Investigation On The Influence Of Oxygen Microalloying On The Structures And Properties Of Al-based Metallic Glasses

Metallic glasses have excellent mechanical properties and have a wide range of potential applications owing to their unique disordered structure.Due to the good oxygen affinity of the metallic elements in the alloy composition,oxidation is difficult to avoid.It significantly affects the properties of metallic glasses and limits their applications in engineering.However,recent studies have found that oxygen can be treated as a microalloying element to strengthen metallic glass.This discovery changes the traditional view that oxygen plays a negative role,and is of great significance to the preparation of low-cost and high-performance metallic glass.Yet the current understanding of the mechanism of oxygen microalloying is still very limited,and systematic research work on it is urgently needed.In this thesis,molecular dynamics simulation and experiment methods are combined to study the mechanism of oxygen microalloying on the structure and properties of Al-based metallic glasses.Based on the molecular dynamics method,the atomic-scale structural change of the oxygen microalloying Al-based glass-forming liquid is studied.It is found that the oxygen quickly reacted with the surrounding metallic atoms after entering the liquid,forming oxide clusters which are small,dispersed,and stable.The covalent bond in the cluster does not match the metallic bond between neighboring atoms,which causes the atoms around the cluster cannot to be densely arranged,and thus increasing the free volume of atoms in this local area.The number of O-centered Voronoi polyhedrons is relatively small,and most of them have the characteristics of low five-fold symmetry.Due to the increase in the free volume of the neighboring atoms of the clusters,the atomic volume in the base matrix is reduced.This promotes the formation of high fivefold symmetry structures with high packing density in the base matrix.Therefore,oxygen significantly improves the average five-fold symmetry of the alloy liquids,which is beneficial to the improvement of the glass-forming ability.The results comprehensively describe the structural characteristics of the oxygen-microalloying metallic glasses and lay a solid foundation for the study of the dynamic and mechanical properties of metallic glasses at room temperature.Based on the molecular dynamics method,the effect of oxygen microalloying on the dynamic characteristics of the Al-based glass-forming liquid is studied.It is found that oxygen increases the diffusion coefficient of the alloy liquid and decreases its viscosity.The temperature dependence of the diffusion coefficient and the viscosity of the liquids deviates from the Arrhenius law at 1.2 times the glass transition temperature,which leads to the breakdown of the Stokes-Einstein equation.The Stokes-Einstein equation of the oxygen-containing alloy liquid has a higher degree of deviation,indicating that oxygen increases the difference between the low-temperature kinetic characteristics of the alloy liquid and those at high temperatures.Due to the different binding abilities of oxygen with different metallic atoms,the difference between the dynamics behavior of the constituent metallic elements are promoted,making the dynamic behavior of the atoms more localized.Moreover,the diffusion coefficient of oxygen-containing clusters is one order of magnitude smaller than that of freely moving metallic atoms.Therefore,the presence of oxygen makes the alloy liquids spatially divided into a slow kinetic zone containing oxide clusters and a fast kinetic zone containing no oxide clusters.This spatial inhomogeneity hinders the long-distance diffusion process necessary for crystallization,which is also the intrinsic mechanism by which oxygen improves the glass-forming ability of the alloy liquid.Combining molecular dynamics simulation with experiments,the effect of oxygen microalloying on the mechanical properties of Al-based metallic glasses at room temperature is systematically explored.The results show that the plasticity of metallic glasses first increases and then decreases with the increase of oxygen contents,and the plasticity of metallic glasses are the best when the oxygen content is 0.7 at.%.By analyzing the microstructure characteristics of metallic glasses during deformation,it is found that the improvement of the plasticity of metallic glasses results from the fact that there is a large amount of free volume around the covalently bonded oxides,and the neighboring atoms are arranged in a loose structure state.Therefore,when an optimal amount of oxygen(0.7 at.%)is present in the metallic glasses,a uniform distribution of oxygen atoms can be achieved,which in turn promotes the formation of a uniformly distributed shear transformation zone during the deformation process.This contributes to slowing down the strain localization and improving the plasticity.The micro-mechanical properties of the oxygen-microalloying metallic glasses were tested via the nanoindentation method,which confirmed the effectiveness of oxygen microalloying to improve the plasticity of the metallic glasses.The research work in this paper is of great significance for the in-depth understanding of the nature of oxygen microalloying of metallic glasses,optimizing the process route of metallic glasses from eliminating the negative effects of oxygen to utilizing the positive effects of it,and developing low-cost and high-performance metallic glasses with strong glass-forming ability.