Structure Prediction Of Al-Co And High-Pressure Synthesis And Characterization Of AlCoNi Quasicrystals

Quasicrystals have attracted much attention owing to their unique structure and physical properties since firstly discovered in 1984. For decades, decagonal AlCoNi and related phases have been extensively studied in this fields as a model system.In this thesis, the potential crystal structures of Al3C0 are explored based on the crystal structure prediction CALYPSO package, which is based on the particle swarm optimization, combined with first-principles calculation methods and a stable orthorhombic Pnma structure has been found. We firstly describe its structure in detail and then calculate the elastic constants, phonon dispersion curves and the formation energy of the new phase in order to determine its mechanical, kinetic and thermodynamic stability. As Co is a late transition metal element, we have also carried out the spin-polarized calculation. The results prove that its ground state is a non-magnetic state. Secondly, the calculations of mechanics properties include bulk modulus (B), shear modulus (G), Young’s modulus (E) and Poisson’s ratio (σ) prove that the new phase has good ductility. Thirdly, the analyses of electronic properties include energy band structure and the density of states show that the new phase is metallic. Finally, thermodynamic properties, such as the Debye temperature, bulk modulus, heat capacity and thermal expansion coefficient was predicted within the quasiharmonic approximation.High pressure and high temperature have been conducted to prepare the decagonal AlCoNi quasicrystal samples by optiming different experiment paremeters, such as time, temperature and pressure. The composition and morphology of the synthesized phases have been determined to be quasicrystal phase’s structure by analysing the XRD, SEM, TEM and EDS results.