Aluminum 5XXX alloys are of industrial importance and interest as they combine a wide range of desirable strength, forming and welding characteristics with a high resistance to corrosion. The presence of Mg in these alloys ensures favorable mechanical properties. However, the room temperature stretching performance of these alloys is limited. Moreover, Al-Mg alloys are known for being susceptible to the Portevin-LeChatelier effect when deformed at room temperature. Nevertheless, improvements in ductility can be achieved through warm forming, especially when the ductility approaches superplastic levels. / The aim of this study was to test for enhanced ductility in three coarse-grained Al-Mg alloys namely, super-pure Al-3%Mg and Al-5%Mg, and commercial AA 5056 alloy. The temperature-dependent flow stress and rate sensitivity behavior of these alloys was investigated by means of tensile testing using ASTM E8M-04 standard samples. Samples were deformed to 10% strain to allow enough deformation to occur such that serrations in the dynamic strain aging (DSA) temperature/strain rate range would be rendered visible on a stress-strain curve. Using this information, the regions of negative and higher-than-normal strain rate sensitivity ('m') were plotted and tensile tests to failure were performed in the vicinity of maximum 'm'. ASTM E2448-06 standard samples for superplasticity tensile testing were used in this case. / A maximum ductility of 170% was recorded with these samples and this was found to increase to nearly 300% when the gage length was shortened. It was observed that the DSA serrations were more prominent at lower strain rates, higher temperatures and higher Mg contents. The results of this study show clearly that if the rate sensitivity is high enough, then enhanced ductility in coarse-grained materials is possible at temperatures well below the maximum test temperature.

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Nanopowders and bulk nanostructred materials have gained large interest in recent years. Bulk nanostructured materials exhibit properties that are far superior in comparison to conventional micron grained alloys. The fabrication of large scale nano-grained materials has been achieved in a two step process: (1) the production of nanostructured aluminium alloy powders and (2) the consolidation of the powder using a electromagnetic shockwave process. / The first part consists of cryo-milling; the milling of powder in an attritor filled with liquid nitrogen. This causes successive welding and fracturing events as the powder is milled, thereby creating the nano-structure. The low temperature prevents the possibility of recrystallization and grain growth. The alloy used for this work was Al 5356 (Al-5%Mg). Two different types of raw source materials were investigated: pre-alloyed powders and a mixture of aluminum with pure magnesium or an Al12Mg17 intermetallic. Experiments have been conducted in order to determine the optimum milling parameters that will simultaneously give a grain size smaller than 100 nm; equiaxed milled particles and mechanically alloyed powder (in the case of the mixture). The optimum milling parameters were established at 15 hours of milling time with a rotational speed of 300 RPM and ball to powder weight ratio of 24:1 in the case of the pre-alloyed powders. For the mixture of pure aluminum with pure magnesium the parameters were 15 hours, 300RPM and 32:1. The parameters for the mixture with the intermetallic were 18 hours, 300RPM and 32:1. / The dynamic magnetic compaction technique was done with a peak pressure of 1.1 GPa. This ultra-high strain rate process minimizes the exposure of the powders to high temperature and therefore reduces the possibility of recrystallization and grain growth. Relative densities of compacted pieces obtained ranged from 86.39% to 97.97%. However consolidation characterized by particle to particle bonding with a melted layer was not accomplished.

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This research focuses on the spall properties of aluminum-magnesium (Al-Mg)alloys.Aluminum alloy 5083 (Al 5083) was used as a model alloy for the work performed in this study. Al-Mg alloys represent a light-weight and corrosion resistant alloy system often used in armor plating. It is desirable to process armor plate material to yield a microstructure that provides maximum resistance to spall failure due to blast and projectile impacts. The blast and impact resistance has often been quantified based on the measurement of the spall strength and the Hugoniot elastic limit (HEL). The spall properties of Al-Mg alloys were measured for four different microstructural states resultant from varying processing conditions. The four microstructures include: (a) textured grain structure from a rolled Al 5083-H116 plate, (b) sub-micron grain structure produced using equi-channel angular pressing (ECAP),(c) equiaxed grain structure, and (d) precipitation hardened microstucture from an Al-9wt.% Mg alloy. The overall results show that grain size is not the most dominant microstructural feature affecting spall strength in aluminum alloys, when the impact conditions are the same. Texture, especially if brittle inclusions align along the grains, appears to have the most dominant effect resulting in decreased spall strength. Furthermore, one-dimensional modeling shows that the inclusion size and distribution is the controlling factor for void formation during spalling. Grain size does affect the decompression rate dependence of each microstructure, whereby smaller grain sizes result in a larger power law exponent for fits of spall strength versus decompression rate. Unlike the spall strength, the HEL shows an increasing trend with decreased grain size, as would be expected from a Hall-Petch type effect, indicating that a smaller grain size is best for penetration resistance. Samples processed using ECAP alone provide the best combination of spall strength and HEL and therefore the most promise for improved blast and penetration resistance of aluminum-magnesium alloy armor plates.

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Thesis (Ph.D)--Materials Science and Engineering, Georgia Institute of Technology, 2006. / Ken Sandhage - Committee Chair, Robert Snyder - Committee Co-Chair, G. Paul Neitzel - Committee Member, Preet Singh - Committee Member, Robert Speyer - Committee Member Part of the SMARTech Electronic Thesis and Dissertation Collection.

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