Study On Dynamic Fracture Toughness Of Bulk Amorphous Alloy

In this paper,the dynamic three-point bending mechanical behavior of Zr_41.2Ti_13.8Cu₁₀Ni_12.5Be_22.5?Vit 1?and Zr₅₆Co₂₈Al₁₆ bulk amorphous alloys was measured by using a self-improved Hopkinson pressure bar device.By observing the surface and fracture morphology of the amorphous alloy,and combining with the microscopic deformation theory model of the amorphous alloy,the dynamic three-point bending behavior of the bulk amorphous alloy was measured.The dynamic fracture mechanism of Zr_41.2Ti_13.8Cu₁₀Ni_12.5Be_22.5amorphous alloy and its strain rate sensitivity,and the mechanism of the increased fracture toughness of Zr₅₆Co₂₈Al₁₆ bulk amorphous alloy by local pre-deformation are discussed by the analysis of free volume and Shear Transformation Zone?STZ?model.Under dynamic loading condition,the maximum load,shear band and fracture toughness of Zr_41.2Ti_13.8Cu₁₀Ni_12.5Be_22.5 bulk metallic glass increase with the increase of the stress intensity factor rate,mainly showing the sensitivity of positive stress intensity factor rate.The shear bands formed and extended at the tip of the notch increase with the increasing of the deformation degree of the plastic zone,and the number of shear bands is proportional to the fracture toughness.Under the condition of high strain rate,a large amount of elastic strain energy stored in metallic glass is released instantly,which induces adiabatic heating in the shear band and forms a viscous rheological layer,which leads to the flocculating structure of the choroid pattern on the fracture surface.The combination and connection of free volume lead to the formation of a large number of holes and micro-cracks.The pre-compressive deformation of Zr₅₆Co₂₈Al₁₆ bulk amorphous alloy resulted in preexisting shear bands,which guide and deflect the cracks in the dynamic fracture process.The dynamic notched toughness of the samples with pre-compression loading of 2.5 KN is 54.44 MPa m^1/2,which is 18%higher than that of the as-cast specimen.Pre-compression deformation treatment can effectively improve the fracture toughness because the pre-existing shear bands can guide and offset cracks under both quasi-static and dynamic loading conditions.In addition,the crack can be deflected beyond the plastic zone and propagated along the preexisting main shear band.However,cracks in as-cast specimens cannot be offset beyond the plastic zone.The smooth region of the pre-compressed specimen is obviously larger than that of the as-cast specimen,which indicates that the crack tip blunting occurs during the crack propagation.Therefore,the local deformation and prefabricated shear band of Zr₅₆Co₂₈Al₁₆bulk amorphous alloy can be produced by pre-deforming the notched specimen,and the deformation and fracture behavior near the notch can be partially changed.Thus,the dynamic fracture toughness of brittle metallic glass was improved significantly.This method is simpler than the whole pre-deformation method such as cold rolling,and provides a new idea for improving the dynamic fracture toughness of materials.