Zr-Cu合金快速凝固过程中微观结构演化及动力学性质的模拟研究

发布时间：2018-02-05 00:47

本文关键词： 非晶合金分子动力学快速凝固微观结构粘度扩散　出处：《燕山大学》2015年博士论文　论文类型：学位论文

【摘要】：非晶合金具有优异的物理化学性能,是一类应用前景广阔的新型先进材料。近20年来,Zr基非晶合金的研究工作取得了巨大进展,其具有高强度、高韧性、抗辐照、耐低温和抗原子氧腐蚀等特殊性能,广泛的应用于核工业和航空航天等领域。Zr基非晶合金优异的性能源于特殊的微观原子结构。本文选取Zr-Cu二元合金为研究对象,采用分子动力学模拟方法,对合金熔体快速凝固过程中,过冷液态、玻璃转变过程和非晶固态三个温度区间内的原子结构演化及其对动力学性质的影响进行了研究。具体内容如下:运用双体分布函数、配位数、键对、键角和Voronoi多面体等结构分析方法,研究了Zr-Cu合金凝固过程中的微观结构演化规律。双体分布函数及配位数分析发现Zr原子和Cu原子周围的局域结构的差异以及异质原子的强烈交互作用。Voronoi多面体和键对分析发现Zr-Cu合金处于过冷液态时,团簇表现出五次对称较熔融态时加强的特点,但团簇相互链接性不强。Zr-Cu合金在玻璃转变过程中,具有更高五次对称性的团簇含量增加,团簇配位数增加且具有饱和性键对的团簇结构增加;二十面体团簇相互链接性显著增加并且以嵌套链接为主。以上几种结构参数的变化初步阐明了微观结构的演化与玻璃转变之间的内在关联。本文利用格林库伯公式对过冷熔体的粘度进行模拟计算,在微观结构演化对粘度影响的计算过程中发现,合金熔体在快速凝固过程中Voronoi熵减小,粘度提高。不同的冷却条件对粘度的影响明显,冷却速率增加或压力减少,导致粘度下降,且Voronoi熵增加,在模拟范围内粘度对温度最敏感,压力和冷速对粘度的影响较小。计算过程中验证了熔体粘度与温度的关系符合Arrhenius关系:????/expRTEA。粘度与Voronoi熵的关系符合Adam-Gibbs关系:??C??expTSCA,揭示出玻璃转变过程中合金热力学与动力学的相关性。进一步结构分析发现,Voronoi熵降低的主要原因是系统形成以二十面体团簇为主的结构,增加了非晶合金的稳定性和有序度,使原子集团重排和跃迁更加困难,导致粘度增加。本文研究了Zr-Cu合金快冷凝固过程中微观结构演化对扩散的影响,模拟发现随着冷速降低、压力增强和温度降低都使体系中五次对称性增加,尤其在玻璃转变温度附近,而且五次对称性在空间的分布不均匀。通过计算原子均方位移发现五次对称性小于0.3的原子具有较快的扩散行为,在0.7和1之间的原子扩散缓慢,验证了五次对称结构的稳定性。利用爱因斯坦公式对合金自扩散系数进行模拟计算,发现随着温度降低、压力升高或者冷速降低自扩散系数减小,尤其在玻璃转变温度附近;通过非高斯参数的计算发现扩散行为在空间上是不均匀的。扩散激活能的减小是导致自扩散系数增加的重要原因。扩散系数与五次对称性符合公式D=Aexp(C/Td5)的关系,揭示了快速凝固过程中合金微观结构与动力学具有内在联系。
[Abstract]:Amorphous alloys have excellent physical and chemical properties, is a kind of new advanced materials has broad application prospects. In the past 20 years, the study of Zr based amorphous alloys have made great progress, which has high strength, high toughness, anti radiation, low temperature resistance and anti corrosion of atomic oxygen, widely used in the nuclear industry and aerospace fields of.Zr based amorphous alloy excellent in atomic energy structure. This paper selects Zr-Cu two alloy as the research object, using molecular dynamics simulations on the melt rapid solidification process, supercooled liquid state, the glass transition of amorphous solid and three temperature range the evolution of the atomic structure and its influence on the dynamic properties were studied. The specific contents are as follows: using the pair distribution function, coordination number, the key, analysis method of bond angle and Voronoi polyhedron structure of Zr-Cu Alloy Condenser The evolution of microstructure in the process of solid law. Analysis of the pair distribution function and coordination number found differences in local structure around Zr atoms and Cu atoms and strong interaction of heterogeneous atoms.Voronoi polyhedron and the pair analysis found Zr-Cu alloy in the supercooled liquid, strengthen the characteristics of clusters exhibit five fold symmetry is the melting of however, the clusters are linked to each other is not strong.Zr-Cu alloy during the glass transition has increased five times higher symmetry clusters in clusters, the coordination number increases with cluster structure to increase the saturated bond; twenty surface clusters linked significantly increased and the nested links changes. The above several structural parameters preliminarily explained the relation between evolution and microstructure of the glass transition. By using the Green Kolb formula of undercooled melt viscosity were simulated in microstructure To calculate evolution process on viscosity of Voronoi alloy melt in rapid solidification process of entropy decreases, the viscosity increased. Effects of different cooling conditions on viscosity obviously, the cooling rate increase or reduce the pressure, resulting in decreased viscosity and Voronoi entropy increase in the simulation range of viscosity is the most sensitive to temperature, pressure and cold the speed had little effect on the viscosity. The calculation process to verify the relationship between viscosity and temperature conforms to the Arrhenius relationship:???? /expRTEA. viscosity and Voronoi entropy is consistent with the Adam-Gibbs?? C?? expTSCA, reveals the correlation between thermodynamics and kinetics of glass transition in the process of alloy structure. Further analysis found that the main reason Voronoi is the system structure entropy decreased to twenty face clusters mainly, increase the stability of amorphous alloy and the order of the group, the atomic rearrangement and transition more difficult, leading to viscosity Increase. This paper studies the Zr-Cu alloy diffusion effect on the microstructure evolution of the rapid solidification process, simulation shows that with the cooling rate decreased, and the decrease of temperature the pressure increase five times increase the symmetry of the system, especially in the glass transition temperature near five times and the distribution of symmetry in space by computing is not uniform. The atomic mean square displacement found five symmetry less than 0.3 atoms with the diffusion behavior of fast, slow diffusion in 0.7 and 1 between the atoms, verified five symmetrical structure stability. By using the Einstein formula of self diffusion coefficient were calculated and found that with the decrease of temperature, pressure or cooling rate decreased self diffusion the glass transition temperature coefficient, especially in the vicinity; calculated by non Gauss parameters that the diffusion behavior is not uniform in space. The diffusion activation energy decrease is the result of self diffusion coefficient. The important reason for the addition is the relationship between the diffusion coefficient and the five order symmetry conforms to the formula D=Aexp (C/Td5). It reveals that the microstructure and dynamics of the alloy are closely related to the process of rapid solidification.

【学位授予单位】：燕山大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.414

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