The Method Development Of Structure Analysis And Structure/Dynamic Investigation Of Metallic Liquids And Glasses

The theory of metallic liquids and glasses as well as the liquid-glass transition isone of the most important fields in Condensed Matter Physics. Knowledge about thelocal atomic packing order in metallic liquids and glasses is critical for understandingand controlling the transformation pathways in metallic alloys upon rapid cooling fromthe high temperature liquids. However, structural description of disordered systems hasbeen a long standing challenge. Even detailed atomic coordinates of liquids and glassescan be available from reliable computer simulations, the underlying structural orders inthese systems are not easy to discern because of the complexity of atomic structure.Though some methods have been already developed and used for the present shortrange order study （such as Honeycutt-Anderson （HA） index analysis and Voronoi indexmethod）, they have some weakness in description the structure, and reliable and robustdescriptors to characterize various types of short range orders （SROs） as well as themedium range order （MRO） in noncrystalline systems is still lacking.This thesis concerns thus these two aspects: The method development of structureanalysis and structure as well as dynamic investigation of metallic liquids and glasses.The thesis consists of six chapters.Chapter One presents a brie?y introduction on the experiment study as well as theatomic structure mechanism of metallic liquid and glass.Chapter Two introduces the simulation techniques of disordered alloy systems,such as the Molecular Dynamic simulation （MD） and the Reverse Mont Carlo simula-tion （RMC）. The most commonly used analysis methods （i.e. pair correlation function,scatter factor, bond angle distribution, HA index and Voronoi index analysis.） in thestudy of metallic liquids and glass are also presented here.In Chapter Three, the idea and technique details of a new atomic cluster align-ment scheme to identify and characterize the local structure order in metallic liquidsand glasses is presented. The key advantage of our alignment approach is that it di-rectly deals with real atomic coordinates of local clusters, which guarantees the localstructural information be saved during the analysis. Successful applications to the el-ementary Al/Zr systems and binary Zr₃₅Cu₆₅/Zr₃₆Ni₆₄ systems are also shown in thischapter, and the results imply that our method can be a very important and versatilelocal structure analysis tool, which can indeed benefit us on the knowledge of the local structure in liquids and glass.With the new developed atomic cluster alignment scheme, Chapter Four shows adirect structure analysis method to reveal and quantify various aspects of MRO in noncrystallinesystems which have remained poorly resolved with other analysis methods.This method has been applied to characterize the MRO that develops in Cu64.5Zr35.5alloy as temperature decreases. We show that Bergman type MRO and a backbone networkof interpenetrating Bergman clusters develops as the system approaches the glasstransition. The discovery of Bergman type MRO in the metallic glass shed interestinglight on the structural relation between metallic glasses and quasicrystals as well as thenature of the glass transition. This discovery presents a significant advance in fundamentalunderstanding of the glass transition derived from a new approach to study andcharacterize the MRO in glass systemsChapter Five shows a series of structure analysis studies on Al-based alloy systems.In the first work, by using ab initio molecular dynamics simulations and thenewly developed atomistic cluster alignment scheme, we identify and characterize thecompetition between icosahedral and FCC order in both pure and Sm doped Al liquidupon rapid cooling. We show that although the icosahedral short ranger order （ISRO）is much stronger in pure Al liquid, the FCC order develops quickly below the meltingpoint and eventually leads to a rapid FCC crystallization. We also demonstrate that minordoping of Sm exhibits significant effects on the phase selection pathway of the Alliquid upon rapid cooling, where the FCC SRO is suppressed and the ISRO is enhancedin favor of glass formation.In the second work, we use the atomic cluster alignment scheme to quantify theSRO in Al liquid generated by ab initio MD simulations. We formulate the correlationamong dynamics, energetics and cluster structures using a kinetic model. We showthat the activation energies and relaxation times of SRO structures can be effectivelycalculated from ab initio MD simulations though this kinetic model. This approachfor determining the energetics and dynamics of SRO structures in metallic liquids isreadily applied to any kind of MD simulation studies of metallic liquids.In the third work, using ab initio MD simulations and the newly developed atomiccluster alignment scheme, we clearly identify and characterize the structural motif ofAl11Sm3 phase in Al90Sm10 liquid upon rapid cooling. We also show the developmentof icosahedral short range order （ISRO） and FCC order in this alloy system during thequench. The result demonstrates that Al-FCC and Al11Sm3 crystalline phases are thetwo possible crystalline phases that compete with the formation of amorphous phase （ISRO） in the Al90Sm10 liquid, which is consistent with the present experiment findingon Al-Sm alloy in the room temperature.In the forth work, atomic structures of minor doped A_l95X₅ （X=Cu, Ni, Sm and Y）binary liquids are studied by ab initio MD simulations. The in?uence of different dop-ing element on the SRO is investigated. We show that the icosahedral SRO can be pro-moted by doping element with large atomic size ratio. A strong Z9 type Frank-Kasperpolyhedra order around Ni solute atom in Al95Ni5 binary liquid is directly identifiedby alignment analysis. We also show the relative stronger packing order of doping Nielement than Cu in Al solvent and the aggregate tendency of Y element in Al solvent.Chapter Six summarizes the contents of five chapters above.