Mackay icosahedron explaining orientation relationship of dispersoids in aluminium alloys

The orientation relations (ORs) of the cubic icosahedral quasicrystal approximant phase α-Al(Fe,Mn)Si have been studied after low temperature annealing of a 3xxx wrought aluminium alloy by transmission electron microscopy. From diffraction studies it was verified that the most commonly observed OR for the α-Al(Fe,Mn)Si dispersoids is [1\bar 11]α // [1\bar 11]Al, (5\bar 2\bar 7)α // (011)Al. This orientation could be explained by assuming that the internal Mackay icosahedron (MI) in the α-phase has a fixed orientation in relation to Al, similar to that of the icosahedral quasi-crystals existing in this alloy system. It is shown that mirroring of the normal-to-high-symmetry icosahedral directions of the MI explains the alternative orientations, which are therefore likely to be caused by twinning of the fixed MI. Only one exception was found, which was related to the Bergman icosahedron internal to the T-phase of the Al–Mg–Zn system.

Crystal Chemistry and Electronic Structure of Magnesium based Mackay Icosahedron Type Approximants

ABSTRACTIn order to obtain reliable experimental information on the long-range order, the role of the valence electron concentration and the type of disorder for Mackay icosahedron type quasicrystals we have studied binary and ternary Mg based approximant phases. An overview is presented on the crystal structures, homogeneity ranges, physical properties, and electronic structures of the following intermetallic phases: Mg6Pd, Mg57Pd13, Mg56.4Pd13.5, Mg306Pd77, Mg3Pd, Mg5Pd2, Mg2Pd, Ga30-xMg32+xPd21-y, Ga15-x-yMg38+yPd13+x, GaMg3Pd2, Ag17Mg54, Ag7Mg26, and AgMg4.

Formation of Icosahedral Quasicrystals in Mg-Al-X (X=Transition Metal) Ternary Systems

Formation of icosahedral quasicrystalline phase (i-phase) was investigated for Mg-Al-X (X=Co, Ni, Cu, Rh, Ag, Pt and Au) ternary systems. A Mackay-icosahedron (MI) type i-phase was found to form in the Mg-Al-Rh system at the composition Mg8Al68Rh24. The i-phase has the F-type ordered structure and the quasilattice constant is 0.447nm. Differential thermal analysis (DTA) studies showed that the i-phase has a high stability. On the other hand, the formation of a Frank-Kasper (FK) type i-phase was found in the Mg-Al-Pt system at the composition Mg45Al42Pt13. The conditions necessary for the formation of the i-phases are discussed in terms of the atomic radius ratio and the electronegativity as well as the electron-per-atom ratio.

α-AlMnSi phase to a bcc structure by XE+ ion-beam irradiation

AlMnSi alloy system has attracted great attentions since the first report on the icosahedral quasicrystal materials in 1984. The rapidly quenched AlMnSi alloy around the stoichiometry of the α-AIMnSi phase (α-phase below) may form icosahedral quasicrystals. And the structure unit of the α-phase is of near icosahedral symmetry. The α-phase has a cubic structure with space group Pm3 (a= 1.268 nm). In each unit cell there are 138 atoms. The most important structure unit in this phase is the so-called MacKay icosahedron. In such a structure unit, 12 Al atoms are decorating the vertices of an icosahedron. Surrounding this icosahedron is another same oriented icosahedron of 12 Mn atoms, which is about twice the diameter of the former. Furthermore, 30 Al atoms define a shell outside the two inner icosahedra with each atom sitting at the midpoint just out of the Mn icosahedron edges. In the structure of α-phase, the icosahedra are slightly distorted and are connected along those of their three fold axes that coincide with the <111> direction of the cubic lattice.

A comparative electron microscopic study of Al-based and Mg-based quasicrystals

It is shown that both icosahedral Al–Mn–Si and Mg–Al–Zn alloys give rise to the same variety of electron diffraction patterns as documented for icosahedral Al–Mn alloys. Subtle variations in intensity are ascribed to a different decorational motif in terms of the Mackay icosahedron and the Pauling triacontahedron. Icosahedral Al–Mn–Si alloys do not appear to be ordered on a superlattice basis.