Icosahedral quasicrystals and their cubic approximants in the Cd-Mg-RE (RE = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm) systems

Abstract

The formation and stability of icosahedral quasicrystals and their cubic approximants in the Cd-Mg-RE (RE = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm) systems are studied in detail, with a focus on the effects of Mg substitution as well as the dependence on temperature and RE species. The results show that the compositional region of the icosahedral phase in the ternary phase diagram shrinks gradually as the RE element becomes smaller in atomic size. This phenomenon is correlated with the geometric ‘stability’ of a so-called Tsai-type cluster described as a packing of spheres that represent the atomic radii of the constituent elements. Moreover, the icosahedral quasicrystals are found to be stabilized at lower RE concentrations (12–13.5 at.%) and temperatures (T < 773 K) compared to their cubic approximants, contrary to earlier findings for the Cd-Mg-Yb system. Single grains of the icosahedral, 2/1 and 1/1 cubic approximant phases with, respectively, rhombic triacontahedron, octahedron and rhombic dodecahedron morphologies ranging ≈1.5–2.5 mm in diameter are further synthesized using a self-flux method. It is confirmed that the bounding facets of the single grains are {111} and {110} for the 2/1 and 1/1 cubic approximants, respectively, through orientation analysis using electron backscatter diffraction. Finally, microstructures involving the Cd-Mg-RE icosahedral quasicrystal and Cd-Mg hexagonal solid solution are analysed to show parallel alignment of 2-fold rotational axes of the former with the 6-fold and 2-fold rotational axes of the latter. This orientation relationship is further discussed in terms of the coincidence of interplanar spacings between the two phases.