Micro-scale fatigue mechanisms in metals: an in situ high frequency experimental approach

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Johns Hopkins University
More than half of all mechanical failures in engineering structures are classified as fatigue failures. Regardless of the constituting material's strength, the limiting factor in service-life is the amount of cyclic damage that the material can accumulate before fracture. Since metals are the material-of-choice for most structural applications, scientists have strived to gain a fundamental understanding of the mechanisms that lead to their failure under cyclic loading, which is necessary for the design of novel and superior alloys. The reason fatigue of metals is such a challenging problem is that different microstructural features are involved and the controlling mechanisms span many length and time scales. In particular, the dislocation plasticity that precedes micro-crack initiation and the nature of micro-crack growth remain poorly understood. Small-scale testing has recently emerged as a valuable methodology to understand micro-scale deformation. Over the past two decades, this method has made many breakthroughs in understanding the physics of dislocation plasticity. However, extending these methods to cyclic loading has been met with many difficulties. In this thesis, fundamental fatigue mechanisms in face-centered cubic metals at the micro-scale are explored. To achieve this, a novel high-frequency micro-fatigue experimental methodology is developed by combining aspects of electron microscopy, micro-mechanical testing, and acoustic emission detection. These experiments are designed to replicate stress states (e.g., bending and uniaxial) and cycle counts (>10^7) common in bulk fatigue experiments, although at a ~10 micron length scale. All experiments are performed in situ in a scanning electron microscope to acquire real-time observations of surface morphology changes that are not obtainable with bulk-scale fatigue experiments. Using this method, a transition from cyclic hardening to softening is found in nickel-base superalloys, which is attributed to the shearing of precipitates. Also, for the first time, persistent slip bands (PSB) were observed in pure nickel in micron-scale single crystals. However, the number of cycles to PSB nucleation was two orders of magnitude higher than those in bulk crystals, which was attributed to the high surface-to-volume ratio in the tested crystals. Additionally, the experiments have shown that PSBs nucleate locally and propagate in a way that current models in literature do not account for. A new mechanism-based model was proposed to capture these new observations. The PSBs were also found to promote micro-crack initiation in nickel and nickel-base superalloys. Finally, intermittent micro-crack propagation events and acoustic emissions associated with micro-plasticity were quantified and characterized statistically.
Fatigue, PSB, Small-scale testing, Nickel, Micro-beams, Nickel-base superalloys