Beyond packing of hard spheres: The effects of core softness, non-additivity, intermediate-range repulsion, and many-body interactions on the glass-forming ability of bulk metallic glasses

Abstract

When a liquid is cooled well below its melting temperature at a rate that exceeds the critical cooling rate Rc, the crystalline state is bypassed and a metastable, amorphous glassy state forms instead. Rc (or the corresponding critical casting thickness dc) characterizes the glass-forming ability (GFA) of each material. While silica is an excellent glass-former with small Rc < 10 −2 K/s, pure metals and most alloys are typically poor glass-formers with large Rc > 10 10 K/s. Only in the past thirty years have bulk metallic glasses (BMGs) been identified with Rc approaching that for silica. Recent simulations have shown that simple, hard-sphere models are able to identify the atomic size ratio and number fraction regime where BMGs exist with critical cooling rates more than 13 orders of magnitude smaller than those for pure metals. However, there are a number of other features of interatomic potentials beyond hard-core interactions. How do these other features affect the glass-forming ability of BMGs? In this manuscript, we perform molecular dynamics simulations to determine how variations in the softness and non-additivity of the repulsive core and form of the interatomic pair potential at intermediate distances affect the GFA of binary alloys. These variations in the interatomic pair potential allow us to introduce geometric frustration and change the crystal phases that compete with glass formation. We also investigate the effect of tuning the strength of the many-body interactions from zero to the full embedded atom model on the GFA for pure metals. We then employ the full embedded atom model for binary BMGs and show that hard-core interactions play the dominant role in setting the GFA of alloys, while other features of the interatomic potential only change the GFA by one to two orders of magnitude. Despite their perturbative effect, understanding the detailed form of the intermetallic potential is important for designing BMGs with cm or greater casting thickness.

Models and Algorithms for the Next Generation of Glass Transition Studies

“…This functional form ensures that the volume fraction occupied by particles within a given bin size is constant. Such a scaling property has been shown to enhance glass-forming ability in discrete mixtures [57], but we have not tested this hypothesis in great detail for the present systems. Finally, we introduce a second type of continuous particle size distributions, which combine the salient features of both discrete and continuous mixtures.…”

“…Our first analysis of models with continuous polydispersity concerns the role of the particle softness. Previous studies have found that softness can have a nontrivial impact on glass properties, such as fragility [71] and glass-forming ability [57]. Here we simulated polydisperse soft particles varying the softness exponent n using the values n = 8, 12, 18, 24.…”

The correlation between fragility, density, and atomic interaction in glass-forming liquids

“…It will also be interesting to investigate systematically how the varying repulsive steepness of interaction affects fragility when the strength of attraction is held fixed as studied in Ref. 38, and how the varying length and strength of attraction influence fragility with the repulsive steepness of interaction unchanged as done in Ref. 39.…”

Origin of Noncubic Scaling Law in Disordered Granular Packing

“…For metallic glasses, since there must be other important parameters (stiffness of potential, covalent bonding, etc. ), which go beyond the hard-sphere picture and thus will influence this exponent [22]. Also, the rather high temperature at which the scaling law is normally probed in metallic glasses could also influence the exponent.…”

“…In this work, we provide microscopic insight to this problem by studying the three-dimensional packing of spherical granular particles, which is a prototypical hard-sphere glass former and has long been considered as a structural model for metallic glasses [20][21][22]. We identify a non-cubic scaling law in our system, and provide evidences that its origin is local, i.e., without resorting to any fractal structures.…”