Poza, P.; Llorca, J.
The microstructure and tensile properties of an 8090 Al-Li alloy reinforced with 15 vol pet SiC particles were investigated, together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al3Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S' precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands. The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture. The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements during deformation, which led to the early failure at the onset of plastic instability.
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Lemkau, K. L.; Peacock, E. E.; Nelson, R. K.; Ventura, G. T.; Kovecses, J.; Reddy, C. M.
With increasing energy demand and rising prices, the use of cheaper and more pollutant-rich heavy fuel oil is expected to continue to increase. To date, the fate of these fuels in the environment has been rare, especially on a short-term basis (less than one month after the spill). On November 7, 2007 the M/V Cosco Busan struck the Bay Bridge in San Francisco, USA. Two port tanks containing fuel were ruptured in the allision resulting in the release of approximately 54,000 gallons of No. 6 fuel oil. To investigate the fate of the spill oiled, samples were collected from five impacted shorelines in and around the Bay Area weeks to months after the spill. Samples were analyzed for petroleum hydrocarbons using gas chromatography with flame ionization detection (GC/FID) and polycyclic aromatic hydrocarbons (PAHs) using gas chromatography with mass spectrometry detection (GC/MS). Additionally, several samples were examined in greater detail by two-dimensional gas chromatography (GC x GC). The effects of biodegradation, evaporation, water- washing, and photolysis were all observed. Although the extent of weathering was site specific, the most weathered were collected in an exposed location outside San Francisco Bay. Biodegradation was more frequently observed in samples that were collected from mid to low tide. These results provide new insights into the behavior of an understudied but important contaminant in the coastal ocean.
Khor, K. A.; Boey, F. Y. C.; Murakoshi, Y.; Sano, T.
There has been increasing use of Al-Li alloys in the aerospace industry, due mainly to the low density and high elastic modulus of this material. However, the problem of low ductility and fracture toughness of this material has limited its present application to only weight- and stiffness-critical components. Development of Al-Li/ceramic composites is currently being investigated to enhance the service capabilities of this material. The Ti-Al alloy is also of interest to aerospace-type applications, engine components in particular, due to its attractive high-temperature properties. Preparation of fine powders by plasma melting of composite feedstock and coatings formed by plasma spraying was carried out to examine the effect of spray parameters on the microstructure and properties of these materials. Characterization of the powders and coatings was performed using the scanning electron microscope and image analyzer. Examination of the plasma-sprayed powders and coatings has shown that in the Al-Li/SiC composite there is melting of both materials to form a single composite particle. The SiC reinforcement was in the submicron range and contributed to additional strengthening of the composite body, which was formed by a cold isostatic press and consolidated by hot extrusion or hot forging processes. The plasma-sprayed Ti-Al powder showed four categories of microstructures: featureless, dendritic, cellular, and martensite-like.