We investigate Cu-Zr liquid alloys using molecular dynamics simulation and well-accepted embedded atom method potentials over a wide range of chemical composition and temperature as model metallic glass-forming (GF) liquids. As with other types of GF materials, the dynamics of these complex liquids are characterized by "dynamic heterogeneity" in the form of transient polymeric clusters of highly mobile atoms that are composed in turn of atomic clusters exhibiting string-like cooperative motion. In accordance with the string model of relaxation, an extension of the Adam-Gibbs (AG) model, changes in the activation free energy ΔGa with temperature of both the Cu and Zr diffusion coefficients D, and the alpha structural relaxation time τα can be described to a good approximation by changes in the average string length, L. In particular, we confirm that the strings are a concrete realization of the abstract "cooperatively rearranging regions" of AG. We also find coexisting clusters of relatively "immobile" atoms that exhibit predominantly icosahedral local packing rather than the low symmetry packing of "mobile" atoms. These two distinct types of dynamic heterogeneity are then associated with different fluid structural states. Glass-forming liquids are thus analogous to polycrystalline materials where the icosahedrally packed regions correspond to crystal grains, and the strings reside in the relatively disordered grain boundary-like regions exterior to these locally well-ordered regions. A dynamic equilibrium between localized ("immobile") and wandering ("mobile") particles exists in the liquid so that the dynamic heterogeneity can be considered to be type of self-assembly process. We also characterize changes in the local atomic free volume in the course of string-like atomic motion to better understand the initiation and propagation of these fluid excitations.

The morphology of the dark and bright regions observed by transmission electron microscopy for the Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass strongly depends on the ion beam parameters used for ion milling. This indicates that the ion beam could introduce surface fluctuation to metallic glasses during ion milling.

Atomic structures of bulk glass-forming Cu 64.5 Zr 35.5 and the eutectic composition Cu 61.8 Zr 38.2 metallic glasses ͑MGs͒ have been studied by a combination of state-of-the-art experimental techniques and computational methods. Three-dimensional atomic configuration of the Cu 64.5 Zr 35.5 MG is established. It is found that icosahedronlike clusters are dominant in both MGs. However, icosahedronlike clusters centered by Cu atoms are slightly denser packing and less distorted in Cu 64.5 Zr 35.5 , which can enhance the glass forming ability ͑GFA͒ by suppressing atomic movements and increasing the structural incompatibility with competing crystalline phases. The atomic arrangements from short to medium-range order are envisaged and compared between both MGs.

When a material is heated, generally, it dilates. Here, we find a general trend that the average distance between a center atom and atoms in the first nearest-neighbor shell contracts for several metallic melts upon heating. Using synchrotron X-ray diffraction technique and molecular dynamics simulations, we elucidate that this anomaly is caused by the redistribution of polyhedral clusters affected by temperature. In metallic melts, the high-coordinated polyhedra are inclined to evolve into low-coordinated ones with increasing temperature. As the coordination number decreases, the average atomic distance between a center atom and atoms in the first shell of polyhedral clusters is reduced. This phenomenon is a ubiquitous feature for metallic melts consisting of varioussized polyhedra. This finding sheds light on the understanding of atomic structures and thermal behavior of disordered materials and will trigger more experimental and theoretical studies of liquids, amorphous alloys, glasses, and casting temperature effect on solidification process of crystalline materials.metal liquids | bond lengths | contraction T he study of metallic liquid structure is of importance because it is a fundamental issue in materials science and condensedmatter physics due to its critical role in understanding the processes of melting, solidification, and glass transition (1-6). Progress has been achieved in recent years both experimentally (7-24) and theoretically (13,(25)(26)(27)(28)(29)(30)(31)(32)(33). It is widely accepted that metallic liquids are composed of atomic clusters (7-33). However, how these clusters evolve upon external effects (e.g., temperature and pressure) still remains unclear (7-37). Generally, materials undergo thermal expansion and average atomic distance in the first shell increase upon heating. Here, we report a contraction of average atomic distance between a center atom and atoms in the first shell for metallic Al, Zn, Sn, In, Cu, Ni, Ag, and Au melts during heating. The thermal behaviors of metallic melts (pure elements and alloys) have been intensely studied, whereas the anomalous behavior of average atomic distance between a center atom and atoms in the first shell in liquids was usually ignored or not systematically evaluated (7-9, 21, 24). The anomalous behavior is focused upon and systematically investigated here by applying the state-of-the-art advanced synchrotron radiation-based experimental techniques and theoretical methods. Fig. 1 shows the pair correlation function g(r) at different temperatures for Al and Zn obtained by in situ high-temperature X-ray diffraction (XRD). Similar results for Sn and In metallic melts were also obtained in Fig. S1. The g(r) was obtained by Fourier transformation of the structure factor S(q) data, which reveals the average probabilities for finding atoms at a distance r for a given atom. In crystalline phases, atoms are located in discrete shells. However, in disordered structures they usually exhibit a broad distribution. The peak shape at various r values in g...

An intrinsic plastic Cu(45)Zr(46)Al(7)Ti(2) bulk metallic glass (BMG) with high strength and superior compressive plastic strain of up to 32.5% was successfully fabricated by copper mold casting. The superior compressive plastic strain was attributed to a large amount of randomly distributed free volume induced by Ti minor alloying, which results in extensive shear band formation, branching, interaction and self-healing of minor cracks. The mechanism of plasticity presented here suggests that the creation of a large amount of free volume in BMGs by minor alloying or other methods might be a promising new way to enhance the plasticity of BMGs.