(Georgia Institute of Technology, 2018-05)
Wang, Xueqiao
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.
(Georgia Institute of Technology, 2018-05)
Short, Andrew E
Over 1 million tons of oil is inadvertently spilled each year. The economic and environmental costs of these spills are enormous and necessitate further development of environmentally friendly sorbent materials. Here, we demonstrate a vapor phase modification approach to create a new class of oil sorbents composed of cellulosic materials (cotton) coated with a sub-nanometer layer of inorganic oxide. This new cellulosic sorbent remains buoyant in water indefinitely and achieves a selective oil sorption capacity (23 g g-1 or 1.05 g cm-3) that is at least 35x better than untreated cellulose in aqueous environments. This new sorbent particularly excels under “realistic” conditions like continuous agitation (e.g. simulated waves) and pre-soaking in water (e.g., rain or forced immersion). When sorption performance is compared on a per-volume basis—which better captures use conditions than a per-mass basis—this modified natural product becomes comparable to the best sorbents reported in the literature, most of which require further expensive processing.
(Georgia Institute of Technology, 2018-05)
Adstedt, Katarina
The ability of atomic layer deposited (ALD) metal oxide films to serve as protective, encapsulating barriers for biological implants is determined through testing the corrosion resistance and degradation behavior of the films. Using plasma enhanced ALD (PE-ALD), metal-oxides are deposited at 100 oC onto gold electrodes. Through MTT cell proliferation assay, the films are determined to be cytologically compatible and will not cause harm to the implant host. Using electrochemical impedance spectroscopy (EIS), the films establish their relative chemical stabilities within three different biological environments, phosphate buffer solution (PBS), simulated sweat and simulated saliva. The resulting data from the EIS measurements demonstrates the rate of degradation for the four respective films and exhibits which films are best suited as protective barriers for biological implants. ALD Al2O3 is not suitable as an encapsulating layer as it demonstrates no corrosion resistance. Within PBS, ALD TiO2 establishes itself as the most stable film barrier while within simulated sweat and saliva ALD ZrO2 is the most chemically stable. The viability of ALD films in biological solutions and their enhanced corrosion resistances opens up the possibility for a new class of materials that can be used for the protection of bioimplants and wearable devices.