18,003 research outputs found
The thermodynamics of and strengthening due to co-clusters: general theory and application to the case of Al-Cu-Mg alloys
Full text linkCo-clusters in ternary or higher order metallic alloys are metastable structures involving two or more distinct alloying atoms that retain the structure of the host lattice. A thermodynamic model based on a single interaction energy of dissimilar nearest neighbour alloying elements is presented, and a model for the strengthening due to these co-cluster dimers is derived. The model includes a new treatment of (short-) order strengthening relevant to these co-clusters and further encompasses modulus hardening and chemical hardening. The models are tested against data on a wide range of Al-Cu-Mg alloys treated at temperatures between 20 and 220ºC. Both quantitative calorimetry data on the enthalpy change due to co-cluster formation and strengthening due to co-clusters is predicted well. It is shown that in general (short-range) order strengthening will be the main strengthening mechanism
The assessment of GPB2/S" structures in Al-Cu-Mg alloys
No full textBased on experimental data presented in the literature, we propose a new structure for GPB2/S?? with the composition of Al10Cu3Mg3, which is aluminium-rich compared to S phase (Al2CuMg). The proposed structure is coherent with the fcc. Al matrix, is formed by the replacement of some Al atoms with Cu/Mg, and has orthorhombic structure (space group Imm2) with lattice parameters a = 0.405 nm, b = 1.62 nm and c = 0.405 nm. Simulated high resolution electron microscopy images and simulated diffraction patterns are compared with experimental data on a range of Al–Cu–Mg alloys. A good correspondence is found
Estimation of dislocation densities in cold rolled Al-Mg-Cu-Mn alloys by combination of yield strength data, EBSD and strength models
No full textAl-Mg-Cu-Mn alloys have been developed for the packaging industry, in which large cold-working deformations are normally applied that can produce high dislocation densities. In this study, we present a simplified model for the yield strength contributions and apply that to obtain the dislocation densities by determining the orientation factors, which can be obtained via the crystal information of electron backscatter diffraction (EBSD). One alloy subjected to three cold-rolling reductions (10%, 40% and 90%) has been analysed by EBSD, and the density of dislocations are estimated using the strengthening model. This assessment suggests that dislocation densities by the Taylor model are roughly consistent but slightly lower than those determined by transmission electron microscopy
A simple approach of determination of the crystallographic orientation: Applications and accuracy
No full textA simple analytical solution for the crystallographic orientation is described. This method is based on one indexed Kikuchi pair in a known zone rather than the corresponding diffraction spots. The accuracy of this method is shown to be better than 0.1° even for cases in which a zone axis deviates by a large angle (e.g. 10°) from the centre of the beam direction. This approach simplifies experiments beacuse only one pair of Kikuchi lines and a zone axis are needed, and is especially suited when it is difficult or cumbersome to resolve a second pair of Kikuchi lines with sufficient accuracy
Two types of S phase precipitates in Al-Cu-Mg alloys
No full textTransmission electron microscopy (TEM) and differential scanning calorimetry (DSC) have been used to study S phase precipitation in an Al-4.2Cu-1.5Mg-0.6Mn-0.5Si (AA2024) and an Al-4.2Cu-1.5Mg-0.6Mn-0.08Si (AA2324) (wt-%) alloy. In DSC experiments on as solution treated samples two distinct exothermic peaks are observed in the range 250 to 350°C, whereas only one peak is observed in solution treated and subsequently stretched or cold worked samples. Samples heated to 270°C and 400°C at a rate of 10°C/min in the DSC have been studied by TEM. The selected area diffraction patterns show that S phase precipitates with the classic orientation relationship form during the lower temperature peak, and for the solution treated samples, the higher temperature peak is caused by the formation of a second type of S phase precipitates which have an orientation relationship that is rotated by ~4 degrees to the classic one. The effects of Si and cold work on the formation of second type of S precipitates have been discussed
A model for the yield strength of Al-Zn-Mg-Cu alloys
No full textA model for the yield strength of multi-component alloys is presented and applied to overaged Al–Zn–Mg–Cu alloys (7xxx series). The model is based on an approximation of the strengthening due to precipitate bypassing during precipitate coarsening and takes account of ternary and higher order systems. It takes account of the influence of supersaturation on precipitation rates and of volume fraction on coarsening rates, as well crystallographic texture and recrystallisation. The model has been successfully used to fit and predict the yield strength data of 21 Al–Zn–Mg–Cu alloys, with compositions spread over the whole range of commercial alloying compositions, and which were aged for a range of times and temperatures to produce yield strengths ranging from 400 to 600 MPa. All but one of the microstructural and reaction rate parameters in the model are determined on the basis of microstructural data, with one parameter fitted to yield strength data. The resulting accuracy in predicting unseen proof strength data is 14 MPa. In support of the model, microstructures and phase transformations of 7xxx alloys were studied by a range of techniques, including differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD) in an SEM with a field emission gun (FEG-SEM)
Comments on “Modelling differential scanning calorimetry curves of precipitation in Al-Cu-Mg”
No full textHersent et al recently published a paper in which they showed that a Kampmann-Wagner (KW) type model with 5 fittable parameter can fit several aspects of a single DSC curve of a 2024 alloy fairly well. We have much more data than just one DSC curve, most of it published, and can show that the model with fitted parameters does not fit crucial elements of this data. To avoid readers drawing erroneous conclusions from the Hersent et al paper we have produced the present short contribution which highlights the limitations of both the Hersent et al approach and the KW model in general. Hersent et al’s work contradicts our work and that of others on the issue of precipitate sizes in the coarsening stage. Hersent et al also make statements on the small (pre-)precipitates in 2024 alloys which contradict our work and that of several other authors that show the presence of Cu-Mg clusters
Precipitation hardening in Al-Cu-Mg alloys revisited
Full text linkTransmission electron microscopy, differential scanning calorimetry and hardness tests have been used to study the precipitation sequence on artificial ageing of a stretched Al–Cu–Mg alloy. The samples were aged for different times at 150 and 190 °C, respectively. Some orthorhombic GPB2/S? is present in samples aged at 150 °C for 48 h, which is at the very start of the second stage of hardening. The combined experiments clearly show that the second stage hardening is dominated by S phase, which forms a dense precipitate structure at the peak hardness stage, whilst no significant amounts of other phases or zones are detected
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 new model for diffusion-controlled precipitation reactions using the extended volume concept
Full text linkIn this work a new model for diffusion-controlled precipitation reactions is derived, analysed and tested against a wide range of data. The model incorporates elements of the extended volume concept and combines this with a new treatment of soft impingement of diffusion fields. The model derivation involves an integration over iso-concentration regions in the parent phase in the extended volume, which leads to a single analytical equation describing the relation the fraction transformed, ?, and the extended volume fraction, ?ext, as: ? = {exp(-2?ext)-1}/(2?ext) + 1. The model is compared to a range of new and old data on diffusion-controlled reactions including precipitation reactions and exsolution reactions, showing a very good performance, outperforming classical and recent models. The model allows new interpretation of existing data which, for the first time, show a consistent analysis, in which Avrami constants, n, equal values that are always consistent with transformation theory
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