Cu-Zr基非晶合金结构及动力学数值模拟

非晶合金的各种优异性能与其独特的原子结构特征存在内在的、直接的关系。因此,研究非晶合金的原子结构有利于深入地理解非晶合金的各种性能,并且可以加深对玻璃转化现象的认识。
活动房本文主要采用数值模拟方法,研究了Cu-Zr合金中的短程序结构,给出了原子团簇的堆积方式;探讨了短程序结构随着温度的演化以及与成分的关系;研究了合金的动力学和热力学性质,表征了扩散系数和热膨胀系数与成分的关系。结合已有的实验结果,阐明了Zr-Cu-Ni-Al四元合金系的结构与成分的关系,揭示了结构与玻璃形成能力的可能关系。本文的主要研究内容如下:非晶合金中存在二十面体团簇,但是,在二元合金中,二十面体团簇的堆积方式并不清楚。本文研究了Cu60Zr40二元非晶合金中的短程序结构,给出了二十面体团簇的堆积方式。研究结果表明,在该合金中,存在两种短程序团簇,一种是以Zr原子为中心、平均配位数为14.7的团簇,另一种是以Cu原子为中心、平均配位数为11.6的团簇。二十面体团簇都是以Cu原子为中心,并且主要以相互贯穿的方式形成弯曲的链条式结构。链条中二十面体团簇的数量介于3～11个之间,从而形成中程有序结构。这种中程序结构在合金中相互离散地、无规则地分布,使合金中的二十面体团簇区域和非二十面体团簇区域相互交错和镶嵌,表现出原子尺度上的结构不均匀性。Cu60Zr40合金中原子短程序结构随温度的变化关系表明,二十面体团簇主要是在玻璃转变温度Tg以上约200 K的温度范围内形成,在此过程中,体系的平均配位数和最近邻键长基本保持不变,原子的排列方式变得更加有序。对Cu60Zr40合金中原子的动力学性质研究发现,当温度高于1200 K时,原子的弛豫可以分为两个阶段,在短时间段原子作振动,此后,原子的运动表现为液体的流动。当温度低于1100 K时,在β弛豫时间段出现笼子效应。合金中Cu原子的扩散比Zr原子快。根据模式耦合理论(MCT),计算得到了Cu60Zr40合金的临界玻璃转变温度Tc,其数值为1008.2 K。对扩散系数的拟合结果表明,在液态,MCT幂指数方程比Vogel-Fulcher-Tammann方程更适合于描述合金的动力学性质,从而验证了MCT理论。在过冷液相区,体系的动力学具有不均匀性,随着温度的降低,动力学不均匀性逐渐加强。合金中原子具有不同的弛豫机制,Cu原子的扩散存在激活跳跃过程,而Zr原子的扩散则没有这一过程。通过对不同成分合金样品的制备、测试分析,研究了Cu-Zr合金的玻璃形成能力,发现Cu50Zr50合金的玻璃形成能力最大,合金中主要的竞争晶化相是Cu10Zr7相。通过数值模拟分析发现,合金中团簇的化学短程序随成分发生明显改变。在所研究的成分范围内,随着Cu含量的增加,以Cu原子为中心的团簇的平均成分从Cu5Zr8变化为Cu8Zr5,以Zr原子为中心的团簇的平均成分从Cu7Zr9变化为Cu11Zr5。局域化学短程序的变化促进了Cu10Zr7相的产生,从而对合金的玻璃形成能力产生显著的影响。Cu-Zr合金的热力学性质和动力学性质随成分发生变化。对合金热力学性质的研究发现,在液态和玻璃态,合金的热膨胀系数随成分的变化满足不同的线性变化规律,变化率数值分别为0.275和0.015。Cu50Zr50合金液体的热膨胀系数最小,热稳定性最强,从而解释了其强的玻璃形成能力。对合金的动力学性质研究发现,在不同的温度区间,成分变化对扩散系数的影响不同。在高温区(T >1300 K),Cu、Zr原子的扩散系数不随成分的变化而发生变化,并且满足Arrhenius关系。在过冷液相区(1300 K> T >1000 K),用Vogel-Fulcher-Tammann方程对扩散系数进行拟合,得到的激活能数值表明,在所研究的四种合金中,Cu64Zr36合金的动力学稳定性最强。根据同步辐射高能XRD实验结果,采用逆蒙特卡罗方法,构建了Zr53Cu18.7Ni12Al16.3,Zr51.9Cu23.3Ni10.5Al14.3和Zr50.7Cu28Ni9Al12.3三种块体非晶合金的原子结构,研究了三种合金结构的差异,发现随着Cu含量的增加,配位数为CN=11及CN=12的多面体团簇的数量增加,配位数为CN=8及CN=9的多面体团簇数量减少。对各元素的团簇结构分析结果表明,随着Cu含量的增加,以Zr和Al原子为中心的团簇中,配位数为CN=11及CN=12的多面体团簇数量增加,配位数为CN=9及CN=10的多面体团簇数量减少。Cu含量的增加使非晶合金中以Zr和Al原子为中心的团簇转化为原子数量更多的原子团簇。在三种合金中,原子的分布存在不均匀性,表现为局域原子密度分布的离散性与无规则性,并且满足正态分布。合金中原子结构的不均匀性与玻璃形成能力直接相关
【Abstract】 There is a direct relationship between outstanding properties of metallic glasses and specific characteristics of atomic structure. Investigation in atomic structure of metallic glass will not only benefit a deep apprehension of the various properties of metallic glass, but also promote an interpretation of glass transition phenomena. In this thesis, numerical simulations are used to investigate the short-range ordering in Cu-Zr metallic glasses. A packing scheme of atomic clusters is obtained. The structural evolution of short-range ordering with temperature and its relation with composition are explored. The dynamic and thermodynamic properties and the composition dependence of diffusion coefficient and thermal expansion coefficient are investigated. Moreover, the composition dependence of the structure of quaternary Zr-Cu-Ni-Al metallic glasses is explored based on the experimental results, the possible relation between the structure and glass-forming ability is discussed. The following provides the main contents of this thesis.Though icosahedral cluster is believed to exist in metallic glasses, the packing scheme of icosahedral clusters in binary metallic glasses still remains unclear. In this thesis, the short-range ordering in Cu60Zr40 metallic glass is investigated, the packing scheme of icosahedral clusters is constructed. It is found that there are two types of short-range clusters in the alloy, one is centered by Zr atoms with an average coordination number of 14.7; the other is centered by Cu atoms with the average coordination number of 11.6. The icosahedral clusters are entirely centered by Cu atoms and primarily form a flexural sting-like structure by interpenetration of 3-11 clusters, exhibiting medium-range order. The medium-range ordering clusters distribute irregularly and separate from each other, leading to the interpenetration of icosahedral