【作者】 吴臣；

【导师】 沈军；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2014， 博士

【摘要】 非晶合金由于其短程有序而长程无序的原子堆垛结构而呈现出优异的性能。非晶合金微观结构的研究对理解其优异的性能有重要的意义。分子动力学方法可以模拟所研究体系在微观尺度以及短时间内微观结构的变化，因而被广泛地应用于非晶合金微观结构的研究中。本文采用经典分子动力学方法和第一性原理分子动力学方法系统地研究了Zr基和Fe基非晶合金在熔体状态、过冷液态和玻璃态的微观结构；将微观结构与微观动力学关联；深入研究了两种合金体系中微观结构对合金的玻璃形成能力的影响。Zr53Cu18.7Ni12Al16.3(Zr53)和Zr50.7Cu28Ni9Al12.3(Zr50.7)合金熔体中的短程序可用Al原子的配位团簇来表示。Al原子的配位团簇主要包括Kasper多面体团簇和不规则多面体团簇。Kasper多面体团簇的中心原子处于局域势能面上势垒较深的局域势能极小值处，其运动速率较小，有利于提高体系的玻璃形成能力。Zr53中Al含量比Zr50.7更高，导致Zr53中二十面体团簇的数量较Zr50.7低，同时Zr53配位团簇中化学组成更加丰富，进一步造成Zr53中二十面体团簇的畸变程度更大。这两方面微观结构因素决定了Zr50.7比Zr53的玻璃形成能力更高。Fe79C21合金熔体的原子结构可以用C原子配位团簇来表示。Fe79C21合金熔体中C原子周围的Kasper多面体配位团簇可以降低C原子的运动速率，有利于体系中非晶合金的形成。随后，本文提出了短程序对称性参数（Short range order Symmetry Parameter，SSP）来描述体系中团簇的畸变程度。用SSP研究Zr50.7和Zr53合金熔体时发现，Zr50.7中二十面体团簇的对称性更高。Cu50Zr50过冷液态合金的短程序可用Cu原子配位团簇表示。在Cu50Zr50过冷液态合金中，Zr5和Zr4二十面体团簇存在着空间关联作用。这种作用阻碍了过冷液态合金中原子的运动，有利于非晶合金的形成。Zr50.7和Zr53过冷液态合金的微观结构在绝大多数情况下可用Al原子的配位团簇表示。但是，在1000K左右，Zr50.7中出现了不含Al的Ni原子配位团簇。这种团簇相对于其他Ni原子配位团簇具有较低的移动速率，提高了Zr50.7合金的玻璃形成能力。Fe70Mo10B20过冷液态合金的原子结构可以用B原子配位团簇来表示。Kasper多面体团簇（（0，3，6，0）、（0，2，8，0）、（0，2，8，1）和（0，4，4，0））在B原子配位团簇中占绝大多数。这些Kasper多面体团簇具有较长的寿命，有利于非晶合金中动力学延缓以及非晶合金的形成。应用SSP研究Cu50Zr50过冷液态合金中配位团簇的畸变程度时发现，过冷液态合金中SSP表现为宽分布谱，证明具有相同Voroni因子和化学成分的团簇，其结构差异仍然很大。进一步，将SSP与原子尺度剪切应力相关联，可以得出大SSP团簇的中心原子受到的原子尺度剪切应力也更大。随着体系温度的降低，配位团簇的SSP以及配位团簇中心原子受到的原子尺度剪切应力都减小。微观动力学分析的结果表明，对于具有相同Voronoi因子和化学成分的团簇，SSP越小其中心原子的活动能力越弱。从局域势能图景的角度看，配位团簇的SSP越小，团簇的中心原子处于势能面上具有更深势垒的局域能量极小值处，也因此具有更缓慢的移动速度。因此，含有更多小SSP配位团簇的体系具有更大的GFA。采用经典分子动力学方法模拟了温度为1000K的Cu50Zr50和Cu46Zr46Al8过冷液态合金的形核过程；采用第一性原理分子动力学方法研究Fe79C21过冷液态合金中微观结构对形核过程的影响。通过CCE方法识别出系统中的类晶体序结构。在形核过程中，三种过冷液态合金都首先析出类BCC结构。过冷液态合金中Kasper多面体团簇抑制类BCC结构的析出，阻碍晶核的形成。过冷液态合金中类BCC结构自发地聚集以降低原子的平均能量。当聚集体中原子总数接近500时，聚集体可以稳定地存在于过冷液态合金中。Al原子的加入使得Cu46Zr46Al8过冷液态合金中的Kasper多面体团簇更加稳定，更进一步地抑制类BCC结构的析出。因此Cu46Zr46Al8合金的玻璃形成能力大于Cu50Zr50合金。Fe79C21过冷液态合金于液相线以下100K处开始析出类BCC结构。用经典分子动力学方法和第一性原理分子动力学方法研究了几种二元、三元以及四元Zr基和Fe基非晶合金的微观结构。通过对SSP的分析，我们发现了Cu50Zr50非晶合金中局域化学成分对团簇畸变程度的影响。发现Cu-Zr二元非晶合金的玻璃形成能力与合金中团簇的SSP密切相关。团簇的对称度越高，合金的玻璃形成能力越强。通过分析Fe65Mo14C15B6的SSP，发现体系中共价键的存在可以显著地降低团簇的SSP，增加体系的玻璃形成能力。结合计算机模拟与拉曼光谱，证明了Fe65Mo14C15B6、Zr53和Zr50.7非晶合金中存在中程序，这种中程序进一步降低了原子的移动速率，增加了体系的玻璃形成能力。最后对Fe65Mo14C15B6非晶合金的微观结构与弹性性质关系的研究发现，合金的弹性变形主要对应以Fe原子为中心的类二十面体中程序的收缩。更多还原

