Shear Bands In Cu₄₆Zr₅₄ Metallic Glasses And The Effects Of Pores: A Molecular Dynamics Study

Metallic glasses have special mechanical property such as high yielding strength, high elastic limits, and superior strength/weight ratio, is a potential high-performance engineering material. However, the plastic deformation of metallic glasses is usually localized into several major shear bands, leading to catastrophic failure with little ductility. Although several theories like free volume theory and shear transformation zone theory have been developed, the mechanism of shear bands formation and propagation has not been fully understand. The difficulty of understanding shear bands in metallic glasses is largely attributed to its unique atomic structure.In this work, we study the shear bands formation and propagation in Cu₄₆Zr₅₄ metallic glasses with molecular dynamics simulation. A series of samples are simulated by quenching them at different cooling rates and pressure. Then, compression and indentation simulations are performed on different Cu₄₆Zr₅₄ metallic glass samples. The effects of cooling rate and pressure are investigated in terms of mechanical property and shear localization. The formation and propagation of shear bands are clearly observed by visualizing the local deviatoric shear strain.The Cu₄₆Zr₅₄ metallic glasses quenched at low cooling rate have more closely packed atomic structure and show highly localized plastic deformation into shear bands, while those quenched at high cooling rate exhibit evenly dispersed shear transformation zones, but no major shear band forms. The effects of pressure are two-folds as a high pressure can promote the annihilation of free volume and suppress the long-range diffusion at the mean time.Shear bands are stress-induced events that activated by thermal perturbation at certain sites. The local atomic structure determines the materials' formability of shear bans; the local stress field decides the initial position of shear bands and the path they propagate; the thermal motion have certain effect on the degree of shear localization and the magnitude of mechanical property . Shear softening happens after a major shear band forms. Metallic glasses with different atomic structure show different plastic deformation modes. The introduction of pores into metallic glasses improves toughness and compressive ductility. The pores in metallic glasses serve as stress concentrators and facilitate the formation of shear bands. Metallic glasses of different porosity deform distinctively as the pores affect the atomic structure in different ways. Metallic glasses of relatively low porosity slip along individual major shear band, resulting in low strength after softening. Metallic glasses of relatively high porosity show both single-shear-band slipping mode and multi-shear-bands slipping mode during uniaxial compression. The pores in the path of shear bands collapse when the shear deformation achieves a certain value, accompanied by a slight increase in strength. The shear band stopping is also observed when two shear bands compact against each other, which result in great increase in strength.