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The morphology of the fracture surfaces of a bulk metallic glass (BMG) tested under compression was systematically studied. Experimental results showed that the fracture surface always comprises two kinds of zones, starting with a relatively smooth zone followed by the second zone with vein patterns. It implies strongly that the plastic deformation of BMGs always starts with a cooperative shear. The following catastrophic fracture characterized by the vein patterns may or may not occur, depending on the magnitude of this shear, which is controlled by the sample size and machine stiffness. This phenomenon was interpreted based on the temperature rise resulting from the work done during the cooperative shear. It revealed that for small samples, the shear is so small that the temperature rise is insignificant, accounting for the extensive serrated flow, while the temperature rise in samples beyond a critical size is sufficiently high so that the temperatures in the shear band are higher than the glass transition temperature or even the melting temperature, leading to catastrophic fracture. I. INTRODUCTION

Normally with limited apparent plastic deformation, the fracture in metallic glasses (MGs) under roomtemperature uniaxial compression/tension is generally believed to be a catastrophic instability process,1,2 usually along a dominant shear band.3,4 However, despite extensive studies on their deformation and fracture behaviors, the detailed fracture mechanism of MGs still eludes us. Right from the beginning of the first few studies4,5 on the mechanical behaviors of MGs, two morphologically distinct zones have been observed on the fracture surface of MG samples under tension. The first one is a smooth, featureless zone,4–10 which is immediately followed by the second zone with vein or river patterns,4–10 as schematically shown in Fig. 1(a). It can therefore be envisioned that the plastic deformation process through shear-band instability comprises two stages according to these two distinct zones. The reported smooth, featureless zone4–10 ranging from submicrometers to a few tens of micrometers in width at the edge of the fracture surface has been proposed to result from a shear event (i.e., a cooperative shear11–13), serving as the initial stage of the plastic deformation. Furthermore, it is noted that in some cases,14–17 this smooth zone contains a regularly spaced striation pattern, indicating that the shear occurred in an intermittent fashion, suggesting a complexity of the shear stage. The vein patterns in the

following zone, on the other hand, are indicative of the reduced viscosity leading to the softening of the shear band.18–21 Basically, this fractographic feature signifies a catastrophic instability process of the shear band, that is, the eventual fracture of MGs. Based on these observations, it is believed that the plastic deformation process of MGs should begin with one (or multiple) shear event(s) and then probably proceed to a catastrophic fracture, as schematically shown in Fig. 1(b). In principle, the