WEB Shear band propagation and networking as a response of tribological deformation of a metallic glass
When metallic glasses are plastically deformed, deformation prevails with the highly concentrated plastic flow and is localized in a specific zone and thin band, i.e., shear transformation zone and shear band. Understanding the formation and propagation of the shear bands in metallic glasses is essential to establish a general theory of deformation. However, studying dynamics of shear transformation in metallic glass is not straightforward because the processes are rapid, non-steady in time and inhomogeneous in space. Furthermore, under a macroscopic deformation, shear bands and micro-cracks are often mix-observed and barely distinguished. Due to these difficulties in observation of shear bands, the reported geometrical information of shear bands in metallic glasses is somehow not straightforward to physically interpret. Here, we use a tribological method “scratching” to create a high density of shear bands and their network, analyzed by using TEM, STEM and EDX. Owing to a high spatial resolution and versatile imaging technics with quantitative analysis in STEM, we could clearly observe the geometry of inset shear bands and their networks.
Scratching on the top surface results in micro-cracking beneath the surface as shown in Fig. 1. The micro-cracks are observed with inhomogeneous characteristics. The rightmost micro-cracks propagates more than 2 µm in length and 1 µm in depth from the scratched surface while the micro-cracks in the middle only propagates less than 0.3 µm. The leftmost micro-crack as shown in Fig. 1b propagates with V-shaped geometry with a high tip angle. The opening width of the micro-crack at the surface is observed more than 200 nm and it becomes sharply thinner to the crack tip. Interestingly, a melted trail with a liquid blending pattern is also observed at the shallow surface < 100 nm. In figure 2, the dark contrast lines in the STEM HAADF images correspond to the shear bands. Various shear bands and their network were observed, e. g., as named, Primary Shear Band (P-SB) in Fig. 2c, Shear Band Branches (SB-B), i.e. secondary shear band, Bended Shear Band (B-SB) in Fig. 2a. It is clearly shown that the P-SB acts as the main stem or shear wall where the SB-B nucleate or blocked for further propagation by secondary shear bands.
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