Medium-range Order of Zr 54 Cu 38 Al 8 Bulk Metallic Glass
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Medium-range Order of Zr54Cu38Al8 Bulk Metallic Glass Pei Zhang,1 M. F. Besser,2 M. J. Kramer,2 and P. M. Voyles1 1
Materials Science and Engineering, University of Wisconsin, Madison, Madison, WI 53711
2
Ames Laboratory, Ames, IA 50011
ABSTRACT We have previously reported, based on fluctuation electron microscopy (FEM) data, that Zr50Cu45Al5 bulk metallic glass (BMG) contains significant icosahedral and crystal-like mediumrange order. Here, we report similar finding for Zr54Cu38Al8 BMG, which is a poorer glass former. Like Zr50Cu45Al5, Zr54Cu38Al8 contains icosahedral and crystal-like structures. In the ascast state, the crystal-like peak in the FEM data is stronger than icosahedral-like peak. After annealing at 0.83Tg (573 K), the icosahedral-like peak increases, but, unlike Zr50Cu45Al5, the crystal-like peak does not decrease. This tendency toward stronger, more thermally stable crystal-like order may be associated with the poorer glass forming ability of Zr54Cu38Al8. INTRODUCTION Medium range order (MRO), which occupies the length scale between short range order (SRO) and long-range order, plays an important role on the properties of metallic glass. Molecular dynamics simulations of Zr-based bulk metallic glasses (BMGs) often contain MRO comprised of connections between icosahedral or near icosahedral SRO clusters [1-3]. That icosahedral-like MRO is related to the plasticity of metallic glass [4] and the glass transition behavior [5]. In simulations of other glass-forming systems, Frank-Kaspar local order [6] or planar structures [7] have been observed. We use fluctuation electron microscopy (FEM) to measure MRO in glasses [8]. Compared with the X-ray and neutron which can measure SRO by two-body correlation functions, FEM is an effective way to detect MRO structure through three and four-body correlation functions [913]. In FEM experiments, the statistically normalized variance V is calculated from the spatiallyresolved diffracted intensity I, measured using electron nanodiffraction in a scanning transmission electron microscope (STEM) as a function of scattering vector magnitude k and the coherent spatial resolution R. (
)
( (
) )
where values within are averaging over the position on the sample. The spatial resolution R, which is typically 1-2 nm, can be defined by the Raleigh Criteria, R=0.61/Q [14], in which Q is the radius of the virtual objective aperture in the STEM. The position in k of peaks in V(k) is related to the interatomic distances inside MRO clusters of BMG and the magnitude of V(k) is associated with the degree of MRO, including the size, volume fraction, and degree of structural perfection of MRO clusters [15]. We have recently combined experimental data from fluctuation electron microscopy (FEM) and information from an empirical interatomic potential [3] in a hybrid reverse Monte Carlo
study of MRO in Zr50Cu45Al5 BMG [16]. We observed both icosahedral-like and crystal-like SRO clusters which organize with similar clusters to form icosahedral-like and crysta-like MRO [16]. The cryst
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