According to the theory of Turnbull and Cohen,  certain open volume or space in glass-forming liquids exist around each atom. When this volume reaches a critical value, less energy is needed to move the atom in and out of it, making possible diffusion and flow. Such a space or volume is called “free volume” (FV). For instance, Liu et al.  proposed that the atomic structure of Zr2Ni metallic glass is essentially an association of ordered clusters and FV. The ordered clusters in size of ~ 1.5 nm consist of a densely packed core (i.e. icosahedral or fcc-type packing) and are surrounded loosely by large FV.
The method based on FFT and inverse FFT developed by Miller and Gibson  and extended by Li et al.  and Jiang and Atzmon  later was applied to analyze the SRO (short range ordering) clusters by Zhu et al. . They employed a core–shell model to investigate the shear bands in as-cast Zr64Nb6Cu13.5Ni8.5Al8 specimen (Figure 1698). In Figure 1698 (b), the bright-yellow part represents the zone with more free volume
(FV), while the dark-blue part represents the zone with less FV. The core is proposed as SRO clusters with a different coordination number (CN)  and the local FV is shell. It is defined that SRO clusters with CN of 12 or more than 12 have no FV, while ordering clusters with CN < 12 have certain FV .
Figure 1698. (a) HRTEM image of an as-cast sample for Zr64Nb6Cu13.5Ni8.5Al8 alloys (the inset presents fast Fourier transform (FFT) pattern), and (b) The corresponding Fourier-filtered, threshold filtered and inverted image. Adapted from 
The concept of defects in metallic glasses is suggested in free-volume theory. In this theory, a defect is defined as a site at which the free volume exceeds a critical value that is on the order of an atomic volume.
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