Pair Distribution Function Analyses of Structural Relaxation in a Zr-Based Bulk Metallic Glass
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Pair Distribution Function Analyses of Structural Relaxation in a Zr-Based Bulk Metallic Glass Mark L. Morrison1, Wojtek Dmowski1, Timothy W. Wilson1, Peter K. Liaw1, Chain T. Liu2, James W. Richardson3, Evan R. Maxey3, Raymond A. Buchanan1, Cang Fan1, Hahn Choo1, Takeshi Egami1, and Wallace D. Porter2 1. Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200, USA 2. Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376, USA 3. Intense Pulsed Neutron Source Division, Argonne National Laboratory, Argonne, IL 604394814, USA ABSTRACT Zr-based alloy ingots with nominal compositions of Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 (at.%), Vitreloy 105, were isothermally annealed below the glass-transition temperature at 630 K for 10, 20, 30, 40, and 60 minutes in vacuum to obtain samples with various states of structural relaxation and compared to the as-cast state. Structural studies were performed using time-of-flight neutron diffraction followed by pair distribution function (PDF) analyses. Differential scanning calorimetry (DSC) was conducted to examine changes in the specific heat, which were correlated to the amount of structural relaxation in the various samples. These samples exhibited increasing structural relaxation with longer annealing times, which was evidenced in the atomic PDF. Relaxation related to the exothermic peak results in changes in the PDF that are consistent with the elimination of short and long inter-atomic distances. Further annealing led to rearrangements in the second atomic shell that may be related to local phase separation. INTRODUCTION Historically, metallic glasses were obtained by rapid quenching from a melt with cooling rates on the order of 105-106 K/s. Bulk metallic glasses (BMGs) were originally developed in the 1990’s [1,2,3,4] and have been the focus of intensive research among multiple groups around the world. These alloys have good glass-forming ability with low critical cooling rates on the order of 1-100 K/s. They exhibit a unique collection of promising properties such as near-net-shape formability [5,6,7], high strength [8], high elasticity [9,10], and good corrosion resistance [11,12], among others. In addition, BMGs can also be utilized as a precursor to nanostructured materials [13,14]. Upon cooling through the glass-transition temperature (Tg), the viscosity of the BMG melt increases by several orders of magnitude, and the supercooled liquid forms a glass. The resultant glass is metastable: it can transform to the crystalline phase but also can undergo subtle structural changes if annealed at low temperatures. The latter effect is called structural relaxation, which can affect the elastic, magnetic, and electrochemical properties, in addition to others. This relaxation phenomenon can be directly observed through structural studies using neutron, X-ray, or electron scattering [15]. Since the cooling rates for BMGs are so low, one would expect that annealing at a low-temperature would have a small eff
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