Microstructural Investigation Of Cd-and Zr-Based Icosahedral Quasicrystal Related Systems

Quasicrystalline phase is characterized by the absence of an average Bravais lattice and/or the observation of’forbidden’crystallographic symmetry. Quasicrystals have lots of unusual physical properties, for instance, hardness, low thermal conductivity, low friction and remarkable electronic properties. Considerable efforts have been devoted to the investigation of the unique structure and the related defects, which is also the main content of this thesis. The main body of the thesis consists of two parts; the corresponding results are summarized as follows:In the first part, we studied the spontaneous formation of filamentary whiskers and degradation of icosahedral quasicrystals in Cd-Mg-RE alloys. Preliminary transmission electron microscopy (TEM) analyses show that, Cd-Mg-Tb, Cd-Mg-Dy, Cd-Mg-Ho, Cd-Mg-Er and Cd-Mg-Lu exhibit micro-scaled heterogeneous contrast. Conversely, Cd-Mg-Yb alloy shows the nearly homogeneous contrast with the average composition is approximately Cd6055Mg23.12Yb16.33, which approaches the nominal composition Cd65Mg2oYb15. The selected area electron diffraction (SAED) patterns of the Cd-Mg-Yb alloy indicate the dominant phase is quasicrystal with a primitive lattice. Concerning the kinematic effects, calculated SAED patterns by using simplified spherical atomic surface model are coincident with the experimental ones.The whiskers appeared spontaneously after the Cd-Mg-Yb alloy was held in a glass dryer for about2months at room temperature. It is found that:(1) the whiskers are hexagonal Cd with growth direction along<2110>cd;(2) CdO particles are frequently observed on surfaces of whiskers and the alloy; there exist four types of orientation relationship between Cd matrix and CdO, resulting in the complicated SAED pattern;(3) based on the analysis of double diffraction, the interfaces between them are shown to accommodate large lattice misfit, which are well explained by near coincidence site lattice model;(4) nanosized Cd particles are observed to aggregate around the whisker root, providing a convincing experimental evidence for the long-range atomic diffusion.The whisker nucleation and growth are induced by selected oxidation of Cd. CdO particles nucleate resulting in the displacement of Cd atoms within the alloy and volume expansion, which cause heterogeneous compressive stress. When the amount of Cd atoms and the stress accumulate to a certain level, the atoms are extruded out of the surface and the whiskers begin to grow. The gradual decreasing of Cd component in the alloy leads to the degradation of the quasicrystal quality.In the second part, we studied the linear phason strains of quasicrystalline precipitates in Zr-Al-Ni-Cu-Nb bulk metallic glass matrix composites. It is found that most precipitates are icosahedral quasicrystals containing various linear phason strains. The distortions of the SAED patterns caused by linear phason strains are summarized by theoretical calculation and corresponding experimental examples. After meticulous measurement, quantitative fitting and calculation of the diffraction spots’ shifting, the phason strain matrices can be determined. The calculated diffraction patterns using the as obtained phason strain matrices match the experimental ones perfectly.Some precipitate particles contain complicated linear phason strains that are inhomogeneous within a single particle. The splitting of the diffraction spots from the whole particle implies the existence of tiny sized domain structures with varied linear phason strains, which can be validated by SAED analysis combined with bright field and dark field imaging. High resolution electron imaging provides supportive evidence of this result as well. Domain structures form as a result of various linear phason strains along directions with equal probability. These phason strain variants can counterbalance each other to produce low macroscopic strain for the whole grain.The obtained values of the linear phason strains are at10-2magnitude, which suggests that there are preexisting plastic strains in the quasicrystalline precipitates.