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Equilibrium properties of the MgO-Al₂O₃-SiO₂ (MAS) system are modeled using techniques from statistical and quantum mechanics. The fundamental structural units in this model are the closed shell ions: Mg²⁺, Al³⁺, Si⁴⁺, and O²⁻. The equilibrium properties of the MAS system are determined by the interactions among these ions and by the environment (i.e. temperature and pressure). The interactions are modeled using coulombic, dispersion, and repulsive forces. Two parameters appearing in the repulsive terms for each cation-oxygen interaction are fitted from properties of quartz, corundum, and periclase crystals. The effects of the environment on the liquid and solid compositions found in this system are calculated using a Monte Carlo technique involving the generation of a Markov chain of configurations; each configuration being a "snapshot" of the particles in the liquid or solid material being studied. The properties of the material are derived from averaging appropriate quantities over all the configurations. Enthalpies of formation, heat capacities, and volumes of seven compositions in the MAS system have been calculated using this method. All are within three percent of the corresponding experimental values. Radial distribution functions for these runs show the competition among the cations for the common anion, oxygen, under charge and mass balance constraints. The electronic structure of several molecular clusters in the MAS system are examined using ab initio linear combinations of atomic orbitals (LCAO) techniques. The assumptions used in LCAO calculations are examined and a small, balanced basis set for the MAS system is presented. The Mg-, Al-, and Si-O interactions are all found to be highly ionic using this basic set. Using a first principles technique, the two body effective pair potentials assumed for the Monte Carlo calculations were shown to be physically reasonable.

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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Cast Al-Si and Al-Si-Mg alloys containing up to 12.6 wt. pct. silicon and 1.0 wt. pct. magnesium were prepared. The changes in electrical conductivity/resistivity of these alloys due to strontium additions have been investigated and explained in terms of variations in microstructure. The conductivity behaviour of strontium-containing and strontium-free alloys was found to exhibit marked differences, depending on the silicon and magnesium contents and the rate of solidification. The electrical conductivity of single phase alloys containing less than 1.60 wt. pct. Si decreased with increasing silicon and magnesium levels. However, strontium had no effect on the conductivity of these solid solution alloys since it does not dissolve appreciably in the aluminum matrix or change the solid solubility of silicon and magnesium in aluminum. Silicon precipitation processes in the supersaturated solid solution alloys of Al-Si and Al-Si-Sr have been examined using the Johnson-Mehl-Avrami equation and found to be isokinetic. Strontium, however, retarded the growth rate of silicon precipitates. Strontium did not affect the kinetics of G.P. zone formation in Al-Si-Mg alloys but it suppressed the formation of stable Mg$ sb2$Si precipitates during subsequent aging at 175$ sp circ$C. Unlike the single phase alloys, two phase Al-Si and Al-Si-Sr alloys, in the range of 2.0 to 12.6 wt. pct. Si, exhibited different electrical conductivity behaviour. The strontium-containing alloys showed a higher conductivity than alloys with no strontium, and this conductivity difference increased as the silicon and magnesium contents were increased and the solidification rate was decreased. It has been demonstrated this difference is due to changes in the silicon morphology. Electron scattering at the interface between the aluminum matrix and the eutectic silicon phase contributes significantly more to the resistivity of unmodified alloys than that of modified alloys. In addition, the resistivity of

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