Master of Science
Mechanical and Materials Engineering
Hexagonal closed-packed (HCP) metals have been extensively used in various industrial sectors, eg, zirconium in nuclear industry, magnesium in transportation industry, and titanium in aerospace industry. Understanding deformation mechanisms of HCP metals is crucial for developing predictive models that can be used for performance and failures analysis of engineering components. The two main deformation modes in HCP polycrystals are slip and twinning. While deformation by slip is well understood, many fundamental questions about twinning are still remained unanswered. The aim of this research is to employ a novel experimental technique, three-dimensional synchrotron X-ray diffraction, on two different HCP metals and acquire a statistical data that can help us understand fundamentals of deformation twinning. After a literature review in chapter two, the steps used for preparing samples, conducting the experiment, and post-processing the collected diffraction patterns are explained in chapter 3. The results of the experiments on zirconium and magnesium samples are provided and discussed in chapters 4, and 5, respectively. This is followed by conclusions and future work. By updating and developing new procedures for grain matching, more than 19000 grains are investigated individually. It is shown for the first time that twin variant selection is preferential in the plastic zone, yet not at the early stages of plasticity. It is shown that due to local and macroscopic stress configurations, twins in zirconium are generally relaxed along the loading direction, but not in magnesium. In this thesis, the very first in-situ cyclic compress-tension experiment on magnesium was conducted where twinning and de-twinning were fully observed in 3D. Understanding the mechanism of twin nucleation and propagation can help us update our existing numerical models and improve our predictions.
Summary for Lay Audience
A broader view of this research topic is deformation of metals. Most metals are solid in nature and consist of closely packed atoms. The packing of these atoms varies from one metal to another, like face centered cubic (FCC) in aluminum and hexagonal close packed (HCP) in magnesium. The collection of crystals form lattice that can have point or line defects (dislocations) due to missing atoms. The population of these defect change with plasticity, effecting hardening. Ideally, the lattice of atoms would all be packed in a single direction however most metals that exist in nature are polycrystalline in nature. This means the metal consists of several lattices or single crystals of different sizes and directions. Most metals, like FCC and body centered cubic (BCC), deform in the plastic region through movement of dislocations. However, in HCP metals, another deformation mechanism, known as twinning, can occur when the lattice is oriented in a specific direction. Twinning is a mechanism that occurs in single lattice causing a portion of the lattice to re-orient in another direction. Twinning can be advantageous, in terms of improving the ductility of the material, and disadvantageous, in terms of providing susceptible sites to crack nucleation. Therefore, the first step to understand twinning is investigating the parameters affecting its initiation. Most of the studies conducted before to understand twinning have viewed twins in 2D in terms of shape and stresses. The experiment conducted in this research looks at twins in 3D and uses the synchrotron 3D-XRD method.
Louca, Karim, "Three Dimensional Characterization of Deformation Twins Using Synchrotron X-Ray Diffraction" (2019). Electronic Thesis and Dissertation Repository. 6374.
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