Bulk metallic glass (BMG) possesses unique mechanical properties, such as high strength, high hardness and large elastic strain limit, etc. However, due to the lack of crystalline defects including dislocation and twinning, plastic deformation of metallic glasses occurs in the form of highly localized shear bands. Consequently, they generally do not exhibit work-hardening, but occur strain softening and/or thermal softening. This leads to the premature catastrophic fracture of BMGs, which greatly hinders their widespread engineering application. However, in recent years, apparent work-hardening behaviors have been observed in some monolithic BMG materials which has attracted significant interest in the field of engineering, and triggered scientific studies on the origin of work-hardening in BMGs. So far, the understanding of how the structure of metallic glasses affects their performance and deformation behavior remains quite limited, thus, the origin of work-hardening of BMGs is still a hot topic of controversy, In general, the work-hardening behavior of BMGs is closely related to their internal structural changes caused by external stress (energy), including the formation of multiple shear bands, the evolution of free volume and the behavior of nanocrystallization, and ultimately associated with the shear-band behavior during plastic deformation. Cu47.5Zr47.5Al5 was the first reported plastic BMG which has extensive work-hardening capability. Relevant studies suggested that the chemical and/or structural inhomogeneity with different scales existing in the alloy promoted the formation and multiplication of multiple shear bands during material deformation; while the interaction of massive shear bands in three dimensions increased the flow stress of the materials, resulting in the work-hardening behavior. which is ascribed to the work-hardening mechanism of BMGs. This theory was first proposed by Das et al., and later confirmed by a variety of other studies. Afterwards, strain hardening and softening phenomena were observed in some BMGs' loading-unloading cycle nanoindentation tests, and a "free volume model" of BMG work-hardening was proposed, they believed that the change of external shear stress lead to the change of net free volume in amorphous structure, and then the variation in hardness of the material caused by the shear band behavior of plastic deformation micro-zone. Chen et al. excluded structural inhomogeneities of amorphous Cu50Zr50 ribbons, detected in situ nanocrystallization behavior at shear bands of bent Cu50Zr50ribbons, and proposed the strain hardening mechanism associated with deformation induced nanocrystallization based on the experimental observation of strong interactions between shear bands and nanocrystallites. These works enriched the basic principles and developed the related research method of BMGs work-hardening. This paper briefly introduces the methods/parameters commonly used to evaluate the work-hardening capability of metal materials, and outlines the shear bands behaviors in BMGs. On the basis of this, the work-hardening behaviors observed in typical BMGs are analyzed and the possible work-hardening origin mechanisms of BMGs are discussed, which could be the reference for studying the mechanical behavior of BMGs and developing plastic BMGs as structural materials with excellent performance. © 2019, Materials Review Magazine. All right reserved.
