties such as strength and electrical conductivity of Al-Zn-Cu-Mg 7xxx series alloys has been studied by considering the formation, dissolution, and incipient melting of S and T phase, dissolution of ?? phase as well as the formation of ? phase. Nine Al-Zn-Mg-Cu alloys have been studied through microstructural examination and detailed DSC analysis. The correlation between the properties and the DSC data of the selected alloys has been analyze
Embrittlement of Al-Li-Cu-Mg alloys at slightly elevated temperatures: microstructural mechanisms of hardening
Get PDFWhen underaged, damage tolerant Al-Li based alloys are exposed to slightly elevated temperatures (60 to 100ºC), their strength increases and toughness decreases. To study the first stages of this process, 8090 sheet in three underaged heat treatment conditions, including T3, T81 and a multi-stage temper, was exposed for 1000 h at 70°C. Using a model for strengthening in combination with a quantitative analysis of differential scanning calorimetry data, it is shown that strengthening can be explained by an increase in the volume fraction of the delta' phase and, to a lesser extent, to the formation of Guinier-Preston (GPB) zones. For the T3 temper, coarsening of delta' precipitates is the main strengthening mechanism during exposure at 70ºC. The solvus of delta' and GPB, which to a large extent defines this embrittlement, is discussed. Toughness reduction during exposure at slightly elevated temperatures can be minimised via a multi stage temper or via a reduction of the Li content of the alloy
Li and Li isotopic composition of hydrothermally altered sediments at Middle Valley, Juan De Fuca
No full textLi and Li isotopes have been measured in the clay fraction of sediments recovered from the Middle Valley hydrothermal site on the Juan De Fuca Ridge. The Li content of pure detrital clays is 51 ppm while hydrothermal clays and carbonates have lower Li (22±11 ppm). However, there is no clear relationship between the mineralogy of the hydrothermal alteration products and their Li content. The δ7Li value of the detrital clays is +5.8‰. Hydrothermal clays and carbonates have δ7Li in the range of -3.9‰ to +7.8‰; these values do not seem to be dependent on the temperature at which they formed. Modelling of the Li and Li isotope systematics indicates that the fluid from which the alteration products form is significantly enriched in Li (higher than 10,000 µmol kg-1) relative to pore fluids recovered from within the sediments (up to 589 µmol kg-1; [Wheat, C.G., M.J. Mottl, 1994. Data report: trace metal composition of pore water from Sites 855 through 858, Middle valley, Juan De Fuca Ridge. In Mottl, M.J., Davis, E.E., Fisher, A.T., Slack, J.F. (Eds.), Proc. ODP, Sci. Res. 139: 749–755]), and that this Li is derived from sediment. Thus, the alteration products are not in equilibrium with their conjugate pore fluids; rather, the alteration minerals formed at lower water/sediment ratios. This suggests that fluid flow pathways at Middle Valley were more diffuse in the past than they are today
Microstructure and strengthening of Al-Li-Cu-Mg alloys and MMCs: II. Modelling of yield strength
No full textA detailed quantitative model for the strengthening of monolithic alloys and composites due to precipitation strengthening, solution strengthening, grain and subgrain strengthening, strengthening by dislocations and load transfer to ceramic inclusions is presented. The model includes a newly derived description of the effect of a precipitate free zone (PFZ) around the reinforcing phase incorporating strain hardening of the PFZ. The model is successfully applied to model the experimental data for the proof strengths of four Al Li Cu Mg type alloys and composites aged to obtain a wide range of microstructures and all strengthening contributions are quantified. It is shown that PFZ formation in the 8090 MMC causes a drastic reduction in the proof strength (about 100 MPa), but it has little influence on the time required for peak ageing. In all alloys strengthening due to GPB zones is more important than strengthening due to delta' (Al3Li) phase
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