RESUMO
The Acoustic Emission of deformation twinning in Magnesium is investigated in this article. Single crystal testing with combined full field deformation measurements, as well as polycrystalline testing inside the scanning electron microscope with simultaneous monitoring of texture evolution and twin nucleation were compared to testing at the laboratory scale with respect to recordings of Acoustic Emission activity. Single crystal testing revealed the formation of layered twin boundaries in areas of strain localization which was accompanied by distinct changes in the acoustic data. Testing inside the microscope directly showed twin nucleation, proliferation and growth as well as associated crystallographic reorientations. A post processing approach of the Acoustic Emission activity revealed the existence of a class of signals that appears in a strain range in which twinning is profuse, as validated by the in situ and ex situ microscopy observations. Features extracted from such activity were cross-correlated both with the available mechanical and microscopy data, as well as with the Acoustic Emission activity recorded at the laboratory scale for similarly prepared specimens. The overall approach demonstrates that the method of Acoustic Emission could provide real time volumetric information related to the activation of deformation twinning in Magnesium alloys, in spite of the complexity of the propagation phenomena, the possible activation of several deformation modes and the challenges posed by the sensing approach itself when applied in this type of materials evaluation approach.
RESUMO
A section of the Upper Enoree River in South Carolina, USA, was contaminated with chemical waste in 1985, and high concentrations of zinc persist decades later. In this study, we examined the zinc concentrations in the water, the accumulation of zinc in a variety of fish tissues, the effects of the contaminated water on fish sperm motility in vitro, and the mortality rates of introduced fish. Zinc concentrations in the water samples collected from six sites decreased as distance from the spill site increased, ranging from 7.3 to 0.34 mg/L (p < 0.001). The zinc concentrations of tissues from native fish were highest in liver (mean across sites of 110 ppm/g tissue) and gills (77.4 ppm/g tissue), followed by gonads (30.7 ppm/g tissue) and muscle (6.9 ppm/g tissue) (p < 0.001). The duration of fast motility of Salmo trutta sperm was significantly diminished in sperm activated in samples from the contaminated stream compared with the control stream (p < 0.05). To further evaluate the ability of fish to survive at the sites with different zinc concentrations, groups of Gambusia holbrooki were placed in traps at a reference site (uncontaminated local tributary), and three sites along the contaminated stream. Rapid mortality was observed in the two sites closest to the spill, including one site in which native fish had been found. The introduced G. holbrooki expressed higher zinc concentration in gills than gonads or muscle (p < 0.001), and water zinc concentration significantly affected fish mortality (p < 0.001). The results from these experiments indicate that zinc contamination of streams can have sublethal effects on populations and physiology of fish that are able to survive in the contaminated water.
