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Deformation Behaviour of Nanocrystalline Magnesium
Hwang, Steve,McCormick, Paul G . 대한금속재료학회(대한금속학회) 1998 METALS AND MATERIALS International Vol.4 No.3
The deformation behaviour of nanocrystalline magnesium was studied. Nanocrystalline magnesium powder was prepared by high energy ball milling in a dry inert gas atmosphere and vacuum cold pressed to form fully dense cylindrical specimens. Compression tests were carried out at room temperature. The samples exhibited remarkably high values of ductility, and a significant improvement in yield strength in sintered specimens. Large yield drops were observed on the stress-strain curve of sintered specimens. The strain rate sensitivity of the flow stress was also measured and compared to the values for commercially available pure magnesium bars. The values of activation volume as well as the stress exponent were found to be significantly lower in the nanocrystalline samples. It is speculated that grain boundary sliding controls the deformation mechanism in the nanocrystalline magnesium samples.
Park, Jinkyu,McCormick, Sean P.,Cockrell, Allison L.,Chakrabarti, Mrinmoy,Lindahl, Paul A. American Chemical Society 2014 Biochemistry Vol.53 No.24
<P/><P>The majority of Fe in Fe-replete yeast cells is located in vacuoles. These acidic organelles store Fe for use under Fe-deficient conditions and they sequester it from other parts of the cell to avoid Fe-associated toxicity. Vacuolar Fe is predominantly in the form of one or more magnetically isolated nonheme high-spin (NHHS) Fe<SUP>III</SUP> complexes with polyphosphate-related ligands. Some Fe<SUP>III</SUP> oxyhydroxide nanoparticles may also be present in these organelles, perhaps in equilibrium with the NHHS Fe<SUP>III</SUP>. Little is known regarding the chemical properties of vacuolar Fe. When grown on adenine-deficient medium (A↓), ADE2Δ strains of yeast such as W303 produce a toxic intermediate in the adenine biosynthetic pathway. This intermediate is conjugated with glutathione and shuttled into the vacuole for detoxification. The iron content of A↓ W303 cells was determined by Mössbauer and EPR spectroscopies. As they transitioned from exponential growth to stationary state, A↓ cells (supplemented with 40 μM Fe<SUP>III</SUP> citrate) accumulated two major NHHS Fe<SUP>II</SUP> species as the vacuolar NHHS Fe<SUP>III</SUP> species declined. This is evidence that vacuoles in A↓ cells are more reducing than those in adenine-sufficient cells. A↓ cells suffered less oxidative stress despite the abundance of NHHS Fe<SUP>II</SUP> complexes; such species typically promote Fenton chemistry. Most Fe in cells grown for 5 days with extra yeast-nitrogen-base, amino acids and bases in minimal medium was HS Fe<SUP>III</SUP> with insignificant amounts of nanoparticles. The vacuoles of these cells might be more acidic than normal and can accommodate high concentrations of HS Fe<SUP>III</SUP> species. Glucose levels and rapamycin (affecting the TOR system) affected cellular Fe content. This study illustrates the sensitivity of cellular Fe to changes in metabolism, redox state and pH. Such effects broaden our understanding of how Fe and overall cellular metabolism are integrated.</P>