Influence Of Stress Concentration And Structural Relaxation On Bending Fracture Behavior Of A Zr₅₅Cu₃₀Ni₅Al₁₀Metallic Glass

Bulk metallic glasses（BMGs）, kinds of potential engineering materials, have attracted a flood of interests from material researchers because of their outstanding mechanical, physical and chemical properties. Owing to the absence of long-range order in atomic arrangement, there is no microdefects such as grain boundaries and dislocations in BMG alloys. Meanwhile, macroscopic imperfections like composition segregation, inclusions and second phases are absent. However, catastrophic fracture makes the BMGs trapped in test and study but not suitable for practical project. Up to now, the nature of BMGs’ deformation and fracture has not been fully understood and the theoretical understanding of the fracture mechanism remains uncompleted. Therefore, the BMG with the composition of Zr₅₅Cu₃₀Ni₅Al₁₀, which owns strong glass forming ability and contains no precious metals or toxic elements, is chosen as the research object in this thesis. Combining with engineering application, single-edge notched beam method is applied to measure fracture toughness of BMGs. The evolution of fracture process was obtained from observing the variation of fracture surface morphology, by transforming the state of stress concentration through changing the radius of notch and altering the structure of alloy by using vacuum isothermal heat treatment. And then the influence of stress concentration and structural relaxation on the Zr-based metallic glass fracture behavior in bending situation are under research and the results are shown as follows:The load-displacement curve of Zr₅₅Cu₃₀Ni₅Al₁₀ metallic glass shows approximate linear, suggesting the deformation process is linear elastic and little ductility in the as-cast sample. The stress concentration has an influence on the fracture behavior of Zr-based metallic glass, resulting in the increasing of bending displacement, critical load and fracture toughness with the decreasing degree of stress concentration. The fracture surface can be divided into two regions: crack spreading zone and rapid fracture zone. There is a smooth region with a width of 10μm in the first region, which is the distance of single shear bands move steadily. Rapid fracture zone contains dimple-like patterns pointing to the direction of crack propagation. The results of 3D morphologyshow that fracture morphology goes coarse with notch radius increasing. The height of dimple patterns can be seen as a half of crack opening distance, which can be applied for estimating critical fracture load.Structural relaxation at the temperature of 50 ℃ below glass transition temperature（Tg） will not result in the crystallization of amorphous alloy. Nonetheless, this treatment will lead to the annihilation of free volume and the longer the time of structural relaxation, the greater the degree of ordering. The relaxation process leads to the change of mechanical properties of BMGs. The hardness increases but fracture toughness gets lower. The annihilation of free volume leads to the reduction of bond length and the increasing of force between atoms, which leads to a higher hardness. The decline of fracture toughness can be attributed to the decrease in the number of shear bands. The fracture surface morphology shows great changes after relaxation process, appearing river-like patterns and quasi cleavage interfaces. The results of 3D morphology show the reduction of surface roughness, suggesting the occurrence of ductile-to-brittle transition.