Abstract: Ultrafine faulty C14-MgZn2 Laves phase precipitates containing quasicrystalline clusters and demonstrating the formation of binary quasicrystalline precipitates with Penrose-like random-tiling were observed in the over-aged FCC matrix of a commercial 7N01 Al-Zn-Mg alloy, using high angle annular dark field scanning transmission electron microscopy. The evolution from C14-Laves phase to quasicrystalline clusters is illustrated, and five-fold symmetry can be found in both real and reciprocal spaces. Our findings reveal the possibility of quasicrystalline formation from Laves phase in a highly plastic metal matrix like Al and demonstrate the structural relationship between Laves phase and quasicrystals.

Abstract: Al-Zn-Mg-based high strength alloys are widely used in aerospace applications due to their low density and excellent mechanical properties. A systematic study of the structural evolution of the nano-precipitation phase and its growth mechanism is an important guide for the design of new high-strength alloys. In this work, the Laves structure precipitates in Al-Zn-Mg(-Cu/Y) alloy was systematically characterized. Based on the structure evolution, the structure of submicron Laves particles and quasicrystalline particles in the alloy at microscale, as well as the regulation of the precipitation behavior after adding Y at nanoscale were further investigated. The main innovative results are summarized as follows: (1) Investigation on coexistence of defect structures in Laves structural nanoprecipitates. Three types of Laves structures can coexist within the η-MgZn2 precipitates: C14, C15 and C36, and the Laves structure transition sequence of C14→C36→C15 in this system was determined. Meanwhile, it was found that there are diverse defect structures in the MgZn2 phase, including stacking faults, planar defects and five-fold domain structures, which have significant effects on relieving the internal stress/strain of the precipitates. (2) Investigation on multiple phase transition of Laves structural nanoprecipitates from C14 to C36 and from C14 to quasicrystal clusters. It is found that C14 precipitates can be completely transformed into the C36 precipitates. And it is also found that the C14 Laves phase structure can also transform into quasicrystalline clusters. These investigations on various phase transition mechanisms among Laves phases provide theoretical support for the microstructural characterization of materials containing multi-scale Laves phases. (3) Characterization of Laves and quasicrystal structural particles in submicron scale. Submicron-scale quasicrystal particles were obtained in conventional casting Al-Zn-Mg-Cu alloys for the first time. Industrial impurity elements Fe and Ni can induce the formation of quasicrystalline particles. When there is no Fe/Ni enriched in particles, the structure is characterized as C15-Laves phase. When Fe/Ni is as quasicrystalline core, a stable core-shell quasicrystalline structure with Al-Fe-Ni nucleus and Mg-Cu-Zn shell can be formed. (4) Investigation on the regulation of nanoscale Laves precipitates’ growth. To regulate the defect structure of the precipitates, rare earth element Y was added in Al-Zn-Mg alloys and its influence on the precipitation behavior was investigated. The addition of Y element can dynamically combine with different alloying elements during aging process, which can refine the size of precipitate and further improve the nucleation rate and precipitation rate of the precipitates.

Abstract: The spiral magnetic structure and underlying spin lattice of BiMnFe2O6 are investigated by low-temperature neutron powder diffraction and density functional theory band structure calculations. In spite of the random distribution of the Mn3+ and Fe3+ cations, this centrosymmetric compound undergoes a transition into an incommensurate antiferromagnetically ordered state below TN≃220 K. The magnetic structure is characterized by the propagation vector k=[0,β,0] with β≃0.14 and the P221211′(0β0)0s0s magnetic superspace symmetry. It comprises antiferromagnetic helixes propagating along the b axis. The magnetic moments lie in the ac plane and rotate about π(1+β)≃204.8-deg angle between the adjacent magnetic atoms along b. The spiral magnetic structure arises from the peculiar frustrated arrangement of exchange couplings in the ab plane. The antiferromagnetic coupling along the c axis cancels the possible electric polarization and prevents ferroelectricity in BiMnFe2O6.

Abstract: LaAMnSnO(6) (A = Sr, Ba) have been synthesized by high temperature solid-state reactions under dynamic 1% H(2)/Ar flow. Rietveld refinements on room temperature powder X-ray diffraction data indicate that LaSrMnSnO(6) crystallizes in the GdFeO(3)-structure, with space group Pnma and, combined with transmission electron microscopy, LaBaMnSnO(6) in Imma. Both space groups are common in disordered double-perovskites. The Mn(3+) and Sn(4+) ions whose valence states were confirmed by X-ray absorption spectroscopy, are completely disordered over the B-sites and the BO(6) octahedra are slightly distorted. LaAMnSnO(6) are ferromagnetic semiconductors with a T(C) = 83 K for the Sr- and 66 K for the Ba-compound. The title compounds, together with the previously reported LaCaMnSnO(6) provide an interesting example of progression from Pnma to Imma as the tolerance factor increases. An analysis of the relationship between space group and tolerance factor for the series LaAMnMO(6) (A = Ca, Sr, Ba; M = Sn, Ru) provides a better understanding of the symmetry determination for double perovskites.

Abstract: LaSrMnNbO6 has been synthesized by high temperature solid state reaction under 1% H2/Ar dynamic flow. The structure is determined by Rietveld refinement of the powder X-ray diffraction data. It crystallizes in the monoclinic space group P21/n with the unit cell parameters: a=5.69187(12), b=5.74732(10), c=8.07018(15) Å and β=90.0504(29)°, which were also confirmed by electron diffraction. The Mn2+ and Nb5+ ions, whose valence states are confirmed by X-ray absorption near-edge spectroscopy, are almost completely ordered over the B-site (<1% inversion) of the perovskite structure due to the large differences of both cationic size (0.19 Å) and charge. The octahedral framework displays significant tilting distortion according to Glazers tilt system a−b−c+. Upon heating, LaSrMnNbO6 decomposes at 690 °C under O2 flow or at 775 °C in air. The magnetic susceptibility data indicate the presence of long-range antiferromagnetic ordering at TN=8 K; the experimentally observed effective paramagnetic moment, μeff=5.76 μB for high spin Mn2+ (3d5, S=5/2) is in good agreement with the calculated value (μcalcd=5.92 μB).