Synthesis and behavior of a thermally stable bulk nanostructured aluminum alloy

Much controversy surrounds the true mechanical behavior of nanostructured materials due to the significant lack of fully dense, bulk material, with which standardized mechanistic testing might be employed. In this thesis, a commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot-isostatically pressed, and extruded. This processing technique was found to produce a thermally stable nanostructured aluminum alloy that maintained an average grain size of 30–35 nm through several processing steps up to 0.61 Tmp. The thermal stability was attributed to Zener pinning of the grain boundaries by AlN and Al₂O₃ particles, and solute drag of numerous atomic species. The n-5083 Al alloy powder was subjected to various thermal heat-treatments in an attempt to understand the fundamental mechanisms of recovery, recrystallization and grain growth as they apply to nanostructured materials. A low temperature stress relaxation process associated with reordering of the grain boundaries was found to occur at 158°C. A bimodal restructuring of the grains occurred at 307°C for the unconstrained grains and 381°C for the constrained grains. An activation energy of 5.6 kJ/mol was found for the metastable nanostructured grains, while an activation energy of approximately 142 kJ/mol was found above the restructuring temperature.; The consolidated n-5083 was found to have a 30% increase in yield strength and ultimate strength over the strongest commercially available form of 5083 with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few, large, single crystal aluminum grains acting as crack blunting objects. Preliminary creep results from a near-nanostructured Al - 4 Mg alloy indicate that creep of this material is similar to that of dispersion strengthened Al alloys. A high creep resistance was found and was attributed to the strong interaction of grain boundary dislocations with dispersoid particles present in the grain boundaries. The apparent activation energy of 80 kJ/mol was found to be in good agreement with the activation energy for grain boundary diffusion in Al, 82 kJ/mol.