Magnesium alloys are increasingly being used in structural applications due to their excellent strength-to-weight ratio, particularly in applications where reductions in weight can result in significant improvements to fuel efficiency. Extending the service lives of these components into the very high cycle fatigue (VHCF) regime (>107 cycles) requires an understanding of the complex role local microstructure plays in determining fatigue behavior. In this thesis, ultrasonic fatigue has been used to characterize the effects of microstructure on VHCF behavior in the hot-rolled magnesium alloy WE43.Crack initiation, short crack growth, and VHCF life behavior have been investigated for three microstructural conditions of WE43 magnesium. As-received (T5) WE43 with a relatively fine grain size was solution treated and aged to produce precipitation strengthened coarse-grained microstructures in the underaged and peak-aged (T6) conditions. Ultrasonic axial fatigue tests with a cyclic frequency of 20kHz were conducted using cylindrical specimens. Crack growth behavior and fatigue crack-grain boundary interactions were investigated in high resolution using a unique combination of ultrasonic fatigue instrumentation and scanning electron microscopy (UFSEM). The UFSEM system was also used to investigate fatigue crack propagation in vacuum, in situ. Fatigue crack-grain boundary interactions were more closely studied using thin (150 μm) foil specimens under positive mean axial stress in ultrasonic fatigue. Post-mortem measurements of a microstructurally small fatigue crack were made using X-ray computed tomography to provide high-resolution reconstructions of the 3-D crack surface morphology, and near-field high-energy X-ray diffraction microscopy to provide 3-D grain geometries and orientations adjacent to fatigue-crack surfaces.A number of findings have resulted from these experiments and analyses, providing new insight into the role of local microstructure on fatigue behavior. Heat treatment was shown to have a strong effect on fatigue strength, with the fine-grained, strain-hardened T5 condition exhibiting much higher values than the coarse-grained underaged and T6 conditions of WE43. Differences in precipitation strengthening between the underaged and T6 conditions resulted in no significant difference in fatigue strength. It was found that average short crack growth rates for the three conditions were similar and had no clear dependence on microstructural condition, despite the significant differences in grain size. Crack initiation lifetime was shown to be particularly sensitive to grain size and occurred through cyclic slip deformation in particularly large and favorably oriented grains in each condition. Environment was shown to have a significant effect on crack growth rate, with rates in vacuum nearly two orders of magnitude lower than in laboratory air. Investigations of fatigue in in situ and foil specimens revealed that current models of the fatigue crack-grain boundary interaction based on boundary and microstructural parameters are inadequate to describe the complex interaction between the fatigue crack and local microstructure. Results indicate that more comprehensive modeling of three-dimensional microstructures using crystal-plasticity finite-element analyses is necessary to accurately predict fatigue behavior. The combined use of X-ray tomography and near-field HEDM allows for the high-resolution characterization of three-dimensional fatigue behavior, specifically fracture surface morphology and crystallography, a first step to three-dimensional modeling of fatigue and microstructure interactions.
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Investigating Microstructural Effects on Short Crack Growth and Fatigue Life Behavior of WE43 Magnesium
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