Structures Of Quasicrystal Approximants In Al-Cr Alloys

With the fast expanding of modern metal-based industry, there is a great pressure todevelop a new generation of metallic materials with excellent performances. Traditionalmetallic materials are confined to elementary metals and binary metallic alloys to which anumber of additional elements are added in small quantities to tailor them for particularproperties. Ternary or quarternary alloys in which three or four components, at comparablequantities, determine the basic properties (like in the precipitate-hardened nickel alloys) aremuch less frequently used. CMA (Complex Metallic Alloys) concentrate on a subset of thesemultinary alloys with giant unit cells containing many tens, up to more than a thousand atomsper cell. Within the huge unit cells, the atoms are arranged in clusters. These stronglyinfluence the electronic structure and the lattice dynamics. This, in turn, leads to a uniquenovel feature of these giant-unit cell materials. Knowing the crystal structures.would assist theperformance anticipation and design of CMA.At the Al-rich portion of Al-Cr binary alloy diagram, exist a number of decagonalquasicrystal approximants with large unit cell parameters, containing a lot of icosahedralatomic dusters. Their structural analysis will not only help us understanding the structure ofdecagonal quasicrytal, but also understanding the combination of atomic dusters in CMA.Furthermore, the method of solving Al-Cr quasicrystal approximants may make it possible tosolve similar complex structures in CMA.. This is how our research project started.Single crystals ofη-Al₁₁Cr₂ phase have been found in holes of ingots with nominalchemical composition of Al₁₁Cr₂, confirming the existence of this phase. Through theexperiments of selected area electron diffraction and single-crystal X-ray diffraction, wrongdetermination of unit cell parameters in history has been explained and accurate parametersare obtained: a=1.77348(10) nm, b=3.04615(18) nm, c=1.77344(10) nm,β=91.0520(12)°, space group C2/c. The chemical composition ofη-Al₁₁Cr₂ is Al_83.8Cr_16.2. Theperitectic reaction ofμ-Al₄Cr+L(Al)(?)η-Al₁₁Cr₂ has been observed in heat treatedAl₁₁Cr₂ alloy. Meanwhile, the observation of eutectoid reaction ofη-Al₁₁Cr₂(?)μ-Al₄Cr+θ-Al₇Cr corresponds to the latest result of thermodynamic calculation in reference.Through single-crystal X-ray structural analysis, the structures of two giant and complexquasicrytal approximants monoclinicη-Al₁₁Cr₂ and hexagonalμ-Al₄Cr have been solved. Theη-Al₁₁Cr₂ has an independent icosahedral layer block (PFP'), piling along the normaldirection of (101) in the period of four layers. There are five parallel icosahedral chainsα,β,γ,γ^* andβ^* in each layer block, whereγ^* andβ^* can be generated through n-glide plane at b= 1/4. Theμ-Al₄Cr has two types of icosahedral layer blocks (P₁F₁P₁') and (P₂F₂P₂^m), piling inthe c axial direction alternately in period of four layers. Thus exists a simple relation: c_μ≈2d_(101)η≈2.485nm. The layer block of (PFP') inη-Al₁₁Cr₂ and the layer block of (P₁F₁P₁') inμ-Al₄Cr have a lot of features in common. Both structures are composed of parallelingicosahedral chains in different combination. Connecting centers of these chains can result inperiodically gathered Penrose fat and thin rhombi. Atomic coordination comparison betweenμ-Al₄Cr andμ-Al₄Mn confirms the assumption thatμ-Al₄Cr is isostructural toμ-Al₄Mn andtotal displaceability between Cr atom and Mn atom inμphase.In high temperature situ-observation of TEM, super lattice week diffraction points ofη-Al₁₁Cr₂ in the three main axial zones are weakening and vanishing, at the same time, themonoclinic angleβare changing from 91°to the right angle, indicating the generation of anew orthorhombic phase O-Al₁₁Cr₂, which is the unordered state ofη-Al₁₁Cr₂ under highertemperature. O-Al₁₁Cr₂ is also a new decagonal quasicrystal approximant because of itspseudo-decagonal electron diffraction patterns.