|Alternative Title||Fracture behaviors of Metallic glasses|
|Place of Conferral||北京|
|Keyword||金属玻璃 断裂准则 断裂角 尺寸效应 屈服强度 塑性|
Metallic glasses (MGs) is a kind of amorphous material which process long-range disorder and short-range order structures.They have attracted wide attention of many scientists due to their other excellent properties. However, due to the absence of dislocation glide, brittle fracture induced by nanoscale shear band often occurs in bulk metallic glasses under high stress at room temperature. This phenomenon has brought great challenges to the practical application of bulk metallic glasses. Many researches have tried to study the laws of deformation and fracture of metallic glasses and to improve their plasticity. Some researches show that the plasticity of metallic glasses can be improved by adjusting their composition. The shear band of metallic glasses would be suppressed in small size so that the metallic glasses deformed uniformly with considerable plasticity. These developments have brought broad application prospects to metallic glass, and also brought new problems and challenges to the study of traditional fracture criteria and microscopic deformation mechanisms. In this paper, the deformation and fracture behavior of metallic glasses under complex loading and different sizes are systematically studied. The main work is as follows:
A general formula of fracture angle is proposed to predict the location of fracture surface of bulk metallic glasses under complex loading. By combining the fracture angle formula with the ellipse criterion, the fracture behavior of round bar specimens of two typical metallic glasses was studied, which are under axial tensile loading and confining pressure. The results show that the fracture of bulk metallic glasses under tension loading is caused by the interaction of tensile stress and shear stress. In addition, by using the fracture angle formula, we obtained a new criterion, which takes the mean normal stress and maximum shear stress as independent variables. Since the mean normal stress and maximum shear stress can be obtained directly after knowing the external loading, the new ellipse criterion will be more convenient to use.
Since bulk metallic glasses with large plasticity were prepared, their plasticity can no longer neglected after material yielding. In addition, the bulk metallic glasses show ideal elastic-plastic behavior after compression yielding, while the nominal stress remains unchanged, and the Cauchy stress decreases as the cross-sectional area increases. At this time, the traditional fracture criterion will not be suitable for describing the failure of metallic glasses. Based on these conditions, we proposed to use strain to describe the fracture behavior of metallic glasses, and put forward hyperbolic strain criterion and generalized Mohr-Coulomb strain criterion. The new fracture criterion not only consider the pressure-sensitivity of materials, but also describe the plasticity of different materials and the influence of deformation state to the fracture behaviors. By combining with the fracture angle formula,, we analyze the metallic metallic round bar specimens under complex compression loading, and reveal the influence of confining pressure on the strength and plasticity of metallic glasses. The theoretical results are in good agreement with the experiments.
Based on the energy change of metallic glasses (from liquid to amorphous state), the functional relationship between the volume fraction of the soft zone in metallic glasses and the characteristic size of the material is obtained. By combining this relationship with Gurson's void model, the yield criterion related to the size of metallic glasses is obtained. The results show that the strength of metallic glasses increases with the decrease of size and remain unchanged while the size is small to a critical vaule. These results are in good agreement with the experimental results. Furthermore, combined with the shear band conditions proposed by Rice, we obtain the stress-strain states while shear bands instability of different sizes samples. It is found that the plastic strain while shear bands instability increases significantly with the decrease of sample size. It causes the change of deformation modes (shear band--uniform deformation) of metallic glasses when their size decrease.
|喻立. 金属玻璃的断裂行为研究[D]. 北京. 中国科学院大学,2019.|
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