A Mechanism Based Investigation Of The Dynamic Behavior Of Pure Magnesium

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Johns Hopkins University
The global emphasis on improving energy efficiency in recent years has propelled the development of new engineering solutions to meet the high efficiency standards. The push from the transportation industry to achieve weight reduction of automobiles/components (in order to meet the higher standards for fuel efficiency) has renewed interest in lightweight materials such as magnesium (Mg). Mg alloys have low density and high specific strengths which makes them attractive in applications where their increased use can lead to significant energy savings. In order to successfully use these materials in applications in which they will be subjected to dynamic loading such as automotive, aerospace and defense applications, we need to assess and understand their behavior under such conditions. The deformation behavior of this HCP material is complicated by the richness of the slip and twinning modes and their interactions and dynamic loading may introduce additional complexities. High strain rate (Kolsky bar) and normal plate impact experiments were performed on pure extruded magnesium to analyze the mechanical behavior. Microstructural analysis of the pre and post deformation samples was performed in order to understand the dominant deformation mechanisms and their evolution. Optical microscopy and electron back scattered diffraction (EBSD) was performed to analyze deformation twinning and transmission electron microscopy (TEM) was performed to analyze the dislocation structures. It was observed that under uniaxial stress compression at high strain rates (10^3 /s) in the ED (induced by Kolsky bars), extension twinning, <a> slip and <c+a> slip are necessary to accommodate plastic strains. The microstructural observations show that extension twinning reorients the material by 86^0 to a harder orientation and causes a significant change in the texture. The insights gained from the analysis of the evolution of the dominant deformation mechanisms are used to develop a simple mechanism based constitutive model which captures the behavior of single and polycrystalline Mg. The normal plate impact experiments conducted (on both single crystal and polycrystalline Mg) at 60-70 m/s impose a uniaxial strain loading for very short durations (~2 μs). Elastic-plastic plate impact simulations performed in Abaqus/Explicit show that the stress, strain and strain rate in the specimen thickness are inhomogeneous in these experiments. Deformation twinning is observed under this loading of very short duration although the characteristics of the twins formed under this loading are different than those induced by the Kolsky bar loading (~200 μs). The twins induced by the plate impact loading are thinner and occupy smaller volume fraction as compared to the ones induced by the Kolsky bar loading. The observations from both the experiments indicate that the differences in the deformation twinning characteristics arise from the differences in the stress states, strain rates and the loading durations in the two cases. These observations help us further our understanding of the mechanisms controlling the dynamic behavior of pure magnesium.
Magnesium, Twinning, Dynamic behavior