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.

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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.

Intermolecular dipole-dipole interactions were once thought to average to zero in gases and liquids as a result of rapid molecular motion that leads to sharp nuclear magnetic resonance lines. Recent papers have shown that small residual couplings survive the motional averaging if the magnetization is nonuniform or nonspherical. Here, we show that a much larger, qualitatively different intermolecular dipolar interaction remains in nanogases and nanoliquids as an effect of confinement. The dipolar coupling that characterizes such interactions is identical for all spin pairs and depends on the shape, orientation (with respect to the external magnetic field), and volume of the gas/liquid container. This nanoscale effect is useful in the determination of nanostructures and could have unique applications in the exploration of quantum space.

Out-of-plane, nanoscale periodic corrugations are observed in the dynamic fracture surface of brittle bulk metallic glasses with fracture toughness approaching that of silica glasses. A model based on the meniscus instability and plastic zone theory is used to explain such dynamic crack instability. The results indicate that the local softening mechanism in the fracture is an essential ingredient for controlling the formation of the unique corrugations, and might provide a new insight into the origin of fracture surface roughening in brittle materials.

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Hot-filament-assisted CVD deposited a-Si:H with low H concentration and low defect density has been examined by 1 H NMR. It is demonstrated for the first time that H microstructures can be altered significantly in device quality a-Si:H films. In the present films, large H clusters account for 90% of the 2-3 at. % H atoms, with the remaining H more dispersed, but still aggregated in a small volume fraction of the material. These results suggest that an ideal a-Si:H network with low defect density and high structural stability may not necessarily be homogeneous. [S0031-9007(96)01099-X]