Structural Evolution And Thermodynamical Properties Of Metallic Liquids And Glasses Studied By Ab Initio Molecular Dynamics Simulations

The structural evolution and thermodynamical properties of metallic liquids and glasses have been the long-standing topics in materials science and condensed matter physics. The metallic liquids are usually located in the high-temperature place of the phase diagrams, and mostly interpreted to be identical and homogeneous phase. The metallic liquids transform into periodic-packing metallic crystals during the equilibrium cooling process, while vitrifying into metallic glasses if rapidly quenched. However, when digging deeply into the atomistic structure, liquid-to-liquid phase transition, an intriguing but still elusive phenomenon, has been reported in a large number of metallic liquids, such as pure liquid Ce, supercooled liquid Zr41.2Ti13.8Cu12.5Ni10Be22.5 and liquid La50Al35Ni15 alloys. According to the structure-property relationship, the atomic structure of metallic glasses plays a decisive role in the superior mechanical and related properties. To this end, the detailed structural evolution and thermodynamical properties of metallic liquids and glasses are of significant importance to facilitate the improvement from both scientific research and industrial applications.In this thesis, the main research is carried out based on these two viewpoints. We systematically study the structural evolution and thermodynamic properties in Al-rich Al-Cu binary alloys, and comprehensively analyze the liquid Al2Au, Al2Ag and Al2Cu systems. We also study the liquid-to-liquid crossover of liquid Ga at high temperature zone, and liquid structure and dynamical properties of liquid AgGe alloy at the cooling stage. We focus on studying the metallic liquids and glasses via the synchrotron-based experiments and ab initio molecular dynamics simulations. Then the systems have been interpreted by pair-correlation functions, structure factors, Honeycutt-Anderson index, coordination number, mean square displacement and diffusion coefficient, bond-orientational order, Voronoi tessellation and atomistic cluster alignment methods. As such, the conclusive results have been listed as follows.(1) Complementary with synchrotron X-ray diffraction (XRD) in Shanghai Synchrotron Radiation Facility (SSRF) and electrostatic levitation (ESL) in Korea Research Institute of Standards and Science (KRISS), we conduct the ab initio molecular dynamics (AIMD) simulations of the systems of Al-rich liquid Al-Cu alloys. We calculate the compositions ranging from Al93Cu7, Al87Cu13, Al82Cu18, Al80Cu20, Al77Cu23, Al75Cu25, Al73Cu27 and Al70Cu30 alloys, and carefully analyze the structural evolutions and thermodynamical properties. We find that:(?) the structure factors and pair-correlation functions obtained from ab initio molecular dynamics simulations remarkably agree with those of synchrotron XRD measurements, and the density results obtained from ESL measurements are consistent with the calculation data, which validate the accuracy of the atomic analyses in the simulations; (?) taking the liquid Al75Cu25 alloy for example, no abnormal changes are experimentally detected in the specific heat capacity over total hemispheric emissivity and density curves in the studied temperature ranging from 800 K to 1600 K measured by ESL technique; the atomic structure evolves continuously and higher-coordinated Voronoi polyhedrons (VPs) of<0,2,8,1>,<0,2,8,2>,<0,3,6,3> and <0,3,6,4> prevail in the liquid; (?)hypoeutectic alloy of Al93Cu7, eutectic alloy of Al82Cu18, and hypereutectic allo of Al73Cu27 liquids share the similar trend in the bond-orientational order analysis and prefers the Al2Cu-like symmetry in the local atomic packing during cooling, indicating the existence of Al2Cu-like precursor phase structure in Al-rich Al-Cu liquids; (?) the self-diffusion coefficients of Al and Cu undergo an increasing deviation from Arrhenius behavior by tuning the fraction of Cu from 7 to 30 atomic percentage, revealing a non-Arrhenius phenomenon in diffusivity; the local ordering causes the non-Arrhenius behavior during cooling; with increasing Cu content from hypoeutectic, eutectic to hypereutectic compositions, local atomic packing is at odds, correlated to the deviation from Arrhenius behavior in liquid Al-Cu alloys.(2) Instructed by the structural evolution of binary Al-Cu liquids, we select the system of liquid Al2Au, Al2Ag and Al2Cu alloys, in order to study the similarities and differences of structural evolution when Al alloys with Au, Ag and Cu elements of the same column in the periodic table. We find that:(?) for Al2Au stable and supercooled liquid states the icosahedral-like clusters are negligible, and the liquid prefers clusters with the HA indices of 131 and 120, and VPs of<0,4,4,0>,<0,3,6,0> and <0,4,4, 2> with coordination numbers of 8,9 and 10, respectively; (?) these clusters are similar to the local atomic structures in the CaF2-type Al2Au crystal, illustrating the existence of structure heredity between liquid and crystalline phase in Al2Au alloy; (?) according to the structural comparison between Al2Ag and Al2Cu liquid alloys, these liquids generally expand their volumes as increasing the temperature, but their atomic distance on the first shell shrinks; Cu atoms incline to join in Al-centered clusters rather than Cu-centered clusters below 1000 K, forming the Al2Cu-like local structure and inheriting local structure to the crystalline crystal; but Ag atoms aggregate in the Ag-centered clusters, the competence between Ag atoms and Al atoms in liquid Al2Ag alloy increase the heterogeneity of local structure at the cooling stage; Ag and Al atoms possess lower activation energy exhibiting higher mobility and forming both the hexagonal close-packed AlAg2-like and face-centred cubic Al-like crystalline structure, which contribute to the eutectic characteristics and absence of intermetallic compound in liquid Al2Ag alloy.(3) We further detect the temperature-induced liquid-to-liquid crossover in liquid Ga and analyze the structural evolution and dynamical properties in liquid AgGe alloy. We demonstrate that:(?) an intriguing liquid-to-liquid crossover at the critical temperature of about 1000 K, is discovered in liquid Ga based on synchrotron XRD experiments and AIMD simulations. Remarkable agreements of structure factors and pair-correlation functions between experiments and simulations provide solid support for the analyses of atomic configurations. Below the critical temperature, the liquid Ga has an activation energy of 8.12 kJ/mol for self-diffusion coefficient, a large increasing slope of peak position shift of the first peak in structure factor and pair-distribution functions, an distinct increase of bond-orientational order (BOO) Q6, heat capacity, and weight average string length with decreasing temperature, resulting from the relative larger fractionis of high coordination polyhedral clusters. Above this temperature, the liquid exhibits an activation energy of 15.78 kJ/mol for self-diffusion coefficient, a small reducing slope of peak-position shift of the first peak in structure factor and pair-distributions functions, an increase of BOO Q6 with elevating temperature, and flat temperature-dependent heat capacity and insensitive weight average string length for forming the relative higher fractions of low coordinated polyhedra. (?) The icosahedral-like short-range order is enhanced during cooling and dominate in the undercooled Ag74Ge26 liquid. The similar atomic-packing order which was formed for amorphous phase at ambient temperature persists in the undercooled liquid. In addition, it is found that the diffusion coefficient of Ag is lower than that of Ge over the studied temperature range. Highly dominated short-range order associated with neatest-neighbors shell could mobilize the interactions and connections between clusters and empower the formation of medium-range order. Furthermore, there is a weak attraction between Ag atom and Ag or Ge atoms, but a stronger repulsion between Ge atoms in the liquid. It seems that Ge atoms disperse into the system surrounding Ag-centered clusters to form the short-range order and strengthen the tight connections after the liquid enters undercooled state, which contribute to the formation of local-packing order in the amorphous state.