Investigating Microstructural Gradients and High Strain Rate Deformation Mechanisms in Industrial Magnesium Alloy Components

U.S. auto manufacturers are considering the use of extruded magnesium alloys in the front end of their automobiles. The low density of magnesium alloys (1.74 g/cm3) makes them attractive for this application due to improved fuel economy and vehicle performance through reduced weight. However, the ability of magnesium alloys to absorb energy and provide adequate crashworthiness is unknown. Understanding the microstructural gradients that can evolve as a result of the extrusion process, as well as examining the deformation mechanisms that dominate at high rates strain can help improve component design and production methods. Two studies were performed on industrial AZ31 and AM30 extrudates to achieve this goal. In the first study, the effects of localized deformation conditions on an industrial magnesium alloy AZ31 extrusion are investigated. The evolution of the fine and coarse grained microstructure is explained in terms of dynamic and static re-crystallization and is related to local deformation conditions and specimen geometry. In another study, the role of anisotropy at various deformation rates was investigated within samples from an AM30 extrudate strained in tension at high impact velocities. It was observed that the tensile direction had a significant impact on the loading behavior as well as the deformation mechanisms. This can be attributed to the highly anisotropic yielding behavior of magnesium hexagonal close packed crystals combined with a strong extrusion texture. Additionally, voids forming on specific twin planes are believed to induce premature failure, which has been shown previously to occur at low strain rates. The results of these experiments add to the knowledge base for determining crashworthiness of magnesium alloy components.