Study Of The Structures Of Quasicrystals And Their Approximants In Al-Cr-(Si) Alloys

The quasicrystals and their approximants in Al-rich Al-Cr-(Si) alloys with a Si content < 15 at.% have been studied. Two intermetallic phases: η-Al₁1Cr₂ and ε-Al₄Cr in Al-Cr alloy system, which almost have the same composition and were not distinguished in the past, have been confirmed by the combination methods of EMPA, DSC and XRD of single crystal. η-Al₁1Cr₂ is an orthorhombic phase whereas e-AUCr is a hexagonal one. A new monoclinic cell of the intermetallic compound Al₄5Cr₇ was chosen with a = 2.0595 nm, b = 0.7574 nm, c = 1.0949 nm and β = 107.34°. The new monoclinic cell has the same space group C 2/m and unit-cell volume as the old one, but has a smaller a parameter and also a smaller β angle. Furthermore, the β angle is very close to 108° of the pentagons in the (010) plane of the Cr3 icosahedron.Two kinds of stable decagonal quasicrystals (DQC) and 16 approximants have been found in Al-Cr-(Si) alloys by means of selected-area electron diffraction(SAED), high resolution electron microscopy (HREM) image and X-ray single crystalline diffraction. Among the 16 approximants, seven of them are new phases. The stable decagonal quasicrystal with a period of 3.72 nm along the tenfold axis is reported for the first time.The basic structural unit of the DQC and the approximants is a homocentric decagon cluster. The connections between two decagon clusters are six, which belong to two larger groups.These connections can form DQC and approximants. The decagonal structural unit can overlap confirming the cover theory of P. Gummelt. Three different planar structural modes of 5 approximants have been put forward by using pentagon, decagon, flat hexagon and boat-shape respectively.A structural mode of the hexagonal μ' phase was deduced from the hexagonal X phase by using the approach of electron crystallography and a structural mode of the hexagonal Ï„(μ) phase was obtained by X-ray single crystalline diffraction. A Ï„-inflation relationship exists among the a parameters of the hexagonal P, H, μ(μ') and Ï„(μ) phases, where x = (1 + V5)/2: aÏ„(μ) = Ï„ aμ = Ï„² a_H= Ï„ aβ. In a PFP' layer block, there are interpenetrated icosahedral chains consisting of 2, 4, (4+2, 3+3) and (4+4+2, 3+3+4)icosahedra, respectively, in β, H, μ(μ'), and Ï„(μ) phases. The periodicity of these chains, namely the lattice parameter a, forms a Ï„-inflated series. We have summed up a rule that the hexagonal approximants can be classified as three by the strong diffraction spots in their [001] EDPs and the direction of the icosahedra chains in their (001) layers is the structure nature of this phenomenon.The crystal structure of the primitive orthorhombic ε'-(Al,Si)4Cr phase was determined by single-crystal X-ray diffraction analysis. The morphological units in ε-Al₄Cr and s'-(Al,Si)4Cr are not only the same but also have the same arrangement and e'-(Al,Si)4Cr can be considered as an ordered one of the e-AUCr. Both the structures of s-Al₄Cr and ε'-(Al,Si)4Cr are closely related to that of the hexagonal μ-Al₄Mn/μ-Al₄Cr. The icosahedral chains in the pseudo hexagonal directions in the ε-Al₄Cr and e'-(Al,Si)4Cr reveal the identity...