Doctoral Dissertations

Date of Award

12-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Ke An, Yanfei Gao, John D. Landes

Abstract

Magnesium (Mg) alloys have received great attentions in the past several decades, due to their unique properties of low density, high strength-to-weight ratio, and high specific stiffness. Previous work on fatigue behavior of Mg alloys typically relies on ex situ microstructural characterization and crack-growth monitoring with replica techniques. The primary challenge is thus the lack of an in situ, non-destructive measurement on microstructural length scales, which prevents us from linking the stress analyses (top-down point of view) to the failure mechanisms on inter- and intra-granular scales (bottom-up point of view). A unique opportunity that resolves many of these difficulties and challenges is provided by in situ neutron and synchrotron X-ray diffraction methods, which provide the unprecedented information on inter- and intra-granular deformation characteristics at different length scales from mm to sub-μm.

The primary objective aims to identify deformation mechanisms during strain-path changes, low-cycle fatigue, and fatigue crack growth tests of Mg alloys from in-situ diffraction and micromechanical studies. We took advantage of the state-of-art VULCAN engineering materials diffractometer of Spallation Neutron Source (SNS), Oak Ridge National Laboratory (ORNL) to achieve the real in-situ neutron diffraction measurement. Instead of the “traditional” step loading method, the neutron diffraction measurements were performed under continuous loading condition at a slow loading rate, which provided the detailed information related to the plastic deformation dynamics of the wrought magnesium alloy. Full-field mapping around fatigue cracks can be performed at ISIS facility, Rutherford Appleton Laboratory, UK. The multiscale nature of deformation near the fatigue crack tip permits a scale-bridging modeling method. Moreover, the twinning and detwinning behavior in an individual grain inside a polycrystalline wrought Mg alloy has been investigated using sub-micron level synchrotron X-ray microbeam diffraction at 34ID-E, Advanced Photon Source (APS), Argonne National Laboratory (ANL), USA.

Critical issues lie on the deformation dynamics, twinning-detwinning behavior at the grain level, and fatigue crack growth mechanisms.

The principal outcome of this research will be the improved microstructural level understanding on deformation dynamics and fatigue mechanisms with which materials scientists can improve the practical applications of Mg alloys.

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