Multi-Scale Calculation And Simulation Of The Microstructure, Phase Transition And Property In Metallic Alloys

During the past decades, various nonequilibrium materials processingtechniques have been developed and successfully employed to fabricate agreat number of nonequilibrium materials featuring unique properties.Consequently, developing new materials theory to clarify the correlationsamong the microstructure, processing and property of the nonequilibriummaterials has become a strong challenge and an urgent demand. In responding,researchers in the fields of materials and condensed matter physics have paidmuch effort to develop atomic scale theory based on inter-atomic potentialgoverned simulations, and to develop electronic scale theory based onquantum mechanics, namely first-principles and/or ab initio calculations,aiming to formulate new materials theory in a quantitative scheme.The present dissertation was dedicated to study the correlations amongthe microstructure, phase transition and property of nonequilibrium phases inthe binary metallic systems by means of first principles calculationsemploying the well-established Vienna Ab initio Simulation Package (VASP)and atomic scale simulations based on the proven realistic n-body potentials.Firstly, first principles calculations were carried out to study thestructural stability of the possible nonequilibrium crystalline phases in fiveselected binary metallic systems, i.e., Y-Mo, Y-Nb, Co-Ag, Co-Cu, and Fe-Cusystems, which are all characterized by positive heats of formation. Thecalculated results predicted that the nonequilibrium phases with close packedstructures (HCP and/or FCC) usually had relatively higher stabilities than theother structures concerned and the predictions were not only confirmed by theexperimental observations, but also supported by those deduced frommacroscopic thermodynamic calculations, suggesting that such kind of firstpriniciples calculations could serve as a theoretical guidance in developingnew nonequilibrium materials.Secondly, first principles calculations were performed to acquire somephysical properties to assist in deriving the n-body potentials in the Ni-W andCu-Ta systems, in which there is no sufficient physical property necessary forfitting the cross potentials. The Ni-W and Cu-Ta potentials were accordinglyconstructed under the Finnis-Sinclair formula and embedded-atom-method,respectively, and the potentials were able to reproduce those importantphysical properties and the reproduced data matched well with those obtainedby first principles calculations or from experiments. Moreover, based on theconstructed potentials, interfacial stability and some other properties of thealloy phases were obtained by molecular dynamics simulations and the resultsalso agreed well with the experimental observations, lending support to thefeasibility of the first principles assisted potential construction approach.Thirdly, based on the projector augmented wave method, spin-polarizedfirst principles calculations revealed that the 3d transition metals of Fe, Coand Ni could all be ferromagnetic in their respective nonequilibrium statesand that the magnetic moment of Fe in a large-sized FCC structure couldsignificantly be enhanced. It was also found that the magnetic moment of theferromagnetic atoms would be enhanced with expanding of their volumes,while reduced with increasing of their nonmagnetic neighbors. The magneticmoment of the ferromagnetic atoms will therefore be balanced by the abovetwo factors and, interestingly, in the Fe-Cu alloys, the high spin ferromagneticstate of FCC Fe can well be reserved.Fourthly, for the Mo-Hf system, first principles calculations using asupercell model revealed that both the surface energy and work function of thenonequilibrium crystalline phases were pertinent to the orientation of thecrystal surface, exhibiting an anisotropic behaviour, and to the composition ofthe crystal phase, featuring an approximately linear dependence on the alloycomposition.