Microstructure-sensitive investigation of plasticity and fatigue of magnesium alloys

This dissertation identifies and quantifies the correlation between strain localizations at different scales and both macro- as well as microplasticity of Magnesium (Mg) based alloys. The extension of the work in the case of cyclic mechanical loading further enabled the investigation of reversible and irreversible microstructural processes that are ultimately linked to progressive fatigue damage development. To accomplish these goals, this dissertation presents a systematic experimental mechanics methodology combining multi-scale mechanical testing, in situ nondestructive evaluation (NDE) and targeted microstructure quantification. The presented research benefited from the novel integration between mechanical testing and multimodal NDE comprising both full field deformation measurements by using the digital image correlation method and time-continuous recordings of acoustic. Specific contributions of this work include the direct identification of the dominant effect of twinning in early stages of plasticity which is demonstrated in this research to be responsible for macroscopic effects on the monotonic and cyclic plasticity, as well as for microscopic processes that include slip-twin interactions and fatigue crack incubations. Such observations both enabled and were validated by careful texture evolution and grain-scale effects including pronounced intrusions/extrusions on the surface which are demonstrated to be responsible for micro-level strain accumulations that eventually, under cyclic loading conditions, lead to the onset of cracking. Surface morphology changes were found to be attributed to an evolving twinning-detwinning-retwinning activity which operates from early stages of the low cycle fatigue life up until later stages, while it was found to be associated with progressive damage development. Furthermore, the role of twinning in plasticity and fatigue of Mg alloys was verified using a Continuum Dislocation Dynamics Viscoplastic self-consistent (CDD-VPSC) polycrystal model. The simulation results reveal that the detwinning mechanism is in fact responsible for the anisotropic hardening behavior for various imposed strain amplitudes. Experimental results were further used to modify strain-based modeling approaches of fatigue life estimation. A number of the insights enabled with this research were further verified by performing a mechanical behavior characterization investigation of Mg alloys with Strontium (Sr) additions, which are currently considered for industrial applications. The presented results demonstrate that the major research accomplishments described in this dissertation could improve current manufacturing processes, which further allow extensions and applications of this research in fundamental and applied aspects of plasticity and fatigue of polycrystalline metals.