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We report a brittle Mg-based bulk metallic glass which approaches the ideal brittle behavior. However, a dimple structure is observed at the fracture surface by high resolution scanning electron microscopy, indicating some type of "ductile" fracture mechanism in this very brittle glass. We also show, from the available data, a clear correlation between the fracture toughness and plastic process zone size for various glasses. The results indicate that the fracture in brittle metallic glassy materials might also proceed through the local softening mechanism but at different length scales.

Metallic glasses are commonly brittle, as they generally fail catastrophically under uniaxial tension. Here we show pronounced macroscopic tensile plasticity achieved in a La-based metallic glass which possesses strong β relaxations and nanoscale heterogeneous structures. We demonstrate that the β relaxation is closely correlated with the activation of the structural units of plastic deformations and global plasticity, and the transition from brittle to ductile in tension and the activation of the β relaxations follow a similar time-temperature scaling relationship. The results have implications for understanding the mechanisms of plastic deformation and structural origin of β relaxations as well as for solving the brittleness in metallic glasses.

For glasses, the structural origin of their flow phenomena, such as elastic and plastic deformations especially the microscopic hidden flow before yield and glass-to-liquid transition (GLT), is unclear yet due to the lack of structural information. Here we investigate the evolution of the microscopic localized flow during GLT in a prototypical metallic glass combining with dynamical mechanical relaxations, temperature-dependent tensile experiments and stress relaxation spectra. We show that the unstable and high mobility nano-scale liquid-like regions acting as flow units persist in the glass and can be activated by either temperature or external stress. The activation of such flow units is initially reversible and correlated with b-relaxation. As the proportion of the flow units reaches a critical percolation value, a mechanical brittle-to-ductile transition or macroscopic GLT happens. A comprehensive picture on the hidden flow as well as its correlation with deformation maps and relaxation spectrum is proposed.

The mechanisms of plastic deformation of glassy solids and structural origin of ␤ relaxation are two fundamental issues. We provide compelling experimental evidence that the activation of shear transformation zones ͑STZs͒ and ␤ relaxations in metallic glasses are directly related, and the activation energy of the ␤ relaxation and the potential-energy barriers of STZs are nearly equivalent. Our results suggest an intrinsic correlation among potential STZs, ␤ relaxation, and the inhomogeneous atomic structure of metallic glasses, which has implications for understanding the deformation mechanism and structural origin of ␤ relaxation in glasses.Plastic deformation of metallic glasses ͑MGs͒ far below glass transition temperature T g is a long-standing issue. [1][2][3][4] Microscopically, MGs are proposed to deform by plastic rearrangements of atomic regions involving tens of atoms termed shear transformation zones ͑STZs͒, 3,4 and a consequence of formations and self-organizations of STZs that induce macroscopical shear banding of MGs. As recognized by Johari et al., 5 the relaxation of supercooled liquids and glasses are governed by two main processes: a fast process, that is the ␤ relaxation which is a locally initiated and reversible process, and a slow process, termed the ␣ process, which is a large-scale irreversible rearrangement of the material. The ␤ relaxation, which is an Arrhenius process, 5-7 persists from supercooled liquid regime to glassy states, and is separated from the non-Arrhenius ␣ relaxation at a crossover temperature. 5-7 It has been proved to be an intrinsic and universal feature of glasses but poorly understood. 5,6 Usually, it is related to localized motions with cooperative nature, a reminiscent of STZs in MGs. From the theory of potential-energy landscape ͑PEL͒, 7,8 the ␤ relaxations were identified as hopping events across subbasins within an inherent megabasin ͑inherent structure͒ while ␣ relaxations entail escape from one megabasin and eventually jump into another ͑e.g., see Fig. 1 in Ref. 8͒. Experimentally, the activation energy of the ␤ relaxations, E ␤ , can be determined by dielectric spectroscopy, 6 differential scanning calorimeter, 9 and by dynamic mechanical spectroscopy ͑DMS͒. 10 Since MGs are good conductors, dielectric method that commonly used in nonmetallic glasses is not feasible. The DMS, which is widely used in field of polymer glasses, 6 has been employed for studying the ␤ relaxation in MGs. 10 Based on the PEL theory and the Frenkel's analysis of shear strengths in dislocation free solids, Johnson et al. 11 proposed a cooperative shear model ͑CSM͒ to understand the deformation mechanisms and rheological properties of MGs. 11,12 The CSM gives rise to functional relations between viscosity and shear modulus ͑e.g., see Ref. 12 for a review͒. According to the CSM, activation of isolated STZs confined within elastic matrix could be associated with the ␤ relaxation. 13 However, the validity of this correlation, which is the fundamental conceptual standpoint of CSM, 11,13 re...

In multicomponent metallic glasses, we demonstrate that diffusion and secondary (β) relaxation are closely related. The diffusion motion of the smallest constituting atoms takes place within the temperature and time regimes where the β relaxations are activated, and, in particular, the two processes have similar activation energies. We suggest cooperative stringlike atomic motion plays an important role in both processes. This finding provides additional insights into the structural origin of the β relaxations as well as the mechanisms of diffusions in metallic glasses.

We report a close correlation between the dynamic behavior of serrated flow and the plasticity in metallic glasses (MGs) and show that the plastic deformation of ductile MGs can evolve into a self-organized critical state characterized by the power-law distribution of shear avalanches. A stick-slip model considering the interaction of multiple shear bands is presented to reveal complex scale-free intermittent shear-band motions in ductile MGs and quantitatively reproduce the experimental observations. Our studies have implications for understanding the precise plastic deformation mechanism of MGs.

Bulk metallic glasses (BMGs) are multicomponent alloys with typically three to five components with large atomic size mismatch and a composition close to a deep eutectic. Packing in BMG liquids is very dense, with a low content of free volume resulting in viscosities that are several orders of magnitude higher than in pure metal melts. The dense packing accomplished by structural and chemical atomic ordering also brings the BMG-forming liquid energetically and entropically closer to its corresponding crystalline state. These factors lead to slow crystallization kinetics and consequentially to high glass-forming ability. This article highlights the thermodynamic and kinetic properties of BMGs and their contributions to extraordinarily high glass-forming ability. Some possible links with mechanical properties are also suggested.