Metallic glasses: Family traits.

The disordered atomic structure of metallic glasses gives rise to unique mechanical and physical properties such as strength and elasticity, and are being considered for a wide range of applications. However, to understand and ultimately control these properties, it is important to study their disordered structure. Seen on the macroscale, metallic glasses are isotropic and homogeneous. On the microscopic scale, however, it is clear the situation is far more complex. There is a short-range order on the atomic scale with clusters of solute atoms surrounded by a majority of solvent atoms. There is also a nanoscale medium-range order that can be modelled by highly structured superclusters consisting of interconnected smaller clusters1,2. Both shortand medium-range orders affect the properties of metallic glasses, but they are extremely difficult to distinguish. The way the atoms pack inside metallic glasses and the role the local atomic order plays in their deformation remains a mystery; our understanding of the glassy structure so far relies to a large extent on models and simulations. In one of the few experimental studies on this topic, Dong Ma and colleagues now show how the metallic glasses inherit the elastic moduli from their solvent components3. Elastic modulus describes the reversible shape change of solids under an applied stress. For example, the shear modulus is related to the strain response of a solid under shear stress and represents its shear strain resistance. The most commonly used elastic modulus in engineering is Young’s modulus, named after the nineteenthcentury British scientist Thomas Young, and is defined as the ratio of uniaxial stress to strain in the solid’s elastic regime. Microscopically, both types of moduli reflect the inherent stiffness of atomic bonds4–6. To explore the structural origin of the moduli in metallic glasses, Ma and colleagues carried out an in situ neutron diffraction study on an elastically deformed metallic glass to determine the response of the local atomic structures to stress. By comparing the Young and shear moduli of various metallic glasses with their base elements (that is, their solvent components), they found that the moduli of the glasses were almost equal to their solvents (Fig. 1a). This indicates that the elastic moduli of the metallic glasses are primarily determined by their solvent metals and that it is the solvents that are responsible for the overall stiffness and rigidity. This is surprising, because the base element normally makes up only about half of such glasses. Furthermore, careful data analysis enables the decomposition of the data into a strain-sensitive part that is related to medium-range order (the superclusters), and a strain-insensitive part related to short-range order (the solute-centred clusters). They further demonstrate that elastic deformation in metallic glasses mainly occurs at the solvent–solvent junctions among solute-centred clusters and/or superclusters, and that the moduli are essentially determined by the weakest solvent–solvent bonding. This implies that metallic glasses have a rubberlike structure, which can be viewed as consisting of stiff solute-centred clusters (similar to the molecular units in rubber) and much weaker solvent–solvent bonds linking the clusters, as shown in Fig. 1b. The experimental results support the so-called random-cluster-packing model for metallic glasses, and furthermore reveal the hierarchical atomic bands and inhomogeneous microstructure of metallic glasses. However, although this represents an important step towards revealing the structural secret of metallic glasses, caution needs to be taken. For some systems such as Pd-, Cuand Co-based metallic glasses, their moduli are markedly different from their base elements (Fig. 1a). Nevertheless, such deviations could help to understand different glass structural features. For example, the moduli of Cu60Zr20Hf10Ti10 glass are not close to those of its base element Cu, but is close to those of Zr, indicating that Zr and not Cu is its solvent6. The findings also provide valuable insight into the structural origin of the superior elasticity of metallic glasses. The conventional theory on the elasticity in glasses is that it occurs uniformly by straining the material on all length scales. However, as Ma and colleagues now show, not every component in a metallic glass contributes equally to the overall modulus (Fig. 1b). Only the least stiff spring (the weakest solvent–solvent bonds) accommodates the strain, and the elastic deformation is inhomogeneous. The elastic moduli also have strong relations with the broader mechanical METALLIC GLASSES