Experimental And Numerical Study On Mechanical Properties And Deformation Behavior Of Bulk Metallic Glass By Laser Shock Peening

Bulk Metallic Glasses (BMGs) are a new family of attractive materials with excellent mechanical properties which make them candidates for structural materials. BMGs have been widely used in many fields such as machinery, electronics, defense and aerospace. However, their poor plasticity significantly impedes their industrial application. Therefore, how to improve the plasticity of BMGs at room temperature has become a significant item in the fields of materials science. Laser shock peening (LSP) is an innovation surface modification technique which uses Q-switched laser pulse with high energy to generate favorable compressive residual stress on the surface of components. It has been effectively used in improving the mechanical properties of a number of metals and alloys. Compared with conventional shot peening, LSP is capable of introducing much greater depths residual stresses, more uniform stress distributions and less microstructure modifications to metals and other materials.In the present thesis, we report that LSP could introduce compressive residual stresses and dense shear bands to the surface of Zr35Ti30Cu8.25Be26.75 BMGs to improve the plasticity at room temperature without the introduction of crystalline phases. Experimental study and finite element method based on free volume model were implemented to investigate the mechanical properties and deformation behavior of laser peened BMGs in various process conditions. The effect of laser parameters on the performance of laser peened BMGs had been concluded. The bending behavior of BMG beams and the compression behavior of BMG pillars had been studied. The relationships between the plasticity with shear bands and the distribution of residual stress were established. The law of LSP induced plastic deformation and the failure mechanism of BMGs were studied by SEM. The effect of LSP on the mechanical properties of BMGs and its intrinsic mechanism were revealed by FEM. This study provides a new way to improve the plasticity of BMGs at room temperature.The main contents and conclusions are as follows:(1) Based on free volume theory, Mohr-Coulomb yield criterion and laser shock wave loading curve, finite element method was used to predict the deformation behaviors of laser peened BMGs through dynamic explicit and static implicit analysis. The distributions of displacement, stress and strain had been achieved. The macroscopic and microscopic deformation and the residual stress distribution of laser peened BMGs had been discussed. The effect of laser parameters on the performance of laser peened BMGs had been concluded. The results showed that the parabolic-shaped dents had formed in the laser irradiation of the specimen. And the macroscopic deformation could be observed. The compressive residual stress was introduced to the surface and in the interior of BMGs. The simulations of multiple-LSP on BMGs had been carried out in different lapping rates. When the overlap rate increased from 0% to 50%, the surfaces of laser peened BMGs tended to smooth and would reduce roughness caused by LSP. Besides, the maximum residual compressive stress and the depth of compressive residual stress affected layer had been increased. Therefore, reasonable laser parameters and peening schemes are the key factors to obtain the optimal distribution of residual stress.(2) The experimental system of LSP on BMG beams had been built. And experimental investigation of the bending deformation of as-cast beams and laser peened beams were performed by three-point bending test at room temperature. Bending fracture morphologies of BMG beams were also examined by SEM. Finite element method was used to predict the bending behavior of laser peened BMG beams. The results showed that the laser peened beams presented better bending behavior than that of as-cast beams before the final fracture. The plastic flexural deflection of two-sides peened beams had been improved 16.8%. Besides, the larger laser irradiated region was, the better bending plasticity of BMG beam would be. According to the bending fracture morphologies, the as-cast BMG beams exhibited obvious shear fracture characteristics, and would break along the main shear band. LSP was conducive to the generation of multiple shear bands in the tensile region of BMG beams. Besides, the compression residual stress induced by LSP could reduce the tensile stress during bending which could prevent the rapid expansion of the main shear band, and finally improve the bending plasticity of BMG beams.(3) The experimental system of LSP on BMG pillars had been built. And experimental investigation of the compressive deformation of as-cast pillars and laser peened pillars were performed by compression test at room temperature. Compression fracture morphologies of BMG pillars was also been examined by SEM. Finite element method was used to predict the compressive behavior of laser peened BMG pillars. The results showed that the compressive behavior of BMG pillars could be improved by LSP treatment. The compressive plastic strain of laser peened BMG pillars could reach to 1.48% and the compressive strength could reach to 1820MPa, which were better that those of as-cast BMG pillars. Besides, the more the laser impacts were, the better the compressive plasticity of BMG pillars would be. The simulation results showed that laser peened BMG pillars had the higher stress concentration and more free volume at the edge of maximum shear stress surface, which will promote the formation of the secondary shear bands and the formation of the multiple shear bands. It could prevent the rapid expansion of the main shear band, and finally improve the compressive plasticity of BMG pillars.