Conductance in Single DNA Molecules Directly Measured
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RESEARCH/RESEARCHERS
Fe-Based Amorphous Alloy Rods with 12-mm Diameter Developed Amorphous Fe-based alloys show several properties that are superior to those of their crystalline counterparts and structural steels. They have higher strength and hardness, superior corrosion resistance, and better magnetic properties, depending on the application. However, because of their limited glass-forming ability, until now it has only been possible to form rods with maximum dimensions in the range of 4 mm. If these alloys are to be used for structural applications, it is necessary to form much larger blocks of the material. It was recently shown that the addition of yttrium improves the glass-formability of Fe-based alloys. The larger Y atoms induce atomic-level stress that was shown to retard crystal phase formation, with the system left in a vitrified state on cooling from the melt. Two new studies have now used this principle to develop structural amorphous Fe alloys with maximum thickness diameters approaching 12 mm. V. Ponnambalam, S.J. Poon, and G.J. Shiflet of the University of Virginia reported the development of Fe-based bulk metallic glasses with thicknesses of more than 1 cm by alloying Y and Er in the May 2004 issue of the Journal of Materials Research (p. 1320). Z.P. Lu, C.T. Liu, J.R. Thompson, and W.D. Porter of Oak Ridge National Laboratory reported their work on developing amorphous Fe alloy rods with large diameters in the June 18, 2004, issue of Physical Review Letters (245503). The two studies go a long way in forging a path toward the ultimate goal of using amorphous Fe-based alloys for structural applications. Poon and colleagues used an Fe-Cr-MoC-B alloy system with 2% yttrium and erbium, a lanthanide, in the alloy (see Figure 1). The Y/Er-to-Fe atomic size ratio is about 1.4, which is close to the largest practically attainable value in Fe-based alloys. Samples were prepared by injecting the molten alloy into a copper mold under optimized casting conditions. Rods of up to 12 mm in diameter were found to remain amorphous, as confirmed by x-ray diffraction (XRD). The Young’s modulus of the material was ~200 GPa. with the bulk modulus in the range of 180 GPa. Tensile strength, based on microhardness measurements, was ~4 GPa, with hardness values of ~13 GPa. The research team hypothesized that the metastable Fe23C6 phase that forms upon devitrification of Fe-CrMo-C-B becomes less stable when alloyed with Y and Er. Lanthanide elements other than Er, such as Dy, Yb, and Gd, also yielded similar amorphous Fe alloys. For their investigations, Lu and colMRS BULLETIN/AUGUST 2004
Technology reported in the June 18 issue of Physical Review Letters (245504) that the addition of Y to a Cu-Zr-Al system was also shown to yield the formation of amorphous copper alloys with dimensions of up to 12 mm. GOPAL RAO
Conductance in Single DNA Molecules Directly Measured
Figure 1. X-ray diffraction pattern for an Fe-Cr-Mo-Er-C-B sample, revealing its amorphous nature. The inset shows a photo of (top) 10-mm-diameter and (bott
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