Theoretical Investigation On The Microscopic Structure Of Zr-based And Fe-based Metallic Glasses In Liquid/Solid State

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.