【作者】 魏明志；

【导师】 袁晓俭； 胡望宇；

【作者基本信息】 湖南大学 ， 材料物理与化学， 2006， 硕士

【摘要】 由于纳米材料具有一系列不同于传统材料的特殊性能，而成为当今科学研究的热点。本文应用分子动力学方法(MD)结合本研究组的改进分析型嵌入原子模型(MAEAM)，模拟研究了V纳米晶体受温度和尺寸影响的热稳定性，Zr纳米晶体的相变过程以及相变临界温度。研究样品是由Voronoi几何方法构建的，模拟过程用到了径向分布函数(RDF)、键对分析技术(CNA)、晶体截面图以及能量等分析技术和方法。 对V纳米晶体的初始结构分析显示：随平均晶粒尺寸的减小，晶界原子比率明显增加，尺寸为2.8nm的晶界原子数超过了50％。统计平均得到的晶粒内晶格参数随尺寸的减小而增大，晶格出现膨胀现象。分析原子平均能量发现，晶界原子能量不随晶粒尺寸的变化而变化，说明晶界结构没有发生变化；但晶粒内部原子的平均势能随尺寸的减小在增加，这是由晶粒内晶格畸变所造成的。 模拟不同晶粒尺寸V纳米晶体的热稳定性发现：随晶粒尺寸的减小热稳定温度明显降低，对应平均尺寸为2.8nm的V纳米晶体其热稳定温度仅为500K。分析原因最主要的是高比例高能晶界结构的存在，另外，通过分析比较发现，晶粒内部的晶格畸变也是一个不容忽视的原因。对热稳定温度范围内纳米晶体的热膨胀现象模拟得到：其热膨胀系数要明显高于传统晶体，大约是传统晶体的2倍以上；同时，模拟晶粒内的热膨胀发现，其值也要高于传统晶体。分析了热稳定温度以上晶粒的生长现象，主要表现为晶界的迁移；考虑到本文模拟的晶粒数较少且尺寸较小，所以很难观察到晶粒位向旋转引起的生长现象。 模拟研究了平均晶粒尺寸为7.05nmZr纳米晶体的相变。对1600K温度下等温相变过程模拟发现，Zr纳米晶体的相交过程分三部分完成：首先在亚稳态高能晶界区形成新相(BCC结构)晶核，对应能量的迅速降低过程；其次通过相界的迁移完成HCP-BCC的相变，由于相界的增加出现能量的升高；最后是新相的生长，对应晶界原子逐渐减少，系统能量不断降低，变为一个局部存在缺陷的BCC单晶。进一步模拟得到其相变临界温度在1200-1250K间。更多还原

【Abstract】 Nowadays nanocrystalline materials have become very popular due to their special properties. The phase transition of nanocrystalline Zr and thermal stability of nanocrystalline V, including the effect of temperature and grain size, are calculated by means of the modified analytic embedded-atom method (MAEAM) and molecular dynamics (MD) in this paper. The samples are generated with Voronoi cell method. The radius distribution function (RDF), common neighbor analysis method (CNA), nanocrystalline section and potential energy curve are used to analysis the simulation process.Simulating the initial structure, it shows that the percentage of grain boundary atom increases as the reducing of mean grain size. Especially for the sample with a mean grain size of 2.8nm, the fraction of boundary atoms exceeds 50%. With reducing grain size, the lattice parameter of intergrain increases. Comparing the potential energy of atoms, there’re no change for boundary atoms, while increase for the intergrain atoms with the reducing of mean grain size. That is to say, the boundary structure has no change while the intergrain lattice is distorted.The simulation result of thermal stability of nanocrystalline V indicates that the stability temperature reduces clearly as the reducing of mean grain size. Especially for the sample with an average grain size of 2.8nm, its thermal stable temperature is only 500K. After having studied the microstructure of nanocrystalline V, we think that the reasones which causes the grain growth of nanocrystalline V mainly comes from the high percentage and high distorted crystal boundaries structure. In addition, the distorted lattice in the intergrain can’t be neglected. The expanse of the nanocrystalline V below the thermal stable temperatures, is higher than that of coarse crystalline, it’s about twice as many. In addition the thermal expansion of intergrain is higher than that of conventional crystals. The grain growth phenomenon studied above stability temperatures, is reflected mainly in grain boundary migration. Taking into account the small grain size and grain number, it is difficult to observe the grain rotation-induced growth phenomenon.The phase transition of nanocrystalline Zr with an average grain size of 7.05nm has been simulated. The radius distribution function, the fraction of different type atoms differentiated by the common neighbor analysis method and更多还原