The design and deformation mechanism of defect-based microstructure within magnesium
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Date
2022-11-02
Authors
Liu, Yushun
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Abstract
As the lightest structural metal, Magnesium (Mg) has been considered a promising candidate for automobiles and aerospace applications, however, its high mechanical anisotropy and poor ductility severely limit its wide applications. This thesis targets exploiting novel defect structures, i.e., grain boundaries and dislocations, and their deformation mechanisms, using transmission electron microscopy (TEM) and in-situ techniques, to advance the mechanical properties of Mg.
Aiming at developing effective preparation of nano-crystalline samples containing excessive grain boundaries for TEM characterization, a wedge mechanical polishing approach was adopted and successfully produced nano-crystalline with a size of ~100nm. This synthesizing approach can be applied to other Mg alloys and other metals as a simple and fast nano-crystalline preparation.
To further explore deformation mechanisms that require tracking grains re-orientation in real-time, new in-situ setting, integrating nano-indentation tests with hollow-cone dark-field (HCDF) imaging, has been proposed and successfully obtained the rotation of grains/slip planes in real-time, together with morphological observation. Results show that significant grain rotation along with dislocation activity occurs during the indentation, and a possible cooperative deformation mode between them has been experimentally demonstrated for the first time.
Apart from grain boundaries, a novel dislocation structure consisting of periodically spaced <c>-screw dislocations was investigated using pure Mg as the model system to demonstrate its availability in different alloy systems. Results show that simple hot-compression has successfully introduced desired dislocation structures. Its formation mechanism and potential deformation mechanisms were discussed based on dislocation interactions.
To summarize, this thesis explores the possibility of optimizing defect structures to improve the mechanical performance of Mg and its alloys. Our findings provide new insights into synthesizing methods, characterization techniques, and understanding the deformation mechanisms of Mg.
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Keywords
magnesium, transmission electron microscopy, defect engineering, dislocations, grain rotation, nano-grains