Fracture behaviors under pure shear loading in bulk metallic glasses

Pure shear fracture test, as a special mechanical means, had been carried out extensively to obtain the critical information for traditional metallic crystalline materials and rocks, such as the intrinsic deformation behavior and fracture mechanism. However, for bulk metallic glasses (BMGs), the pure shear fracture behaviors have not been investigated systematically due to the lack of a suitable test method. Here, we specially introduce a unique antisymmetrical four-point bend shear test method to realize a uniform pure shear stress field and study the pure shear fracture behaviors of two kinds of BMGs, Zr-based and La-based BMGs. All kinds of fracture behaviors, the pure shear fracture strength, fracture angle and fracture surface morphology, are systematically analyzed and compared with those of the conventional compressive and tensile fracture. Our results indicate that both the Zrbased and La-based BMGs follow the same fracture mechanism under pure shear loading, which is significantly different from the situation of some previous research results. Our results might offer new enlightenment on the intrinsic deformation and fracture mechanism of BMGs and other amorphous materials

Macroscopic tensile plasticity by scalarizating stress distribution in bulk metallic glass

The macroscopic tensile plasticity of bulk metallic glasses (BMGs) is highly desirable for various engineering applications. However, upon yielding, plastic deformation of BMGs is highly localized into narrow shear bands and then leads to the "work softening" behaviors and subsequently catastrophic fracture, which is the major obstacle for their structural applications. Here we report that macroscopic tensile plasticity in BMG can be obtained by designing surface pore distribution using laser surface texturing. The surface pore array by design creates a complex stress field compared to the uniaxial tensile stress field of conventional glassy specimens, and the stress field scalarization induces the unusual tensile plasticity. By systematically analyzing fracture behaviors and finite element simulation, we show that the stress field scalarization can resist the main shear band propagation and promote the formation of larger plastic zones near the pores, which undertake the homogeneous tensile plasticity. These results might give enlightenment for understanding the deformation mechanism and for further improvement of the mechanical performance of metallic glasses

Macroscopic tensile plasticity of Zr-based bulk metallic glass with surface screw thread shaped structure

Zr-based bulk metallic glass (BMG) dog-bone-shaped tensile specimens with designed screw thread shaped (STS) structures fabricated by mechanical turning were tested under uniaxial tensile loading. Obvious macroscopic tensile plasticity and serrated flow behavior appear when the typical size of STS structures-the depth reaches the intrinsic plastic zone size for MGs. Finite element analysis show the introduced STS structure twists the stress field distribution and then hampers the main shear band propagation and promotes the formation of multiple shear bands, which improve the tensile plasticity. The proposed STS designing scheme may shed light on the deformation mechanism and be helpful for structural application of BMGs. (C) 2016 Elsevier B.V. All rights reserved.</p