| Metallic glasses (MGs) have shown widespread potential applications for their high elastic limit, high strength, high wear resistance, high corrosion resistance and excellent magnetic property. However, the MGs typically show near-zero plasticity at room temperature because of the unique short-range order and long-range disorder structure, which significantly restricts their engineering applications and manufacturing. Therefore, how to improve the room temperature plasticity of MGs has become an urgent need to solve the problem. In the present thesis, the Zr41.2Ti]3.8Cu12.5Ni10Be22.5(vitl) bulk metallic glass (BMG) was chosen for the study. The influences of the thermal effect, the mechanical effect and the thermal-mechanical effect of laser shock peening (LSP) on the surface morphology, microstructure, mechanical property and corrosion resistance of vitl BMG were investigated. Some important conclusions of this work were listed as follows:(1) Firstly, the morphology structures on vitl BMG surface induced by the thermal effect of LSP are studied. The process and mechanism of the interaction between nanosecond laser and metallic glass are discussed. The underlying mechanisms of the periodic ripples formation and the fingering phenomenon are explained by the surface tension wave theory, Kelvin-Helmholtz hydrodynamic instability theory and Saffman-Taylor instability theory. Secondly, the periodic ripple structures evolution and the changes of ring structure diameter on vitl BMG surface for different laser energies and repeated pulse number are studied. The results show that the ring structure diameter increases nearly in linear with the laser energy and pulse numbers, but the growth trend decrease with increasing the pulses numbers larger than 10. For the laser energy of 600 mJ with pulse number of 200, many micro-cracks distributed on the whole peened area.(2) The surface integrity of vitl BMG subjected to the mechanical effect of single pulse LSP is investigated systematically. Firstly, the changes of microstructure, optical topography and roughness on the surface of vitl BMG after LSP are studied, and the influence mechanisms are analyzed. Secondly, the effect of LSP on the micro-hardness of vitl BMG is studied, and the analysis is carried out. Finally, the formation mechanisms of micro cracks and shear bands on vitl BMG after LSP are analyzed. The results show that the amorphous structure of specimen after LSP is not changed. A micro-dent is produced in the peened area where the roughness is increased. The reason is that the shock wave pressure induced by LSP is much larger than the elastic limit of vitl BMG. Furthermore, the uniform of laser energy and the surface unevenness of absorbent coating result in the uneven shock wave pressure distribution. The micro-dent diameter, depth and roughness increase simultaneously with the increase of laser energy. The LSP treated area exhibited a significant decrease on micro-hardness, and the maximum reduction is about 13% in the central region of peened area. Analysis shows that the large numbers of shear bands induced by LSP are the main cause hardness decreasing. The formation of micro cracks/shear band on vitl BMG surface is through the holes connection with the action of high shock wave induced by LSP.(3) The microstructure and mechanical property of vitl BMG subjected to the multiple pulses LSP are investigated systematically. Firstly, the effect of the transverse overlapping rate on the surface roughness of specimen in the isosceles-right triangle-style is analyzed. Secondly, LSP is performed on vitl BMG with optimized parameters. The microstructure changes of vitl BMG after LSP are studied. Finally, the effects of LSP on plasticity of vitl BMG are studied by three-point bending tests and uniaxial compression tests. The results show that the surface roughness is the smallest when the transverse overlapping ratio is 29.3%. The vitl BMG remains amorphous structure after multiple pulse LSP treatment. However, large amounts of free volume are introduced in vitl BMG after LSP treatment.The plastic deflection of LSP treated specimen is 1.83 times as large as that of the as-cast specimen in three-point bending tests. Analysis shows that the plastic deformation ability improvement of vitl BMG can be attributed to the increase of free volume induced by LSP treatment, and multiple shear bands are more easily formed in the subsequent deformation. The uniaxial compression tests show that the LSP treated vitl BMG has obvious size effect. The small size LSP treated vitl BMG has higher compression strengthen and better plasticity.(4) The microstructure and properties of vitl BMG subjected to the thermal/mechanical effect of multiple pulsees LSP are investigated systematically. Firstly, laser shock peening without coating technology (LSPwC) is performed on vitl BMG, and the microstructures of vitl BMG after LSPwC are analyzed. Secondly, the effect of LSPwC on the plasticity of vitl BMG is analyzed, and the influence of the thermal/mechanical effect of LSP on the plasticity of vitl BMG is analyzed. Finally, the nano-hardness and the corrosion resistance of vitl BMG after LSPwC are studied. The results show that the surface of vitl BMG after LSPwC treatment is remains amorphous structure. While many crystals uniformly distributed in the depth of about 120 ?m under the surface. Compared with the as-cast specimen, the plastic deflection of the specimen increased by 23% after LSPwC. Analysis shows that there are many kinds of influence factors on the plasticity of vitl BMG in the process of bending deformation:increasing in plasticity by introduction of free volume crystalline and the complex residual stress induced by LSPwC and decreasing by the formation of ablation pit, but the effect of ablation pit is not predominant on the overall plasticity. The nano-hardness of vitl BMG decreases after LSPwC, but the corrosion resistance is enhanced. Compared with the LSPwC treated vitl BMG, the corrosion resistance of the laser shock peening with absorbent coating is better. |
PDF Full Text Request