13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- The material exhibits the maximum fracture strength~1.9GPa and less macroscopic plastic strain at a quasi-static strain rate. It is of interest to note that the fracture strength markedly decreases at high strain rates, which agree well with the previous observations for other Zr-based [8, 13] BMGs. Another interesting observation is that quasistatically deformed samples fractured into two large millimeter-scale blocks, whereas at impulsive loading, the samples broke into many small scale fragments. Fractured samples were examined to understand the failure behavior by SEM. The samples fracture along a single plane at low strain rates, indicating a major shear band dominates the final fracture process. And vein-like pattern spreads over the whole fracture surface and extends in a uniform direction, corresponding to the propagation direction of the primary shear band. On the other hand, the considerable randomly distributed vein-like patterns, corresponding to the different propagation directions of shear bands on surface of fragments formed in dynamic loaded case show that multiple shear bands occur and dominate failure process. The network-like multiple shear bands are observed in the surface of the small fragments. Therefore, the transition from one dominant shear band to network-like multiple shear bands results in the different failure modes. But the mechanisms of transition are unknown. 4. Discussion 4.1 Local evolution model for shear band Room-temperature plastic deformation of metallic glasses has been known to be accomplished through shear bands in which plastic flow driven mainly by shear stresses is localized within a nanoscale zone [14, 15]. As is known, the shear-banding process is a dissipation system [7, 16-20].There are three types of diffusion, i.e. conventional momentum/viscous, thermal/energy and special free-volume dissipation within the propagation shear band. In our previous work, the coupled effect of free-volume softening and thermal softening upon shear-banding evolution in metallic glasses was discussed [6, 9, 21, 22]. Nanometer-scale defects[23], a density fluctuation[24] and then a viscosity fall[9, 10, 18, 25] usually are results of the shear band propagation, suggesting the coalescence of free volume in the band. When the shear stress τat the shear band vanishes (or the shear displacement ψreaches a critical cψ), the shear band is regarded as being fully mature, and a finite amount of energy is expended. Zhang et al [18] proposed a micromechanical model based on momentum diffusion controlling shear band spacing. Additionally, Lewandowski and Greer[26, 27] have experimentally shown a significant temperature rise within and around the shear bands using tin-coated specimens, and some authors [27-29] observed the heat-affected zone (HAZ) around the shear band due to heat conduction which may play an important role in shear-band propagation. As discussed above, momentum and thermal derived from shear band spread to the neighboring elastic medium. Thus shear stress release also occurs in the undisturbed medium and stored energy is released. Based on these analyzes, a theoretical model that takes into account the balance between the energy dissipation within shear band and the stored energy released in the vicinity of the shear band to fuel shear localization is developed. 4.2 Shear-band dissipation energy It has been demonstrated that the following equations properly govern the shear banding in BMGs [1, 9, 30]: 2 2 b b t y γ τ ρ ∂ ∂ = ∂ ∂ & (1)
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