Investigation Of Structural Evolution Of Disordered Alloys By Synchrotron Radiation Techniques And Molecular Dynamics Simulations

Metallic glasses(MGs)have unique properties due to their disordered atomic packing,among which Fe-based MGs have attracted extensive attention for their cheap cost,superior mechanical properties,excellent softmagnetic properties and high corrosion resistance,etc.However,the small critical size owing to the poor glass forming ability(GFA)seriously restricts their wide-spread engineering applications.It is well recognized that the microstructure of materials determines their macro-properties.Therefore,in order to develop MGs with high GFA and outstanding properties,it is necessary to gain a better understanding of their microstructure and atomistic details on mechanism of the glass transition.Moreover,MGs are usually prepared by rapid quenching of their high temperature liquids.However,the atomic structure and nucleation process in metallic liquids is little known.Thus,it may be helpful to study microstructure and its evolution of low-melting metallic liquids under extreme conditions for designing new materials.In this thesis,by using synchrotron radiation X-ray techniques and theoretical calculations,we mainly focus on(I)the effect of minor addition of some particular elements on the GFA and microstructure oT MGs,(ii)the atomic structure,structural evolution and nucleation process in Ga-In metallic liquids under high temperature and high pressure conditions.The thesis consists of six chapters are listed as follows:Chapter one briefly introduces the development,potential applications and the general structural models of metallic glasses.Besides,the structural features and the research progress in metallic liquids are also introduced.Chapter two gives a briefly introduction to the experimental techniques,such as X-ray diffraction and X-ray absorption fine structure,as well as the theoretical methods including classical Molecular Dynamics simulation,ab initio Molecular Dynamics simulation and Reverse Monte Carlo simulation,for structural study of metallic glasses and liquids.The most commonly used structural characterization are also presented here,such as pair correlation function,scatter factor,bond angle distribution,Voronoi tessellation,bond-orientational order parameter,atomic cluster alignment and the nearest-neighbor correlation index analysis.In chapter three,the effect of minor Y and Nb addition on the enhancement of GFA and atomic structure of Fe-B metallic glasses is discussed by x-ray diffraction and x-ray absorption fine structure,combined with Reverse Monte Carlo and ab initio molecular dynamics simulations.It is found that in Fe72Y6B22 and Fe68Nb4Y6B22 MGs,Y atoms likely take the 8c sites of Fe23B6 phase and Nb atoms tend to occupy the 32f sites owing to the relatively small atomic radius compared to Y atoms,which promotes the formation of complex(Fe,Y,Nb)23B6-like phase,leading to the difficulty of complex phase precipitation and improved stability of amorphous state.Besides,minor Y and Nb additions not only increase the local element segregation but also promote the formation of icosahedron-like clusters and their connectivity with large polyhedral,frustrating the geometry of the competing crystalline phases and eventually enhancing the GFA during solidification.In chapter four,by using high temperature in-situ synchrotron X-ray diffraction and ab initio molecular dynamics simulation,we systematically study the microstructure and temperature-induced liquid-liquid crossover in the liquid Ga-In alloys with different compositions.We reveal a reversible structural crossover in Ga20In80,Ga50In50,Ga70In30 and Ga85.8In14.2 liquids above the melting temperatures,in which there exist two kinds of liquid states with different local atomic packings.It is likely ascribed to the atomic rearrangements of Ga and In atoms from a relative random packing at high temperatures to a relative non-uniform packing at low temperatures,in which In atoms prefer to have more In neighbors.Structural evolution in the supercooled liquid region of Ga85.8In14.2 eutectic alloy and subsequent solidification process are also discussed.It is found that no sudden structural change exists in the supercooled liquids,but In-rich regions likely act as the precursors to promote the solidification process.The theoretical results show that the supercooled liquid prefers to solidify into bct-like structure locally similar to bct-Ga-? and bct-In phases.in contrast to a mixture of orthorhombic Ga-? and bct-type In crystalline phases measured by XRD.The difference in final solidification phases could be linked with different cooling rates used in two methods or the small box size and periodic boundary conditions in simulations.In chapter five,we present a combined experimental and theoretical study of high pressure and high temperature behavior of liquid GaIn eutectic alloy.A high pressure and high temperature"phase diagram" of GaIn eutectic alloy is constructed.No pressure-induced abnormal structural change is observed during isothermal compression at low(300 K)and high(700 K)temperatures.The theoretical results reveal that there may exist phase selection during solidification process and different liquid-to-solid transition modes at high and low temperatures.At low temperatures below 600 K(300,350,400,500 and 600 K),the liquid proceeds first into body-centered cubic(bcc)-like local order and transforms into highly symmetric structure with more pronounced face centered cubic(fcc)-like ordered packing upon increasing applied pressure.However,at high temperatures above 650 K(650,673 and 700 K),the liquid directly solidifies into bcc-like structure.In chapter six,we summarize the research work of this thesis and prospect the several useful directions for the next study.