Three Dimensional Characterization of Deformation Twins Using Synchrotron X-Ray Diffraction
Abstract
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.