The issue, composition dependence of glass-forming ability (GFA) in metallic glasses (MG), has been investigated by systematic experimental measurements coupled with theoretical calculations in Cu-Zr and Ni-Nb alloy systems. It is found that the atomic-level packing efficiency strongly relates to their GFA. The best GFA is located at the largest difference in the packing efficiency of the solute-centered clusters between the glassy and crystal alloys in both MG systems. This work provides an understanding of GFA from atomic level and will shed light on the development of new MGs with larger critical sizes.

Glassy materials lacking the periodicity of crystals possess varieties of dynamic behaviors. Boson peak and slow -relaxation (or Johari-Goldstein -relaxation) preceding the α-relaxation are the two intrinsic and markedly different dynamic behaviors of glasses, and their structural origins are long-standing issues. Through alloying a model La-based metallic glass (MG) with different types of elements we find that the low-temperature boson heat capacity anomaly evolves hand in hand with high-temperature slow -relaxation associated with free volume. With the free volume model and the generalized Maxwell model describing the heterogeneous structure, we demonstrate that the slow -relaxation of MGs is a series of localized microscopic flows, which naturally develops from defective regions where boson peak arises. The ]; with a broadband version of picosecond photo-acoustics technique, BP is related to marginally stable regions in glasses [16]; severely deformed metallic glasses (MGs) show intensified BPs related to the increase of highly localized nano-scaled shear transformed zones [17]; theoretical work identifies BP to be hybridized modes between transverse acoustic modes and local librational vibrations in loosely packed nano-regions [11,14]. With Maxwell's theory of viscoelasticity based on the assumption of spatially fluctuating local viscoelastic coefficients, Schirmacher et. al. combine the BP and low-temperature asymmetric α-relaxation anomaly for different types of glasses [18]. At the high temperatures approaching the glass transition temperature (T g ), a slow secondary or -relaxation (also called Johari-Goldstein -relaxation) preceding α-relaxation emerges [19]. Regarded as the elementary process of glass transition, the -relaxation is commonly found in wider classes of amorphous materials such as polymers, pharmaceuticals, proteins and so on [19]. Many experiments support that the β-relaxation is a process of localized motions of a small number of molecules confined in loosely packed regions, i. e., the so-called "islands of mobility" [20]. But the nature of the molecular motion involved in the relaxation varies greatly from one system to another. For instance, the β-relaxation in polymer involves restricted rotation of the bonded repeat unit [21], while in water the process relates to rotation and translation [22]. Experiments reveal that the β-relaxations of MGs with simple atomic structures are associated with diffusion of small atoms [23], short-range collective rearrangements of large solvent atoms [24], cooperative movement of La 2 3 4 5 6 7 8 tables and figures). La was substituted by Cu, Pd, Ti, Si, Sn and Al as these elements have different atomic radiuses, mixing enthalpies (H mix ) and bonding tendencies with La. Rectangular plates of La 70-x Ni 15 Al 15 Cu x (x=0, 2, 5, 8) (2 mm in thickness)were fabricated with the copper mold casting method to detect the elastic properties.The glassy nature of the samples was ascertained by X-ray diffraction (a MAC Mo3 XHF diffractometer with Cu Kα r...

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Atomic level flow plays a critical role in the mechanical behavior of metallic glass (MG) while the connection between the flow and the heterogeneous microstructure of the glass remains unclear. We describe the heterogeneity of MGs as the elastic matrix with “inclusions” of nano-scale liquid-like flow units, and the plastic flow behavior of MGs is considered to be accommodated by the flow units. We show that the model can explain the various deformation behaviors, the transformation from inhomogeneous deformation to homogeneous flow upon strain rate or temperature, and the deformation map in MGs, which might provide insights into the flow mechanisms in glasses and inspiration for improving the plasticity of MGs.

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