Understanding of the Shear Bands in Amorphous Metals

Abstract

Amorphous metals, especially bulk metallic glasses (BMGs), are highly applicable for new structural materials as a substitute for conventional crystalline metals due to superior mechanical properties, and thus occupy a unique niche compared with other classes of engineering materials. However, they usually suffer from a strong tendency for shear localization and macroscopically brittle failure at room temperature, which restricts structural application of BMGs (Schuh et al., 2007). Although the underlying deformation physics of these materials establishes less firmly as compared with crystalline metals, understanding of shear banding is particularly important in amorphous metals because shear bands play a crucial role in controlling plasticity and failure at ambient temperature. First of all, to discuss why the deformation behavior of amorphous metals presents a great contrast to crystalline metals, the atomistic understanding of these two materials is necessary. It has been widely known in soil mechanics that the shear of randomly closed-packed grains causes dilatation phenomena (Reynolds, 1885), e.g., the disappearance of water underneath feet on a wet beach because of the shear induced by the weight. The same mechanism can be applied to the atomic scale deformation in amorphous metals. Fig. 1 shows the atomic structures of crystalline and amorphous metals. When the crystalline metals are exposed to shear stress, they maintain the initial volume during plastic deformation because the periodicity along the slip planes provides identical atomic positions for the sheared material. On the other hand, amorphous metals, which have a random structure, increase their internal free volume as they plastically deform and must leave some voids. In general, the nucleation of nano-voids is more favorable on tensile side than compressive side, which indicates that the hydrostatic component of stress affects void nucleation. As a result of deformation, dislocations and shear bands are formed in crystalline and amorphous metals, respectively (Fig. 2) (http://faculty.virginia.edu/teamhowe/ files/EMFacility.html), which are caused by different micro-mechanisms. Crystalline metals use the dislocation motions pISSN 2287-5123·eISSN 2287-4445