Metastable States During Devitrification of a Zr 70 Pd 20 Cu 10 Metallic Glass
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Metastable States During Devitrification of a Zr70Pd20Cu10 Metallic Glass M. J. Kramer and D. J. Sordelet Metal and Ceramic Sciences Program, Ames Laboratory (USDOE) Department of Materials Science and Engineering, Iowa State University Iowa State University Ames, Iowa 50011-3020, USA ABSTRACT High energy synchrotron x-rays (124.63 keV) are used to investigate the initial stages of the devitrification for the Zr70Pd20Cu10 metallic glass prepared by melt-spinning (MS). Due to the excellent signal:noise, we are able to determine the initial nucleating phase by analyzing the differences in the total scattering function S(Q) as a function of time at a temperature ~ 50 K below the crystallization temperature. The alloy undergoes a structural relaxation prior to nucleation and growth. Devitrification proceeds from nucleation of the icosahedral phase. The differential pair distribution function (dPDF) indicates that the as-quenched alloy may have icosahedral-like order which undergoes local rearrangement to true icosahedral order at an annealing temperature 50 K below the crystallization temperature. More importantly, time-resolved high-energy synchrotron is shown to have excellent sensitivity to the initial atomic rearrangements preceding nucleation in metallic glasses. INTRODUCTION Within the large number of systems that form metallic glasses, several Zr based alloys are unique in that over a large range of compositions, a meta-stable quasicrystalline phase forms as the primary devitrification product [1-6]. The formation of a meta-stable quasicrystal phase from the glass is intriguing since it begs the question whether icosahedral clusters are present in the liquid, as suggested by Frank over 50 years ago [7] and more recently shown in an undercooled melt [8]. Secondly, the parent compound Zr2M, where M can be a variety of late transition elements, provides an ideal system to study the role that atomic size, competing phase assemblages and minor elements such as oxygen have on the formation of metallic glasses [9-13]. Finally, these Zr-based systems are ideal for the study of the role of nucleation and diffusion in multi-component alloys [14,15]. Therefore, the study of the formation, thermal stability and solute effects of these alloys will be necessary to resolve some of the critical issues surrounding the formation of metallic glasses. It was recently proposed that the amorphous Zr66Pd33 alloy undergoes a polymorphic transition to the quasicrystalline phase [6]. In an earlier paper using High Energy X-ray diffraction (HEXRD) coupled using Reverse Monte Carlo (RMC) simulations we demonstrated that the SRO of an amorphous Zr70Pd30 alloy developed during mechanically milling (MM) by high-energy ball milling is distinctly different than the SRO developed in the amorphous alloy produced by melt spinning (MS) [16]. The partial pair distributions based on Reverse Monte Carlo simulations (RMS) suggest that
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both alloys have short Pd-Pd bonds, intermediate Zr-Pd and longer Zr-Zr bonds in the
