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Icosahedral short-range ordering (ISRO) has been considered the main building block of supercooled liquid and metallic glasses [1–4]. For instance, Liu et al. [9] proposed that the atomic structure of Zr2Ni metallic glass is essentially an association of ordered clusters and FV (free volume). 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.
Figure 1703a shows XRD patterns of Zr70-xNbxCu13.5Ni8.5Al8 (x=0, 2, 4, 6, 7, 8, 10) alloys. The broad peaks in the patterns of the samples with Nb content <8 at.% indicate the samples are in amorphous phase. The patterns of the samples with 8 and 10 at.% Nb present an intense double peak and several weak peaks that is indexed in icosahedral phase (I-phase) [6–8].

Figure 1703a. XRD patterns of the as-cast Zr–Nb–Cu–Ni–Al alloys. [5]
Figure 1703b shows a TEM image taken from a Zr62Nb8Cu13.5Ni8.5Al8 specimen. The particles were identified as I-phase. The inset shows SAED patterns taken from three different particles, presenting two-, three- and fivefold symmetry, respectively.

Figure 1703b. A TEM image taken from a Zr62Nb8Cu13.5Ni8.5Al8 specimen. Adapted from [5]
Figure 1703c (a) shows the CN (coordination number) dependence on the FV (free volume) in the Zr2Ni metallic glass (MG). FV increases statistically with decrease of the CN even although the FV is also related to other factors, e.g. atomic size. On the other hand, the atomic configuration of SROs (short range orderings) strongly depends on the CN. In the case of the Zr2Ni MG, the configuration with CN=12 is icosahedral (ICO) or face-centered-cubic (fcc) type SRO with no FV, CN=10 the body-centered-cubic (bcc) or body-centered tetragonal (bct) type, and CN=6 the trigonal prism (TP) as indicated by the insets. Figure 1703c (b) shows a representative ~1.5 nm cluster (including 64 atoms) taken from a densely packed region. The CN decreases gradually from the interior to the exterior area while the FV increases. The red octagons show schematically the distribution of the FV. The yellow circle reveals the core area with no FV.

Figure 1703c. (a) The CN (coordination number) dependence on the FV (free volume) in Zr2Ni metallic glass (MG), and (b) Example of CN and FV distributions. Adapted from [10]
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