Formation of Quasicrystalline Phases and Their Close Approximants in Cast Al-Mn Base Alloys Modified by Transition Metals
The aim of the presented research was to study the influence of Cr, Co, Ni, and Cu additions on the formation of quasicrystalline particles in the 94Al-6Mn base alloy during casting at intermediate cooling rates. Based on the obtained results, Cu and Ni enhance quasicrystalline phase nucleation compared to the unmodified binary composition. In the case of Cu addition, formation of a quasicrystalline phase takes place along whole thickness of the prepared casting, but its fraction and morphology depends on the cooling rates present in different parts of the sample. Based on the previous works on the beneficial effect of Fe addition, a quaternary alloy containing both Fe and Cu was prepared to evaluate the effect of the simultaneous presence of these elements on the microstructure of the obtained castings.
Crystallization and precipitation structures of quasicrystalline phase in rapidly solidified Al-Mn-X ternary alloys
AbstractTranmission electron microscopic observations were carried out to reveal the solidification and precipitation structures of rapidly solidified Al-Mn-X alloys (X=Si or Zr), with particular focus on the formation of the icosahedral quasicrystals. On increasing the manganese content, the distribution of the icosahedral quasicrystals in the rapidly solidified alloys changed from the type of cell-boundary segregation to that of one component in the eutectic and the primarily dendritic phase. The precipitation of the icosahedral quasicrystals from the supersaturated solid solutions of the present alloys was restricted in the surface layers and/or largeangle grain boundaries. The orientation relationships between the icosahedral quasicrystals and the aluminium matrix were such that three mutually perpendicular two-fold axes of the icosahedron lie along the [100],
$$[0\bar 11]$$
, and [011] axes of the fcc crystallographic directions. The 15-fold symmetry diffraction pattern frequently obtained from (111) and/or (233) matrix planes could be explained by considering axial rotation between the five-fold and three-fold symmetry axes of the icosahedron and cubic lattices. It was suggested that the hierarchy of the free energies of the icosahedral phase and the other crystalline phases was very close.
Crystal structure of the monoclinic -Al11Cr2
The crystal structure of the monoclinic phase-Al11Cr2 of the space group C2/c, a ≈ 1.76 nm, b ≈ 3.05 nm, c ≈ 1.76 nm, β ≈ 90 ◦ [L.A. Bendersky, R.S. Roth, J.T. Ramon, D. Shechtman, Metall. Trans. A 22A (1991) 5] has been determined by single-crystal X-ray diffraction. The structure model, refined to a final R value of 0.0441, has the composition of Al83.8Cr16.2. a = 1.77348(10) nm, b = 3.04555(17) nm, c = 1.77344(10) nm, monoclinic angle β = 91.0520(12) ◦ . There are 80 (66Al + 14Cr) independent atomic positions in a unit cell, of which all Cr atom sites and 8 Al atom sites have icosahedral coordination. These icosahedra are interconnected forming icosahedral chains along [1 0 1], (1 0 1) icosahedral layer blocks as well as a three-dimensional icosahedral structure. © 2007 Elsevier B.V. All rights reserved.