【Abstract】 Metallic glass possesses distinguished properties due to its unique atomic packing structure, owning short range order but lacking of long range order. In order to understand its excellent end-properties, it is of great importance to study the microscopic structure of metallic glasses (MGs). Molecular dynamics simulation, which characteriaze the structural changes particularly at microscopic length scale and short time scale, has been extensively implemented to investigate the microscopic structure of MGs. In this dissertation, both of the classical molecular dynamic (CMD) simulation and ab initio molecular dynamic (AIMD) simulation are conducted to systematically study the structure of Zr-based, Fe-based metallic glasses in the melting, supercooled and amorphous states. The microscopic structure has been correlated with the microscopic dynamics. The influence of microscopic structure on the glass forming ability (GFA) of MGs is explored.The short range order in Zr53Cu18.7Ni12Al16.3(Zr53) and Zr50.7Cu28Ni9Al12.3(Zr50.7) melts can be described by the Al-centered coordination cluster (CC). These Al-centered CCs consist of Kasper polyhedral cluster and irregular polyhedral cluster. The central atoms enclosed in Kasper polyhedral cluster are located at the local energy minima with deeper energy barrier which constrains the movement of central atoms. This dynamical slowing down contribute to the GFA of the alloy system. The higher Al-content in Zr53than in Zr50.7makes the icosahedra population of Zr53much less than that of Zr50.7and the distortion of icosahedra in Zr53even stronger than in Zr50.7. Both of the two former factors account for the higher GFA of Zr50.7than Zr53. The atomic structure of Fe79C21melt can be described by the C-centered CC. The Kasper polyhedral cluster around C reduces the velocity of these C atoms, and is propitious to the glass formation. A new parameter, short range order symmetry parameter (SSP), is proposed in order to quantify the degree of distortion for the CC in the system. With the help of SSP, it is found that the CC in Zr50.7possesses larger degree of symmetry.The short range order in Cu50Zr50supercooled alloy can be described by the Cu-centered CC. In Cu50Zr50supercooled alloy, there exists a spatial correlation between the Zr5and Zr4icosahedra. This correlation causes the slowing down of the dynamics in the system and contributes to the glass formation. The atomic structure of Zr50.7and Zr53supercooled alloys can be generally described by Al-centered CC. However, at about1000K, Al-free Ni-centered CC emerges in Zr50.7. This kind of CC corresponds to small velocity, and is good for the improvement of GFA in Zr50.7. The atomic structure of Fe70Mo10B20supercooled alloy can be described by the B-centered CC. Kasper polyhedral cluster ((0,3,6,0),(0,2,8,0),(0,2,8,1) and (0,4,4,0)) accounts for a dorminant part in these B-centered CC. These Kasper polyhedral clusters possess long lifetime, and thus are favored by the dynamical slowing down and glass formation in the system. By means of SSP, the degree of distortion for the CC in Cu50Zr50supercooled alloy is studied. There exist broad SSP distribution-spectra demonstrating that large distinction exists between the CC with identical Voronoi index and chemistry. Moreover, SSP can be used to quantify this distinction. By further analysis, SSP is correlated with the atomic level shear stress. The central atom of large-SSP CC bears large atomic level shear stress. With cooling, both the SSP and atomic level shear stress decreases in the CC. Analysis of the microscopic dynamics show that, for the CC with identical Voronoi index and chemistry, a small SSP is associated with a low mobility of the atom. From the perspective of potential energy landscape, the central atoms of small-SSP clusters are located at the local energy minima with deeper potential barrier. These atoms have slow dynamics. Consequently, alloys with more small-SSP cluster have large GFA.The nucleation process in Cu50Zr50and Cu46Zr46Al8supercooled alloys are simulated by CMD method, the influence of microscopic structure on nucleation in Fe79C21supercooled alloys is studied by AIMD simulation. By means of the characteristic crystallographic element method, the crystalline order is identified from the liquid alloys. Buring the nucleation process, BCC-like structure first precipitates in these three liquid alloys. The Kasper polyhedral clusters inhibit the precipitation of BCC-like structure and the formation of crystal nucleus. The BCC-like structures spontaneously aggregate with each other in order to reduce the enery per atom, forming an aggregation of atoms in the supercooled alloy. When the total number of atoms in the aggregation approaches500, the aggregation can stably exist in the supercooled alloy. By introducing Al atoms, the Kasper polyhedral clusters in Cu46Zr46Al8supercooled alloy are more stable which further inhibit the precipitation of BCC-like structure. Therefore, the GFA of Cu46Zr46Al8alloy is larger than that of Cu50Zr50alloy. The BCC-like structures start to precipitate at about100K below liquidus temperature in Fe79C21supercooled alloy.CMD and AIMD simulations are implemented to study the microscopic structure of binary, ternary and quaternary Zr-based and Fe-based metallic glasses. Through the analysis of SSP, the influence of local chemistry on SSP is presented for Cu50Zr50MG. The intimate relationship between SSP and GFA has been found in Cu-Zr binary alloys. Large GFA corresponds to small SSP of the clusters. Study on the SSP in Fe65Mo14C15B6metallic glasses shows that the existence of the covalent bonds in metallic glass can significantly reduce the SSP of CCs. This reduction improves the GFA of the system. By combination of the computer simulations and Raman spectrum, the evidence of the existence of the medium range order is found in Fe65Mo14C15B6, Zr53and Zr50.7MGs. This kind of medium range order further slows down the mobility of atoms and enhances the GFA. Finally, the correlation between the elastic properties and the microscopic structure in Fe65Mo14C15B6metallic glass is studied. It is found that the elastic deformation in Fe65Mo14C15B6MG originates from the shrink of the icosahedral type medium range order centered at Fe atoms.更多还原