Modeling of the effect of temperature, frequency and phase transformations on the viscoelastic properties of AA 7075-T6 and AA 2024-T3 aluminum alloys Jose I. Rojas a,* and Daniel Crespo a,** a Escola d’Enginyeria de Telecomunicació i Aeroespacial de Castelldefels (EETAC) Universitat Politècnica de Catalunya (UPC Barcelona Tech) Esteve Terradas 7 – 08860 – Castelldefels (Barcelona) – Spain * Corresponding author: email: josep.ignasi.rojas@upc.edu; phone: +34 934 134 130, fax: +34 934 137 007 ** email: daniel.crespo@upc.edu; phone: +34 934 134 130, fax: +34 934 137 007 Abstract The viscoelastic response of commercial aluminum alloys 7075-T6 and 2024-T3 as a function of temperature is presented. Experimental data is obtained with a Dynamic-Mechanical Analyzer (DMA) at different loading frequencies and compared to available Transmission Electron Microscopy (TEM) and Differential Scanning Calorimetry (DSC) data. The effect of successive microstructural transformations (particle precipitation and redissolution) is revealed. An analytical model is developed, which fits the mechanical response up to 300ºC. The model takes into account the concentration of Guinier-Preston Zones (GPZ) and metastable precipitates (Ș’ in AA 7075-T6 and ș’/S’ in AA 2024-T3), allowing us determine the kinetic parameters of these transformations. The activation energies were previously obtained by several authors from DSC measurements and other techniques, showing considerable dispersion. The presented data, obtained with a completely different technique, allows reduce the uncertainty on these data and shows the potential of DMA measurements in the study of microstructural transformations. Keywords viscoelasticity; aluminum alloy; AA 2024; AA 7075; microstructure; phase transformations; Guinier-Preston Zones; storage modulus; dynamic-mechanical analysis 1. Introduction Much research has been devoted to the characterization of most of the mechanical properties of materials. However, the viscoelastic behavior of metals, consequence of internal friction, has received much less attention. The comprehension of the underlying physics of this phenomenon is of high interest as structural materials are submitted to dynamic loads in most applications. Indeed, fatigue is the consequence of microstructural changes induced in a material under repeated loading, and the viscoelastic behavior is intimately linked to the microstructure [1]. This fact has been shown in metallic glasses, where structural relaxations, the glass transition and the crystallization processes have been analyzed by dynamic-mechanical analysis [2]. Accordingly, the characterization of the viscoelastic response of a material offers an alternative method for analyzing