Shineng Sun

Self-Assembly of Two Unit Cells into a Nanodomain Structure Containing Five-Fold Symmetry

Five-fold symmetry was forbidden for the periodic crystals until the discovery of the Al-Mn icosahedral quasicrystal. We report a kind of precipitated rod-shaped nanophase containing five-fold symmetry but not belonging to any crystals or quasicrystals discovered so far. These metastable nanodomain phases, which precipitated in Mg-6Zn alloy during isothermal aging at 200 °C, contain two separate unit cells in the 2D plane perpendicular to the five-fold axis but with periodic atom arrangement along the five-fold axis, that is, 72° rhombus structure and 72° equilateral hexagon structure. The self-assembly of two unit cells under some geometrical constraints into a nanodomain contains the 2D five-fold, C14, and C15 structures. This finding confirms the existence of solid matters in a special structure between the crystals and quasicrystals, and it is expected to provide a way to understand the atomic arrangement and stacking behavior in condensed matters.

Effect of solid solution treatment on in vitro degradation rate of as-extruded Mg-Zn-Ag alloys

Abstract The degradation behaviors of the as-extruded and solution treated Mg-3Zn-xAg (x=0, 1, 3, mass fraction, %) alloys, as well as as-extruded pure Mg, have been investigated by immersion tests in simulated body fluid (SBF) at 37 °C. The as-extruded Mg-Zn(-Ag) alloys contained Mg51Zn20 and Ag17Mg54. While the quasi-single phase Mg-Zn(-Ag) alloys were obtained by solution treatment at 400 °C for 8 h. The quasi-single phase Mg-Zn(-Ag) alloys showed lower degradation rate and more homogeneous degradation than corresponding as-extruded Mg alloys. Degradation rate of solid-solution treated Mg-3Zn-1Ag and Mg-3Zn-3Ag was approximately half that of corresponding as-extruded Mg alloy. Moreover, the degradation rate of solid-solution treated Mg-3Zn and Mg-3Zn-1Ag was equivalent to that of as-extruded pure Mg. However, heterogeneous degradation also occurred in quasi-single phase Mg-Zn-Ag alloys, compared to pure Mg. So, preparing complete single-phase Mg alloys could be a potential and feasible way to improve the corrosion resistance.

Isothermal section of Mg-rich corner in Mg–Zn–Al ternary system at 335 °C

Abstract The phase equilibria and compositions at the Mg-rich corner of the Mg–Zn–Al ternary system at 335 °C were systemically investigated through the equilibrated alloy method by using X-ray diffraction (XRD) and scanning electron microscopy (SEM) assisted with energy dispersive spectroscopy of X-ray (EDS). It is experimentally testified that the α -Mg solid solution is not in equilibrium with the Mg 32 (Al, Zn) 49 ( τ ) ternary intermetallic compound or q quasicrystalline phase, but only in equilibrium with one ternary intermetallic compound Al 5 Mg 11 Zn 4 ( φ ). The whole composition range of the φ phase was also obtained at 335 °C, i.e., 52.5%–56.4% Mg, 13.6%–24.0% Al, 19.6%–33.9% Zn (mole fraction). The solubility of Al in the MgZn phase is remarkably more than that in the Mg 7 Zn 3 phase, and the maximum is about 8.6% Al. Aluminum and zinc are simultaneously soluble in the α -Mg solid solution.