Enterohemorrhagic Escherichia coli O157:H7 is a major foodborne pathogen causing severe disease in humans worldwide. Healthy cattle are a reservoir of E. coli O157:H7, and bovine food products and fresh produce contaminated with bovine waste are the most common sources for disease outbreaks in the United States. E. coli O157:H7 also survives well in the environment. The abilities to cause human disease, colonize the bovine gastrointestinal tract, and survive in the environment require that E. coli O157:H7 adapt to a wide variety of conditions. Three major virulence factors of E. coli O157:H7 have been identified including Shiga toxins, products of the pathogenicity island called the locus of enterocyte effacement, and products of the F-like plasmid pO157. Among these virulence factors, the role of pO157 is least understood. This review provides a board overview of E. coli O157:H7 with an emphasis on pO157.
In this paper, high temperature oxidation behavior of newly developed alloys, Ti-6Al-4Fe and Ti-6Al-1Fe, is examined. To understand the effect of Fe on the air oxidation behavior of the Ti-Al-Fe alloy system, thermal oxidation tests are carried out at 700 o C and 800 o C for 96 hours. Ti-6Al-4V alloy is also prepared and tested under the same conditions for comparison with the developed alloys. The oxidation resistance of the Ti-Al-Fe alloy system is superior to that of Ti-6Al-4V alloy. Ti-6Al-4V shows the worst oxidation resistance for all test conditions. This is not a result of the addition of Fe, but rather it is due to the elimination of V, which has deleterious effects on high temperature oxidation. The oxidation of the Ti-Al-Fe alloy system follows the parabolic rate law. At 700 o C, Fe addition does not have a noticeable influence on the amount of weight gain of all specimens. However, at 800 o C, Ti-6Al-4Fe alloy shows remarkable degradation compared to Ti-6Al-1Fe and Ti-6Al. It is discovered that the formation of Al 2 O 3 , a diffusion resistance layer, is remarkably hindered by a relative decrease of the α volume fraction. This is because Fe addition increases the volume fraction of β phase within the Ti-6Al-xFe alloy system. Activities of Al, Ti, and Fe with respect to the formation of oxide layers are calculated and analyzed to explore the oxidation mechanism.Key words Ti-Al-Fe alloy, oxidation, iron, alloying element, oxide layer.
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