regions and non-icosahedral regions, and thus exhibiting structural heterogeneity on atomic scale.The temperature dependence of short-range order in Cu60Zr40 metallic glass unveils that the icosahedral clusters are mainly formed within a temperature range of about 200 K above glass transition temperature Tg. During this process, the average coordination numbers and the nearest neighbor bond lengths nearly remain unchanged, but the arrangement of atoms becomes more ordering.The investigation in dynamical properties of Cu60Zr40 metallic glass suggests that, there are two processes during the relaxation at temperatures higher than 1200 K. At short times the atoms vibrate, and then at long times perform liquid-like flow. Cage effect is found during theβrelaxation at temperatures lower than 1100 K. The diffusion of Cu atoms is faster than that of Zr atoms. According to the mode-coupling theory (MCT), the critical glass transition temperature Tc is obtained to be 1008.2 K. In liquid state the MCT power-law equation is more suitable in describing the diffusion kinetics than the Vogel-Fulcher-Tammann equation. Therefore, the results validate the MCT. Dynamic heterogeneity is detected and becomes pronounced with the decrease of temperature in the supercooled liquid state. Different mechanisms are found in the atomic relaxation. The activated hopping process takes place for Cu atoms but not for Zr atoms.The glass-forming ability of Cu-Zr metallic glasses is assessed by means of sample preparation and analysis. It is found that Cu50Zr50 metallic glass is the best glass former among the studied binary alloyes, its primary crystalline phase is Cu10Zr7. By numerical simulations, the chemical short-range order is found to change significantly with composition. The average composition of the clusters centered by Cu atoms varies from Cu5Zr8 to Cu8Zr5, while that of the clusters centered by Zr atoms varies from Cu7Zr9 to Cu11Zr5. This variation promotes the formation of Cu10Zr7 phase and brings about great effect on the glass-forming ability of metallic glasses.Meanwhile, the dynamic and thermodynamic properties of Cu-Zr alloyes change with composition. In liquid and glassy states, the composition dependences of thermal expansion coefficients indicate different linear relationships, the corresponding slopes are 0.275 and 0.015, respectively. That the thermal expansion coefficient of Cu50Zr50 metallic liquid is smallest among the studied alloys indicates its largest thermal stability and thus interprets its highest glass-forming ability. Effect of composition on diffusion coefficient varies with temperature. At temperatures higher than 1300 K, diffusion coefficients of Cu and Zr atoms are independent of composition and follow the Arrhenius law. However, at temperatures between 1300 K and 1000 K, the activation energies obtained by fitting Vogel-Fulcher-Tammann equation to diffusion coefficients of both types of atoms suggest that the dynamic stability of Cu64Zr36 alloy is largest in the supercooled liquid region.In addition, atomic structures of Zr53Cu18.7Ni12Al16.3, Zr51.9Cu23.3Ni10.5Al14.3 and Zr50.7Cu28Ni9Al12.3 bulk metallic glasses are investigated using reverse Monte Carlo method. The number of clusters with coordination number of 11 and 12 is found to increase with Cu content, while that with coordination number of 8 and 9 decreases. With increasing Cu content, the number of clusters with coordination number of 11 and 12 increases but that with coordination number of 9 and 10 decreases among the clusters centered by Zr and Al atoms. The clusters centered by Zr and Al atoms become larger. Structural heterogeneity is found on atomic scale in the three metallic glasses. The local atomic density distributes separately and irregularly, following a normal distribution. The atomic heterogeneity of metallic glasses is closely related to its glass-forming ability