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Superplastic aluminum alloys have been the subject of recent research in the automotive industry for several reasons. First, the formability of such alloys under superplastic forming (SPF) conditions is quite good; second, the potential weight reduction associated with formed aluminum sheet as compared to traditional steel sheet is particularly important for automotive applications. Unfortunately, SPF-grade aluminum is relatively expensive, and the typical SPF process is expensive and time-consuming because of the high temperatures and slow strain rates involved. The present study investigates the potential for forming AA5083, the most common commercial alloy for SPF operations, at reduced temperatures and increased strain-rates. The deformation and failure mechanisms under these conditions are examined in a modified AA5083 material and are compared with data from unmodified AA5083. A new method of presenting creep transient data is presented and is used to determine deformation mechanisms at elevated temperatures. A modified AA5083 alloy produced by continuous casting (CC), and containing a small addition of Cu, was studied. This small addition of Cu causes the formation of Mg2Cu and MgCu2 intermetallic particles, which produce incipient melting. It is proposed that these low-melting-temperature phases may reside at grain boundaries and enhance grain-boundary sliding at low temperatures, which may explain the improved ductility of the Cu-containing alloy. Ductility variations for the Cu-containing alloy are also explored. The differences in tensile ductility between the Cu-containing AA5083 and unmodified AA5083 materials, produced by both CC and direct-chill (DC) casting, are related to differences in cavitation behavior. The final part of the present work is a study on the effect of various heat treatment schedules on the constituent particle distribution in AA5083. It is shown that a simple one-day aging treatment at a moderate temperature can effectively reduce the interparticle spacing. This may lead to a finer recrystallized grain size [1] and, thus, improve superplastic properties.
KEY FEATURES: A unified, fundamental and quantitative resource. The result of 5 years of investigation from researchers around the world New data from a range of new techniques, including synchrotron radiation X-ray topography provide safer and surer methods of identifying deformation mechanisms Informing the future direction of research in intermediate and high temperature processes by providing original treatment of dislocation climb DESCRIPTION: Thermally Activated Mechanisms in Crystal Plasticity is a unified, quantitative and fundamental resource for material scientists investigating the strength of metallic materials of various structures at extreme temperatures. Crystal plasticity is usually controlled by a limited number of elementary dislocation mechanisms, even in complex structures. Those which determine dislocation mobility and how it changes under the influence of stress and temperature are of key importance for understanding and predicting the strength of materials. The authors describe in a consistent way a variety of thermally activated microscopic mechanisms of dislocation mobility in a range of crystals. The principles of the mechanisms and equations of dislocation motion are revisited and new ones are proposed. These describe mostly friction forces on dislocations such as the lattice resistance to glide or those due to sessile cores, as well as dislocation cross-slip and climb. They are critically assessed by comparison with the best available experimental results of microstructural characterization, in situ straining experiments under an electron or a synchrotron beam, as well as accurate transient mechanical tests such as stress relaxation experiments. Some recent attempts at atomistic modeling of dislocation cores under stress and temperature are also considered since they offer a complementary description of core transformations and associated energy barriers. In addition to offering guidance and assistance for further experimentation, the book indicates new ways to extend the body of data in particular areas such as lattice resistance to glide.