Five-fold symmetry as indicator of dynamic arrest in metallic glass-forming liquids

Abstract

With sufficient high cooling rates, a variety of liquids, including metallic melts, will cross a glass transition temperature and solidify into glass accompanying a marked increase of the shear viscosity in approximately 17 orders of magnitude. Because of the intricate atomic structure and dynamic behaviours of liquid, it is yet difficult to capture the underlying structural mechanism responsible for the marked slowing down during glass transition, which impedes deep understanding of the formation and nature of glasses. Here, we report that a universal structural indicator, the average degree of five-fold local symmetry, can well describe the slowdown dynamics during glass transition. A straightforward relationship between structural parameter and viscosity (or α-relaxation time) is introduced to connect the dynamic arrest and the underlying structural evolution. This finding would be helpful in understanding the long-standing challenges of glass transition mechanism in the structural perspective.

Computational modeling sheds light on structural evolution in metallic glasses and supercooled liquids

“…Examples include a structural signature of dynamical slowdown as well as the liquid fragility in metallic glass-forming liquids, [26][27][28][29][30][31][32] anomalous thermal contraction of metallic melts in the nearest-neighbor shell, [33][34][35] possible crystallike order in MGs/liquids, [36][37][38][39] a fractal structure model of MGs, [40][41][42] advanced algorithms (such as machine learning methods) to efficiently characterize the structural basis of flow defects and dynamical slowdown in amorphous materials, 43,44 and a new structure parameter that incorporates dynamic (atomic vibration) information beyond the description of static structure/ configuration. [45][46][47][48][49] Computational research has played a key role in formulating these ideas.…”

“…3c) ii) Degree of five-fold local symmetry: Closely related to (i) above is the structural parameter W, defined as the average degree of fivefold local symmetry for the coordination polyhedra at the short-range scale. 27 In this approach, instead of tracking a characteristic SRO around certain species in an alloy, Hu et al 27 track the population of all the fivefold bonds in the alloy such that the metric can be applied to any alloy composition, including those for which icosahedral SRO is not preferable and those where obviously dominant SRO motifs have not been identified. Of course, this approach is expected to yield the same trend as the SRO parameter in (i) above, because any preferable SRO (motif) would be rich in local fivefold topology (the third digit of the Voronoi index is maximized for the coordination number in question).…”

“…27 iii) First peak height in the structure factor, S(q): Mauro et al 28,29 have studied the temperature dependence of the height of the first peak in S(q), as a measure of local structural order developed. This maximum peak intensity evolves with undercooling.…”

Study of Glass Forming on Cu60.0Zr32.5Ti7.5 Alloy by Molecular Dynamics Simulation

“…These high cooling rates produce essentially the same glassy structure. Probably the main differences are the amount of free volume (less relaxed state) 35 and the amount of icosahedral-like clusters 36 and a higher energetic state, which is more prone to present relaxation. Figure 2 displays the evolution of Total EnergyTemperature (TE-T) curves of Cu 60.0 Zr 32.5 Ti 7.5 alloy during heating-up and cooling-down steps to determine the phase transition temperatures.…”

“…The viscosities were evaluated at different temperatures higher than glass transition and were fitted using the Vogel-Fulcher-Tamman (VFT) relationship 45 . The fragility was determined by using the following expression: According to the concept proposed by Angell 46 , the liquids can be classified within the "strong" and "fragile" extremes using T g as a scaling parameter 36,47 . The fragility is a quantitative description of diverse kinetic behaviors, and it quantifies the viscosity dependence on temperature.…”