Load Dependent Fatigue Crack Initiation in High Purity Al
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Fatigue crack initiation sites and mechanisms in metals and alloys have long been investigated, since metal components are often subjected to cyclic loading, and fatigue cracking is one of the major causes of failure. Therefore, understanding the dominant cracking mechanism under different conditions is essential for tailoring the composition and microstructure of metal components for better fatigue resistance under various loading conditions. Load dependent fatigue response in high purity aluminum (Al) is investigated. In low cycle fatigue, extrusions and intrusions are found to form on grain boundaries (GBs), especially prevalently at triples junctions. However, contrary to theories on extrusion formation from persistent slip bands (PSBs), no slip bands are observed in these specimens. Dislocation cells, on the other hand, are observed to form in higher densities and smaller sizes as stress amplitude increases. As extrusion formation occurs only after a threshold number of cycles, it might be a result of the progression of dislocation cell formation. In high cycle fatigue, no extrusions are observed at GBs, while microcracks form within grains. Therefore, high cycle fatigue life may be controlled by mechanisms other than dislocation cell formation, and involves transgranular, rather than intergranular, fracture.