[en] A general statistical model is proposed for describing network-forming systems. The model is based on the representation of the partition function for all possible configurations of a thermoreversible network in the form of a functional integral over a scalar field. According to this model, two types of first-order phase transitions can occur in the systems under consideration: macroscopic phase separation with the structural phase transition due to the change in the configuration of the spatial network and the sol-gel transition due to the formation of a thermoreversible percolation cluster consisting of bound structural units. A detailed analysis is performed of the thermodynamic and structural properties of a solution of monomers that have f functional groups and can form thermoreversible chemical bonds. The influence of specific features of the chemical and volume interactions on the phase diagram of the system is investigated. The mutual position of the sol-gel transition line and the phase diagram is determined for different model parameters. It is revealed that two substantially different regimes of the behavior of the sol-gel transition line in the 'temperature-volume fraction of structural units' plane are observed with a change in the rigidity of chemical bonds.

[en] An efficient description of the structures of liquids and, in particular, the structural changes that happen with liquids on supercooling remains to be a challenge. The systems composed of soft particles are especially interesting in this context because they often demonstrate non-trivial local orders that do not allow to introduce the concept of the nearest-neighbor shell. For this reason, the use of some methods, developed for the structure analysis of atomic liquids, is questionable for the soft-particle systems. Here we report about our investigations of the structure of the simple harmonic-repulsive liquid in 3D using the triple correlation function (TCF), i.e., the method that does not rely on the nearest neighbor concept. The liquid is considered at reduced pressure (P = 1.8) at which it exhibits remarkable stability against crystallization on cooling. It is demonstrated that the TCF allows addressing the development of the orientational correlations in the structures that do not allow drawing definite conclusions from the studies of the bond-orientational order parameters. Our results demonstrate that the orientational correlations, if measured by the heights of the peaks in the TCF, significantly increase on cooling. This rise in the orientational ordering is not captured properly by the Kirkwood’s superposition approximation. Detailed considerations of the peaks’ shapes in the TCF suggest the existence of a link between the orientational ordering and the slowdown of the system’s dynamics. Our findings support the view that the development of the orientational correlations in liquids may play a significant role in the liquids’ dynamics and that the considerations of the pair distribution function may not be sufficient to understand intuitively all the structural changes that happen with liquids on supercooling. In general, our results demonstrate that the considerations of the TCF are useful in the discussions of the liquid’s structures beyond the pair density function and interpreting the results obtained with the bond-orientational order parameters. (paper)

[en] In contrast to the generally accepted practice for searching metallic glassformers by trial-and-error quenching, we propose using density and coefficient of thermal expansion of as-cast polycrystalline alloys as indicators of the glass-forming ability. We study experimentally these characteristics for Cu_100−xZr_x alloys over the whole concentration range and a wide temperature range from room temperature up to 1120 K. In the concentration range of 35 < x < 55 at% corresponding to high glass-forming ability, strongly non-monotonous behavior of both quantities is observed. We reveal that location of the extrema on concentration dependencies of both the coefficient of thermal expansion and the density deviation from the additive law correlates with those for critical thickness of Cu–Zr amorphous alloys. We argue that correlation between glass-forming ability and anomalous behavior of thermal expansion is because as-cast polycrystalline samples of glass forming alloys possess highly distorted structure, which cause phonon anharmonicity. Our findings propose new express method to search metallic alloys with high glass-forming ability. (paper)

No abstract available

[en] The structural and dynamic properties of the three-component Zr_4_7Cu_4_6Al_7 system are subjected to a molecular dynamics simulation in the temperature range T = 250–3000 K at a pressure p = 1.0 bar. The temperature dependences of the Wendt–Abraham parameter and the translation order parameter are used to determine the glass transition temperature in the Zr_4_7Cu_4_6Al_7 system, which is found to be T_c ≈ 750 K. It is found that the bulk amorphous Zr_4_7Cu_4_6Al_7 alloy contains localized regions with an ordered atomic structures. Cluster analysis of configuration simulation data reveals the existence of quasi-icosahedral clusters in amorphous metallic Zr–Cu–Al alloys. The spectral densities of time radial distribution functions of the longitudinal (C̃_L(k, ω)) and transverse (C̃_T(k, ω)) fluxes are calculated in a wide wavenumber range in order to study the mechanisms of formation of atomic collective excitations in the Zr_4_7Cu_4_6Al_7 system. It was found that a linear combination of three Gaussian functions is sufficient to reproduce the (C̃_L(k, ω)) spectra, whereas at least four Gaussian contributions are necessary to exactly describe the (C̃_T(k, ω)) spectra of the supercooled melt and the amorphous metallic alloy. It is shown that the collective atomic excitations in the equilibrium melt at T = 3000 K and in the amorphous metallic alloy at T = 250 K are characterized by two dispersion acoustic-like branches related with longitudinal and transverse polarizations.

[en] We address a relationship between properties of liquid and solid states by comparing structural characteristics and viscosity in Al–Cu–Fe and Al–Cu–Ni melts. The former system forms an equilibrium quasicrystalline phase but the latter does not. We show that the concentration behavior of the viscosity, melting temperature and characteristics of the chemical short-range order correlate with each other. The main structural differences between the melts are related to the peculiarities of their electronic structure, which is the same for liquid and solid states near the melting temperature. (paper)

[en] Using molecular dynamics simulations with embedded atom model potential, we study structural evolution of Cu_64.5Zr_35.5 alloy during the cooling in a wide range of cooling rates γ ∈ (1.5 ⋅ 10⁹, 10¹³) K/s. Investigating short- and medium-range orders, we show that the structure of Cu_64.5Zr_35.5 metallic glass essentially depends on cooling rate. In particular, a decrease of the cooling rate leads to an increase of abundances of both the icosahedral-like clusters and Frank-Kasper Z16 polyhedra. The amounts of these clusters in the glassy state drastically increase at the γ_min = 1.5 ⋅ 10⁹ K/s. Analysing the structure of the glass at γ_min, we observe the formation of nano-sized crystalline grain of Cu₂Zr intermetallic compound with the structure of Cu₂Mg Laves phase. The structure of this compound is isomorphous with that for Cu₅Zr intermetallic compound. Both crystal lattices consist of two types of clusters: Cu-centered 13-atom icosahedral-like cluster and Zr-centered 17-atom Frank-Kasper polyhedron Z16. That suggests the same structural motifs for the metallic glass and intermetallic compounds of Cu–Zr system and explains the drastic increase of the abundances of these clusters observed at γ_min.