High-Accuracy X-Ray Diffraction Analysis of Phase Evolution Sequence During Devitrification of Cu 50 Zr 50 Metallic Glas
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ALTHOUGH the investigation of glass formation in metallic systems has uncovered numerous systems with high glass forming ability, leading to the development of several so-called bulk metallic glasses (BMGs) with noteworthy mechanical, electrical, and chemical properties,[1–4] the effective application of these materials is somewhat hindered by our limited ability to control the complex phase transition dynamics that result in crystalline structures during solidification of a highly undercooled melt or devitrification of a glassy solid. Moreover, several recently reported examples exist of amorphous-crystalline composite (ACC) materials that illustrate the great potential value of controlled devitrification as a means to obtain novel structures of metastable phases.[5–7] In such cases, in situ and ex situ methods have been investigated with respect to their utility in enabling control of phase selection and composite structure.[8–10] Here, we focus on the devitrification behavior in Cu-Zr, a binary system upon which several BMG alloys are based. Of course, reliable prediction and control of phase selection and structural dynamics requires quantification of system thermodynamics as well as an understanding of the kinetic mechanisms involved in solidification/devitrification processes, and advances in the thermodynamics and I. KALAY, Ph.D. Candidate, is with the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011. M.J. KRAMER, Senior Scientist, is with the Ames Laboratory, U.S. Department of Energy, Ames, IA 50011. R.E. NAPOLITANO, Associate Professor, is with the Department of Materials Science and Engineering, Iowa State University, is also a Scientist with Ames Laboratory, U.S. Department of Energy. Contact e-mail: ralphn@ iastate.edu Manuscript submitted April 12, 2010. Article published online November 24, 2010 1144—VOLUME 42A, MAY 2011
structure of Cu-Zr liquids, glasses, and crystalline phases recently have been reported.[11–13] Indeed, with numerous stable and metastable crystalline phases, this system offers an immense spectrum of potentially accessible structures and properties and has attracted much attention related to BMGs,[14–16] crystallization structures,[17–19] and ACC materials.[20,21] A review of prior thermodynamic treatments for the Cu-Zr system was reported recently, and the assessed phase diagram is shown in Figure 1.[22] The crystalline phases observed in this system are summarized in Table I. We focus the present investigation on the Cu50Zr50 composition, for which several stable and metastable phases have been observed upon solidification from the melt and devitrification from the glass.[17– 19] The first systematic study of crystallization from the glass in this alloy was reported by Freed et al.,[46] who used X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) to investigate the crystallization of splat quenched glassy alloy, reporting primary crystallization involving an unidentified metastable phase. Usi
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