Reverse Monte Carlo structural model for a zirconium-based metallic glass incorporating fluctuation microscopy medium-ra
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We used reverse Monte Carlo (RMC) modeling to simulate the atomic structure of a Zr-based bulk metallic glass (BMG), incorporating short-range structural data from the electron diffraction total reduced density function G(r) and medium-range structural data from fluctuation electron microscopy (FEM). Including the FEM data created within the model loosely ordered planar atomic arrangements covering regions 1 nm in diameter without degrading the agreement with G(r). RMC refinement against only G(r) produced no agreement with FEM. Improved simulations are needed to create fully realistic BMG structures, but these results show that including FEM in RMC further constrains the structure compared with G(r) data alone and that the FEM signal in real materials is likely to arise from pseudo-planar arrangements of atoms.
I. INTRODUCTION
Understanding the atomic structure of bulk metallic glasses (BMGs) at the nanometer scale is necessary to understanding their materials properties and transformations. Structural relaxation is one example of a phenomenon, the structural underpinnings of which are not understood. Differential scanning calorimetry detects significant heat flow during relaxation,1 but the accompanying change in short range order (SRO) measured, for example, by the total reduced density function, G(r), from neutron diffraction, is almost undetectable.2 Another phenomenon is inhomogeneous flow of BMGs, which occurs by activation of shear transformation zones (STZs).3–5 Mechanics measurements have shown that STZs consist of a few tens to hundreds of atoms and are a few nanometers in diameter,3,4 but their structure before activation and the structural change of activation are not known. Finally, in some theories of the glass transition, there is a divergent structural correlation length.6,7 Sheng et al.8 have suggested based on simulations of a Zr-based BMG that icosahedral nearest-neighbor clusters percolate to the nanometer scale and beyond as the temperature drops toward Tg, but there is only limited experimental evidence for or against this hypothesis.9,10 Despite recent advances in understanding of the atomic structure of metallic glasses,8,11 their nanometer-scale structure, called medium-range order (MRO), remains elusive, at least in part because of a lack of good experimental data. G(r) (or other varieties of pair distribution a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0386 J. Mater. Res., Vol. 24, No. 10, Oct 2009
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function),12 have been used extensively to study the structure of metallic glass,2,8,13–15 but it is limited to SRO within 3 coordination shells because of an inherent statistical sampling problem in isotropic materials.16 Chemically sensitive probes such as extended x-ray absorption fine structure (EXAF)17 or resonant x-ray scattering18 can, in some systems, extend the physical length scale covered by measuring three chemically selected coordination shells, but these functions still
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