Electrochemical behavior of Ca-based bulk metallic glasses

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SINCE the late 1980s, a basic understanding of amorphous alloy design has led to the discovery of bulk metallic glasses (BMGs) in a variety of systems, including the Pd-, Pt-, Au-, Mg-, Zr-, Ti-, Hf-, La-, Cu-, Fe-, Co, Y-, and Ni-based systems. In addition to these alloy systems, Amiya and Inoue reported the formation of a new class of BMGs in the Ca-based system in 2002.[1,2] With the exception of the Mg-based alloys, this system is the only known nontransitionmetal-based system. To date, the numerous Ca-based bulkglass forming alloys that have been reported in the literature include Ca-Mg-Ag-Cu,[2] Ca-Al-Mg-Cu,[3] Ca-Mg-Cu,[1,4] Ca-Mg-Zn-Cu,[4] Ca-Mg-Al-Ag-Cu,[4] and Ca-Mg-Zn.[4,5] These BMG alloys are of interest because of their low densities and costs; however, few studies have examined the properties of these novel materials. In this study, the thermophysical properties of three Ca-based BMGs were examined. In addition, the electrochemical behavior was investigated and compared to a crystalline, Mg-based alloy. Cyclic-anodic polarization tests were conducted in a 0.05 M Na2SO4 electrolyte, and posttest analysis with scanning electron microscopy (SEM) was conducted to assess the damage to the exposed surfaces. I. EXPERIMENTAL METHODS Three Ca-based BMG alloys, with nominal compositions of Ca65Mg15Zn20, Ca55Mg18Zn11Cu16, and Ca50Mg20Cu30 (atomic percent, at. pct), were fabricated by induction melting of the pure elements (99.9 pct) in a water-cooled, copper crucible in an argon atmosphere. The prepared alloys were induction remelted in a quartz crucible with a 2-mm-diameter hole at the bottom, and the molten metal was injected through this hole into a water-cooled copper mold with a cavity of 15 mm  15 mm  4 mm. It was shown in previous work[6] that the critical thicknesses of the alloys, below which M.L. MORRISON, formerly Graduate Research Assistant with the Department of Materials Science and Engineering, The University of Tennessee, 434 Dougherty Engineering Building, Knoxville, TN 37996-2200, is now Senior Research Engineer with Smith & Nephew, Inc., 1450 Brooks Rd., Memphis, TN 38116. Contact e-mail: [email protected] R.A. BUCHANAN and P.K. LIAW, Professors, are with the Department of Materials Science and Engineering, The University of Tennessee, 434 Dougherty Engineering Building, Knoxville, TN 37996-2200. O.N. SENKOV, Senior Scientist, is with UES, Inc., Materials and Processes Division, 4401 Dayton-Xenia Road, Dayton, OH 45432. D.B MIRACLE, Senior Scientist, Materials and Manufacturing Directorate, is with the Air Force Research Laboratory Wright-Patterson AFB, OH 45433. Manuscript submitted August 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

they are fully amorphous, are 6 mm for the Ca65Mg15Zn20 (at. pct) and greater than 10 mm for the other two alloys. Therefore, it was expected that the 4-mm-thick plates produced for this study should also be fully amorphous. The amorphous state of the cast samples was verified using X-ray diffraction (XRD) and differential scanning calorim