Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 240

In metallic glasses (Chapter 7.10) bonding is directionless and SRO comprises clusters of atoms around 3 Å in diameter [11, 12]. Coordination numbers (CN) are between 10 and 11 – much greater than in directionally bonded glasses. With respect to crystalline metals, the CNs of metallic glasses exceed 8, the value for bcc structure (8), but fall short of 10, the CNs for fcc and hcp structures. Atomic cluster units in metallic glasses are around 5 Å apart, similar to interpolyhedral IRO distances in network glasses. The interatomic correlations between neighboring cluster units are identifiable out to around 15 Å (Figure 2), similar to the establishment of LRO in network glasses. In these densely packed metallic structures, however, the geometry of bond angles and dihedral angles and ring topology is absent. The sequence from IRO to LRO is usually collectively described as MRO [4], but is less well understood than in network glasses.

Furthermore, compared to supercooled network systems, where high shear viscosity and low atomic diffusion stem from the existence of open structures, the glass‐forming ability of densely packed metallic melts is imprecisely understood. In searching for melt compositions that are suitably viscous for conventional glass quenching, those associated with deep eutectics can be a guide, but not exclusively so [4, 23]. An overriding requirement, though, resides in achieving the highest atomic packing density in the supercooled state, which is often achieved by “dissolving” smaller atoms. The atomic size ratio for solute to solvent atoms, which yields the most efficient packing, is frequently found to be approximately 0.9 [4].

Figure 7 Simple two‐dimensional models of glass structure created from sparse (a) and dense (b) packing of spheres. (a) The continuous random network (CRN) model of Zachariasen [13] representing a network glass comprising threefold coordinated cations and bridging anions. Different ring sizes (3, 4, 5, 6, 7, 8) perpetuating extended range order are shown. (b) The dense random packing of atomic spheres (DRPHS) model of Bernal [25] showing variations in icosahedral packing, viz. 5, 6, and 7, that promote homogeneous noncrystalline extended range order.

For complicated high‐density geometries, different packing arrangements in metallic glasses are now modeled with computer simulations – mainly RMC, but also ab initio MD – the aim being to analyze the variety and number of different Voronoi polyhedra present [23]. For an atomic size ratio of 0.9, SRO is predominantly icosahedral, the geometry for tessellated quasicrystal structures. In glasses with pronounced chemical order and atomic size ratio lower than 0.9, pentagonal biprisms replace icosahedra as the dominant Voronoi polyhedra. Because they embody fivefold symmetry, icosahedrons and pentagonal biprisms frustrate crystalline close packing and represent the geometric counterparts to odd‐membered rings in directionally bonded glasses with open structures.

Metallic glasses, like network glasses, are less dense than their crystalline counterparts, the additional free volume being a throwback from the configurational diversity of the supercooled liquid. In contrast to that of network glasses, though, the FSDP in metallic glasses (Figure 3) is considered to derive from scattering from voids interspersed within the SRO of atomic clusters (Figure 7). In Ca–Mg–Cu glasses, Q_FSDP is, for example, about 1.2 Å⁻¹, and the FSDP correlation length about 5 Å (Figure 3). In multicomponent alloys Q_FSDP is affected by the size distribution of the different metals and its intensity by density. In Ni–Zr–Al, Q_FSDP is, for example, about 0.9 Å⁻¹ with a FSDP correlation length of about 7 Å.

Also found in metallic glasses, the excess THz modes at the onset of the VDOS and linear low‐temperature specific heat first observed in oxide glasses at THz frequencies [7] appear to have a common origin in the behavior of collective transverse acoustic modes at the Ioffe–Regel limit, where the phonon mean free path equates with its wavelength [19]. At this point vibrations no longer propagate, which suggests that LRO vibrations are localized. Demonstrated by computer simulation of Lennard‐Jones glass models, collective vibrations in these close‐packed structures replicate the scaling down of I_BP with density referred to earlier and the increase in ν_BP.