Disordered quasi-crystal

This thesis consists of two parts, dealing with different aspects of disordered quasicrystals: (i) A quasicrystal is disordered into an assembly of randomly oriented microquasicrystals, about 20A in diameter, which makes the translational correlation length sufficiently small to make its diffraction pattern appear indistinguishable from that of a metallic glass; (ii) A quasiperiodic structure is disordered into random tilings with two types of Penrose rhombi, the structural units used for the two-dimensional ordered quasicrystal.; In part I, the phase selection upon preparation by different methods, including ion-beam sputtering from composite targets, solid state interdiffusion in multilayered samples, and fast pulsed laser quenching, is investigated in quasicrystal-forming Al-transition metal-based system. Microquasicrystals are formed preferentially over a truly amorphous phase. The microquasicrystalline structure is identified by a calorimetric study of its transformation upon heating. A monotonically decreasing isothermal signal, characteristic of normal grain growth in the microquasicrystalline assembly, was seen. The transformation process of the microquasicrystalline material has a strong composition dependence. Outside a very narrow composition range in which only grain growth of microquasicrystals is observed, transformations to stable intermetallic compounds take place before significant grain growth occurs. The calorimetric analysis of grain growth is developed here for the first time. Kinetic factors, such as the grain growth exponent, the activation enthalpy, and the average grain boundary enthalpy can be determined from calorimetric measurements. In laser melting experiments, an extremely high nucleation frequency, {dollar}sim{dollar}10{dollar}sp{lcub}24{rcub}{dollar}cm{dollar}sp3{dollar}-sec, of the quasicrystalline phase from the melt is found. Such a high nucleation frequency had been predicted from the similarity of polytetrahedral short range order in the quasicrystal and the liquid.; In part II, the configurational entropy of disordered two-dimensional quasicrystal is determined by two methods. An entropy calculation using a technique based on counting the number of available configurations of each tile during physical model building gives 0.495{dollar}ksb{lcub}rm B{rcub}{dollar}, and 0.48{dollar}ksb{lcub}rm B{rcub}{dollar} per tile for the compositionally unconstrained and constrained tilings, respectively. A geometrical model based on the number of configurations around each vertex without correlations between them gives 0.64{dollar}ksb{lcub}rm B{rcub}{dollar} per tile, which illustrates the importance of higher correlations in the space-filling random tiling. A Gibbs free energy diagram for the icosahedral Al{dollar}sb6{dollar}Mn is calculated and used to estimate a residual entropy of 0.12{dollar}ksb{lcub}rm B{rcub}{dollar} per atom. A consequence of the disorder in the random tiling is the occurrence of diffuse diffraction rings and distorted Bragg peaks. Their existence is revealed by optical diffraction and numerical Fourier transformation of the vertices of the tilings.