Early subnanometre cluster formation during quenching of a high-strength AA7449 aluminium alloy was investigated using in situ small angle X-ray scattering. Fast quench cooling was obtained by using a laser-based heating system. The size and number density of homogeneous nucleated clusters were found to be strongly dependent on the cooling rate, while the volume fraction of cluster formation is independent of the cooling rate. Heterogeneous larger precipitation starts at higher temperatures in volume fractions that depend on the cooling rate. V C 2014 AIP Publishing LLC. The fabrication of aluminium alloys involves a number of thermomechanical steps such as solute-heat treatment followed by fast cooling, i.e., quenching. It is well known that the cooling rate influences the homogeneous and heterogeneous formation of precipitates and that the effect of these differences on the mechanical properties cannot be removed by additional aging. The typical precipitation sequence of the equilibrium g phase (Mg(Zn,Cu,Al) 2 ) in the Al-Zn-Mg(-Cu) system is: GPZ ! g 0 ! g. 1 Early studies by Lendvai and L€ offler on Al-Zn-Mg alloys report that vacancy-rich clusters (VRC), acting as nucleation sites for Guinier Preston zones (GPZ), can form during the cooling from the solutionizing temperature. 2,3 The number of VRCs strongly depends on the quench parameters influencing the excess vacancies in the structure. Clusters formed by only a few atoms 4 have been identified using 3D atom probe. Despite their small size, the clusters are effective in increasing the yield strength of the material. 5 In addition to the VRCs, heterogeneous precipitation of the equilibrium g phase occurs at higher temperatures. 1 It is therefore not surprising that when producing thick Al alloy plates, the resultant precipitation size, density, and volume fraction are expected to differ across the plate because of the difference in cooling rates. The latter creates residual stresses at as quenched temper that are reduced by stress relief. The remaining residual stresses at final temper may lead to machining distortions. To investigate the influence of precipitation on residual stresses, thermomechanical models linking solid-state transformations to the final stress distribution have to be developed. Such simulation schemes need input on size, density, and volume fraction of precipitation as function of cooling rates that can only be provided by experimentation.Small angle X-ray scattering (SAXS) has proven to be a useful tool for the investigation of precipitation phenomena. 6 SAXS is a well established technique for investigating clusters with high contrast in atomic number with respect to the matrix. SAXS has been used extensively to study precipitation phenomena in Al alloys ex situ and also in situ during aging. [6][7][8] Providing information on the size and volume fraction of the precipitates, SAXS validated thermodynamic-based precipitation model predictions 7,9 of the effect of aging on precipitation. There is however no information to be gathered...
a b s t r a c tIn this paper, a simple but realistic approach is presented to predict the as-quenched residual stress distribution in thick 7xxx aluminium alloy plates. Instead of modelling precipitation that occurs during quenching, a thermo-mechanical model is used whose parameters are identified using a limited number of tensile tests achieved after representative interrupted cooling paths in a Gleeble machine. The material behaviour law accounts for recovery at high temperature in a simple way and neglects the Bauschinger effect as suggested by a dedicated experiment. The results of this simple approach are compared to residual stress measurements in plates of different thicknesses for two different 7xxx alloys, AA7449 and AA7040.
a b s t r a c tThe formation of Cu-Mg clusters in an Al-Cu-Mg aluminum alloy is observed by small-angle X-ray scattering during cooling. Cooling rates are choosen to mimic the different conditions obtained at the surface and in the center of large forgings. Clusters of 0.45 nm start to form at 250°C. Their volume fraction depends strongly on the cooling rate and the amount of excess vacancies. The difference in cluster kinetics explains the difference in rapid hardening across large forgings.Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Age-hardenable Al-Cu-Mg alloys are widely used for aerospace and engineering applications owing to their high specific strength and good corrosion resistance. Their age hardening response is characterized by two distinct stages. The first stage is defined by a fast initial increase in strength, which is known as rapid hardening. It was shown by 3D atom probe tomography (APT) and small-angle X-ray scattering (SAXS) experiments that the rapid hardening is associated with the formation of Cu-Mg clusters [1][2][3][4]. The second rise to peak hardness is generally ascribed to the formation of the equilibrium S phase (Al 2 CuMg) [4,5].For industrial applications, the Al-Cu-Mg alloys are commonly processed as large components such as forgings or plates. The mechanical properties are tailored by the age-hardening treatment, which also involves a quenching step. Thermal gradients decrease from the surface to the center and give rise to residual stresses (RS) [6]. The magnitude of the as-quenched RS can be very high and even exceed the as-quenched strength of the materials [6,7]. In the case of Al-Zn-Mg-Cu alloys, the high as-quenched RS are caused by the formation of fine hardening precipitates that form during quenching of large components and thereby increase the yield strength of the material [8,9].In order to predict the RS formation in industrial components during quench, the changes of the nanostructure during cooling conditions close to industrial practice needs to be characterized as they highly impact the yield strength and thus the internal stress generation. Further, the influence of excess vacancies on precipitation during cooling needs to be explored. This can be done by adapting the cooling rates at high temperature, which influence the excess vacancy concentration due to annihilation on defects.SAXS is a useful tool to monitor precipitation phenomena during fast time and temperature changes given that a sufficiently high electron density contrast between the precipitate and matrix exists [4,8,10]. SAXS provides the means to collect information on the size and volume fraction of the precipitates that can then be directly compared to predictions of thermodynamic-based precipitation models [11,12]. These models can be coupled with macroscopic finite-element RS simulations to better predict the residual stress formation during the processing of industrial components [9,13] as part of through-process modeling.This work investigates the Cu-Mg cluster ...
The GP(I) zone formation during quench is simulated in an industrial Aluminum alloy AA7449 75 mm thick plate by using a multi-class precipitation model. For this purpose, results of in situ SAXS experiments are reported. A methodology is presented that takes advantage of the collected data to derive i -a thermodynamic description for GP(I) zones from reversion heat treatments by using a solubility product and ii -the influence of excess vacancies on diffusion coefficients. This approach allows reproducing reasonably well the GP(I) zone formation measured during rapid cooling. Further, the simulated as-quenched surface yield strength compares well with experimental results reported in the literature.
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