Modeling of the Effect of Temperature, Frequency, and Phase Transformations on the Viscoelastic Properties of AA 7075-T6
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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 dynamicmechanical analysis.[2] Accordingly, the characterization of the viscoelastic response of a material offers an alternative method for analyzing its microstructure and fatigue behavior. In addition, it enables a deeper understanding of other technologically essential properties, like mechanical damping and yielding.[1] The research reported in this article is aimed at the identification, characterization and modeling, whenever possible, of the effects of temperature, frequency of dynamic loading, and microstructure and phase transformations on the viscoelastic behavior of aluminum alloys (AA) 7075 and AA 2024. These alloys are key JOSE I. ROJAS, Teaching Assistant, and DANIEL CRESPO, Professor, are with the Escola d’Enginyeria de Telecomunicacio´ i Aeroespacial de Castelldefels (EETAC), Universitat Polite`cnica de Catalunya (UPC Barcelona Tech), 08860 Castelldefels (Barcelona), Spain. Contact e-mail: [email protected] Manuscript submitted May 24, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
representatives of their respective families (i.e., AlZnMg and AlCuMg alloys) that, after proper age-hardening processes, feature excellent mechanical properties[3] and are highly suitable to a number of industrial applications, especially in the aerospace sector and transport industry. For instance, these alloys are widely used in aircraft skin panels, especially in military aircraft[4–6] but also in commercial civil aviation aircrafts.[7] Age hardening is based on the formation of intermetallic products from the decomposition of a metastable super-saturated solid solution (SSS), which is obtained by solution treatment and quenching. The interaction between the decomposition products and the dislocations is the main responsible for the hardening.[3] That is, the particular precipitation path and phase transformations, which in turn depend on the alloy composition, quenching conditions, and aging parameters,[8] determine the microstructure, and hence the material properties as well. This is the reason why appreciable research efforts have been focused on the investigation of the transformation sequence during aging in metals. This research has been particularly intense in AlZnMg alloys (series 7000)[9] and in AlCuMg alloys (series 2000) because of their commercial and industrial importance, for the reas
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