Development of a Thermally Stable Magnesium-Based Bulk Nanocomposite
Magnesium has the potential to become an indispensable structural material alongside aluminum due to its low density. However, magnesium is currently held back in its use as a result of its low strength and poor ductility with relatively limited structural applications. This dissertation presents the results for a processing route that was used to achieve a high-strength and thermally stable magnesium-based bulk nanocomposite in order to increase the applicability of magnesium-based alloys. This processing route included the gas atomization of Mg-4Y-3RE (WE43) powders which were treated by cryomilling (mechanical ball milling in liquid nitrogen) followed by consolidation by spark plasma sintering (SPS) in order to obtain a bulk material. The resulting bulk nanocomposite consisted of approximately 15-20% by volume fraction of extremely fine MgO nanoparticles/grains (< 10 nm) with a bimodally distributed Mg matrix consisting of nanocrystalline (~10 nm) and coarse grains (~1 ï¿½m). Besides Mg and MgO, no other phases were detected suggesting that the rare earth elements were elementally segregated to the grain boundaries. The microstructure contained a quasi-duplex structure consisting of two distinct regions with one of these regions containing only the coarse Mg grains with the other region containing all of the nanocrystalline Mg grains, MgO nanoparticles, and rare earth elements. Focused ion beam (FIB) was used to prepare 4 ï¿½m micropillars in order to determine the bulk compressive yield strength along with nanoindentation to evaluate the elastic modulus and hardness. Micropillar compression revealed a compressive yield strength of 325 MPa (compared to 190 MPa for traditional processing consisting of casting and a T6 heat-treatment) with nanoindentation demonstrating an elastic modulus of about 60 GPa and a hardness of 1.25 GPa. The nanocomposite also showed remarkable thermal stability with no observable differences in both its microstructure as well as in its mechanical properties even up to an extremely severe heat-treatment at 450 ï¿½C (0.9Tm) for 100 hours. The thermal stability was attributed to a combination of the MgO nanoparticles as well as the rare earth elements segregated to the grain boundaries acting as grain boundary pinning sites with the thermal stability of the nanocrystalline regions confining the growth of the discontinuous coarse-grained Mg regions.