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Authors: Wilkinson, David S.
Nemcko, Michael J.
Materials Science and Engineering
Issue Date: Nov-2015
Abstract: Carbon dioxide emissions from the transportation sector contribute 25% of Canada’s total greenhouse gas emissions. As a result, the automotive industry aims to reduce emissions by reducing vehicle weight. Magnesium is an excellent candidate for a lightweight structural material. However, the lack of active deformation modes at room temperature limits its formability. This research uses several approaches to examine the deformation and fracture behavior of commercially pure magnesium to help guide the development of materials with acceptable properties. The first approach uses in-situ tensile testing coupled with electron microscopy, applied to thin sheet samples with pre-drilled holes in the gage section. The results reveal the heterogeneous nature of deformation leading to fracture dominated by twin and grain boundary related failure. This is qualitatively different from the damage processes observed in FCC materials such as aluminum and copper. As a consequence, classical theories cannot be used to predict ductile fracture in magnesium. The second approach involves in-situ tensile testing under optical microscopy. Here digital image correlation is used to determine the strain distribution on the surface. Localized deformation is observed at twin and grain boundaries. The results introduce a length scale which is not present in classical continuum theories. Therefore, a crystal plasticity finite element approach is employed to understand the role of the deformation mechanisms during deformation. The last approach couples in-situ tensile testing with x-ray microtomography to observe the fracture processes in sheet materials without pre-drilled holes. Twin and grain boundaries again dominate damage nucleation. Once nucleated voids show rapid growth and linkage. The final fracture occurs by a macroscopic shearing process related to the crystallographic texture. In summary, this project demonstrates how fracture in magnesium is fundamentally different from that in non-HCP metals and what approaches can be used to fully understand this.
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