On the Structural Characterization of a Series of Novel Ni-Nb-Sn Refractory Alloy Glasses
- PDF / 725,103 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 64 Downloads / 171 Views
MM9.5.1
On the Structural Characterization of a Series of Novel Ni-Nb-Sn Refractory Alloy Glasses Michelle L. Tokarz1 and John C. Bilello1,2 1 Center for Nanomaterials Science, Department of Materials Science and Engineering; University of Michigan, Ann Arbor, MI 48109 2 Department of Materials Science California Institute of Technology, Pasadena, CA 91125 ABSTRACT Recently refractory alloy glasses of varying Ni, Nb and Sn concentrations were prepared and studied via several characterization methods, including x-ray diffraction via standard lab and synchrotron radiation sources, SEM, and other complementary techniques. A comparison between x-ray diffraction results obtained from synchrotron sources vs. standard lab sources shows the necessity of a low-divergence source in order to distinguish nanoscale crystallites present within an amorphous matrix. The divergence of both sources was determined by comparing the diffraction patterns of a LaB6 standard and noting the deviation from ideal or theoretical Bragg peaks. The crystallites in these glasses comprised between 0 and 7.5 percent by volume, depending upon the specific composition. The results presented here also show a very good sample-to-sample consistency for any given alloy composition. While xray diffraction results give information about the average structure, SEM was also performed to understand aspects of individual crystallite size and distribution. By studying results of varying alloy concentrations, it is seen that a very small composition range exists for nearperfect glass formers (as defined by a near zero percentage crystallinity). Radial distribution analysis was also performed for each composition and compared to hard sphere models for each alloy composition. This analysis indicated the presence of intermediate range order beyond the first nearest neighbors as indicated by the divergence of the experimental reduced radial distribution functions from that predicted by the accompanying hard sphere models. INTRODUCTION The successful production of bulk metallic glasses presents opportunity for materials with much increased stiffness over traditional crystalline alloys in several applications. Initially glassy metals were processed via a “splat-cooling” method whereby the melted components were catapulted onto a cooled surface in order to bypass the critical cooling rate for glass formation1,2 (~105 K/s). This method nominally produced “ribbons” of 40-60 µm thicknesses. More recent research on bulk amorphous alloys has resulted in materials that can be potentially injection molded into parts with fine dimensional detail and thicknesses up to 3mm3,4. This is accomplished by selecting component mixtures that are characterized by “deep eutectics” and including constituents of varying atomic sizes of the alloy constituents, which allows one to utilize the favorable kinetics of glass formation over the thermodynamically favored eutectic structure. In this way, an alloy with a much lower critical cooling rate (~ 1K/s) is created, thus permitting the large-s
Data Loading...
