Study Of Alloy Phase Formation By Thermodynamics,Molecular Dynamics Simulation And Ion Beam Mixing

In the past decades, various metastable alloys produced bynonequilibrium processing techniques show unique electrical, magnetic,optical and mechanical properties, and therefore have great potential forapplications. The theoretical and experimental studies have been abundant forthe miscible binary metal systems, however, relatively limited for theimmiscible systems. This dissertation is dedicated to conduct theoreticalstudies combining with some experiments concerning the formation of themetastable alloy phases in some selected immiscible binary metal systems.Firstly, under the framework of Miedema's theory, we proposed amodified model for calculating the standard formation enthalpy, a calculationmethod for calculating the structural formation enthalpy including themagnetic energy, and a criterion for predicting the formation of Laves phasefor the binary transition metal systems. Secondly, we developed a friendlysoftware for thermodynamic calculation, which is able to calculate the Gibbsfree energy of a binary metal system and the interfacial free energy of themetal-metal multilayers.In theoretical modeling at an atomic scale, the present study is tocombine ab initio calculation and molecular dynamics simulation for studyingthe formation of the metastable alloy phases. Accordingly, five immiscibleW-based binary fcc-bcc and hcp-bcc metal systems, i.e, Cu-W, Ag-W, Au-W,Sc-W and Y-W systems, and miscible Co-Nb systems, were selected to studythe formation of metastable phase selection and glass formation ability.First, through fitting some ab initio calculation data, the embedded-atomn-body potentials are constructed for the five selected systems. Especially, weproposed a set of formulas and an effective fitting approach together with abinitio calculations for deriving the n-body potentials for the hcp-bcc metalsystems, which have almost not been studied previously. Based on the provenrealistic potentials, molecular dynamics simulations are then performed toreveal the metastable phase selection over the entire composition for theCu-W, Ag-W, and Au-W systems, and to predict the glass-forming ability forthe Co-Nb, Sc-W and Y-W systems.In experimental pursuing, ion beam mixing of multiple metal layers wasperformed for the Cu-W, Ag-W, Au-W, Sc-W and Y-W systems. Theformation of the metastable fcc/bcc super-solid solutions in the five selectedsystems and of the amorphous alloys in the Ag-W, Au-W, Sc-W and Y-Wsystems was in good agreement with those deduced from the theoreticalcalculations. More interestingly, in the Y-W system, a new dual-phasemetallic glass with unique two-dimensional fractal morphology was observedin Y-rich side, and its possible formation mechanism was also studied byatomistic scale simulations.