UC Irvine

A major difference between coarse-grained materials and nanocrystalline materials is the volume fraction of grain boundaries, which is the factor that alters nanocrystalline materials’ physical and mechanical properties. Mechanical properties are also changed due to the fact that dislocation movements are impeded by small grains and other enhanced stabilizing mechanisms (dislocation locking, Zener pinning, etc.), reducing the driving force for deformation. The understanding of physical phenomena that takes place in such microscopic level poses an extremely interesting area of study where grain boundaries can be manipulated to alter physical and mechanical properties of a material. This thesis focuses on grain boundary doping as a mean of manipulation. Nanocrystalline Al and Al-Mg alloys with an average grain size of 24 nm were used to isolate the effect of grain boundary doping on strength. To begin, Mg was added to the Al lattice using mechanical milling, a process that produces materials in nanocrystalline form. This was followed by annealing treatments to induce segregation of solute (Mg) to the grain boundary, therefore changing the overall energy state that affects material properties. A combination of energy dispersive spectroscopy and X-ray diffraction were used to quantify the composition of the grain interiors and grain boundaries, then the relative contributions of solid solution strengthening and grain boundary segregation were extracted. Our results showed that nanocrystalline Al -7 at. % Mg had a maximum hardness of 4.56 GPa, approximately three times the hardness of pure nanocrystalline Al with the same grain size. Microcompression experiments on the strongest powder alloys indicated a yield strength value of 865 MPa and a specific strength value of 329 kN⋅m/kg, making these materials among the strongest Al alloys ever made. This work confirms that grain boundary segregation lowers the overall energy state and increases strength.