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Plastic deformation and surface characteristics of a peak-aged AA6082 alloy have been studied by means of tensile tests, Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD), Transmission Electron Microscopy (TEM) and Atom force Microscopy (AFM). The results showed that a simultaneous enhancement in ductility and strength of the alloy was obtained at 77K in comparison with that at 295K. The enhanced properties at 77K are attributed to higher work hardening accompanied by a more homogeneous slip mode.Moreover, in order to clarify the effect of temperature-induced microstructural changes on mechanical properties, a further investigation, i.e., pre-stretching at 77K and 295K with or without subsequent annealing treatment, followed by tension to fracture at 295K, was conducted. It was found that pre-stretching at both temperatures produce a yield point and followed by different yield drop zones, while pre-stretching and annealing lead to reduced stress levels, much less pronounced yield point behavior and improved ductility compared to their predecessors. Especially, pre-straining at 77K with subsequent annealing demonstrated the highest ductility and work hardening ability among the four cases. It is thought the obtained results are associated with variety of microstructure such as strain localization, β ' /β '' ratio and dislocation densities under different conditions.

Peak-aged AA6082 flat extrusions were cryorolled at liquid nitrogen temperature to three different thickness reductions, the reductions were 21%, 42% and 85%, respectively.For comparison, an identical extrusion was rolled at room temperature to a reduction of 85%.The microstructure and plastic deformation behaviors of the initial as well as the rolled materials were studied by means of electron microscopy and tensile tests at room temperature.The initial material showed fibrous grain structure. After cryorolling to 21% thickness reduction, grains in the alloy were further elongated. In addition, microbands were detected in some grains, while other grains were microband-free. More detailed information was obtained in the sample after 42% thickness reduction. Here, numerous microbands aligned along slip planes with the highest resolved shear stress, leading to the formation of new low angle subgrains and high angle grains. In the case of microband-free grains, multiple slip systems were activated concurrently due to similar and low Schmid factors, hence producing homogeneous deformation within these grains. A cryorolling strain of 85% produced a hierarchical microstructure in the materials volume, consisting of nano-and ultrafine grains with size < 100 nm coexisted with micro-sized grains. The development of hierarchical 2 microstructure could be considered as a consequence of the intersection between families of microbands. As a result, both the strength and uniform elongation of the severely cryorolled alloy, i.e. 85% thickness reduction, are more superior to rest of rolled alloys. Therefore, this study revealed that a simple processing procedure, i.e., cryorolling, can be utilized to tailor for desired properties in favor of both strength and ductility aluminum alloy.

The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.