镁铝合金凝固特性与固液界面结构的分子动力学研究

发布时间：2018-01-15 04:06

本文关键词：镁铝合金凝固特性与固液界面结构的分子动力学研究　出处：《南昌大学》2016年硕士论文　论文类型：学位论文

【摘要】：镁合金作为21世纪极具潜力和应用价值的绿色工程材料,具有好的切削加工性能、比强度高以及抗冲击、阻尼性能好等众多优点,在航空航天材料、军事、医药化工、金属压铸件和汽车工业等领域得到了广泛的应用。但是由于镁合金的塑性和成形性能较差,限制了其应用和发展。同时随着科学技术领域的不断创新与进步,对材料性能的要求越来越高,以实验手段为主的材料研究方法存在许多局限性,已经难以满足现阶段对材料研发的要求,所以通过计算机模拟新技术来深入探索新材料的优良性能成为一种趋势。分子动力学模拟技术可以观察金属熔体在高速凝固过程中原子的微观结构变化,在对材料宏观性质的计算上也具有很高的准确度,逐渐成为材料研究领域的重要手段。本文以镁铝合金作为研究对象,采用基于嵌入式原子法的分子动力学方法,针对镁铝合金凝固特性以及固液界面结构开展理论计算和实验研究,相互验证,以揭示镁熔体中溶质原子对异质形核界面的影响机制。利用基于嵌入式原子法的分子动力学方法,采用径向分布函数、双体分布函数、均方根位移、体系总能量以及VMD分子动力学可视化分析等微观结构分析与表征方法,对纯Mg熔体、Mg-3Al合金、Mg-6Al合金和Mg-9Al合金液固转变过程微观结构的演化机理进行了分析。当冷却速率为1×1013K/s时,Mg-3Al合金、Mg-6Al合金和Mg-9Al合金最终都形成了原子排列有序的晶体结构,只是初始转变温度稍有不同,Mg-3Al合金在体系温度降至497K时已经呈现晶体结构特征,Mg-6Al合金和Mg-9Al合金液固转变温度分别为647K和747K。此外Mg-Al合金中Mg原子跟Al原子之间具有更强的成键倾向。均方根位移(MSD)和系统总能量的分析也进一步验证了径向分布函数(RDF)的结果。通过VMD分子动力学可视化分析可以直观的观察到体系的晶化情况,最终体系都形成了原子排列有序的晶体结构,并且随着合金体系中铝含量的增加,体系的晶化程度也在提高。采用分子动力学方法研究了Mg/Mg-3Al合金体系固液界面的结构,通过可视化软件Ovito记录了不同时刻的原子位置文件,绘出了所模拟体系原子的位置图像,从而能够定性的分析固液界面处原子的排列情况,在温度调整到680K时,从5000步开始,虽然Mg-3Al合金区域原子还是处于无序状态,但是靠近固液界面处的液体区域开始呈现一些有序性,并且发现Al原子有向界面处扩散的趋势。随着弛豫时间的增加,基体上半部分靠近固液界面处的几层原子已经出现了一定的有序度,这也跟液体附着在固体基底时的情况一致,并且合金中的Al原子也处于有序排列状态,占据着Mg原子的位置。固液共存体系弛豫1200000步后,体系处于两相平衡状态,靠近固液界面处的液态区域原子存在明显的有序度。采用OM、SEM和EDS等分析测试手段对镁合金中初生相与共晶相界面的组织与成分分布进行实验观测和分析,讨论了Mg晶粒密排面取向对共晶生长及Al元素分布的影响。结果表明,不同Mg的密排面跟Mg-9Al合金形成的界面对溶质原子分布的影响不一样,纯Mg的(0001)密排面与Mg-9Al合金熔体形成的固液界面更能促进共晶组织的生长,合金液中Al溶质元素沿着垂直于固液界面方向的扩散也更活跃。
[Abstract]:Magnesium alloy as green engineering materials in twenty-first Century and potential application value, has good cutting performance, high strength, impact resistance, good damping property and many other advantages, in aerospace materials, military, medicine and chemical industry, metal castings and automotive industry and other fields has been widely used. But because of the plastic and the poor formability of magnesium alloy, limit its application and development. With the continuous innovation and progress in the field of science and technology, on the material properties of the increasingly high demand, the research methods of materials to experiment mainly means there are many limitations, has been difficult to meet the requirements of materials research and development stage, so to explore excellent the performance of new materials has become a trend of new technology by computer simulation. Molecular dynamics simulation can be observed in high speed metal melt solidification process of atoms in micro structure change In the calculation, on the macroscopic properties of the material also has a high degree of accuracy, has gradually become an important means in the field of materials research. In this paper, magnesium alloy as the research object, using molecular dynamics method based on embedded atom method, based on the theoretical calculation and experimental study, carry out the solidification characteristics of magnesium alloy and the solid-liquid interface structure of mutual authentication in order to reveal the influence mechanism of solute atoms in magnesium melt on heterogeneous nucleation interface. Using molecular dynamics method based on embedded atom method, the radial distribution function, pair distribution function, mean square displacement, total energy and VMD molecular dynamics visualization analysis of microscopic structure analysis and characterization methods of pure Mg melt of Mg-3Al alloy. Mg-6Al alloy and Mg-9Al alloy in liquid-solid transformation process of microstructure evolution mechanism is analyzed. When the cooling rate is 1 * 1013K/s, Mg-3Al alloy, Mg -6Al alloy and Mg-9Al alloy are eventually formed the atomic arrangement of crystal structure and orderly, but the initial transition temperature is slightly different, Mg-3Al alloy to 497K temperature in the system has characteristics of crystal structure, Mg-6Al alloy and Mg-9Al alloy in liquid-solid transition temperature were 647K and 747K. in the bond between Mg atom in Mg-Al alloy with Al the atom has stronger. RMS displacement (MSD) and analysis of the total energy of the system is further validated by radial distribution function (RDF). The results by VMD molecular dynamics visualization analysis can be directly observed crystallization system, final system has formed the atomic arrangement of crystal structure and orderly, and with the increase of aluminum content the alloy system, the crystallization degree of the system is also improved. The structure of Mg/Mg-3Al alloy system of the solid-liquid interface by molecular dynamics method, through visual software O Vito has recorded the atomic location file of different time, draw the position image simulation system of atoms, so as to the arrangement of atoms at the solid-liquid interface and qualitative analysis, the temperature adjustment to 680K, from the beginning of the 5000 step, although the Mg-3Al alloy still in the disordered state of atoms, but the liquid region near the solid-liquid interface start showing some order, and found that Al atoms diffuse into the interface of the trend. With the increase of the relaxation time, the matrix layers of atomic part near the solid-liquid interface has been ordered to some extent, it is attached to a solid substrate with the liquid of the line, and the Al atoms in the alloy in the ordered state, occupy the position of the Mg atom. The solid-liquid coexistence system of relaxation after the 1200000 step, the system in the two-phase equilibrium liquid near the region of the atom at the solid-liquid interface are Ming Order of the display. By OM, SEM and EDS analysis method of magnesium alloy in primary and eutectic phase interface microstructure and composition distribution of the experimental observation and analysis, the influence of Mg grain orientation on the close packed plane distribution of eutectic growth and Al elements discussed. The results show that different Mg close packed plane with the formation of Mg-9Al alloy interface is not the same effect on the solute distribution, pure Mg (0001) dense solid-liquid interface surface and the formation of Mg-9Al alloy melt can promote the eutectic growth of Al alloy liquid solute diffusion along the direction perpendicular to the solid-liquid interface are more active.

【学位授予单位】：南昌大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG292

【参考文献】