Preparation And TEM Study Of Crystal Structures Of Al-Cr-Fe-(Si)Decagonal Quasicrystals

Quasicrystals are aperiodic structures with no translational symmetry.The crystal structure of quasicrystals has always been the focus and difficulty in quasicrystals research because there is no translational unit cell similar to that of traditional crystals.After nearly 40 years of research,the crystal structure of quasicrystals has become increasingly clear,but there are still some important scientific problems need to be solved.For example,can quasicrystals have a single repeating unit cell similar to crystals?How can quasicrystal self-similarity be realized experimentally?What is the mechanism of quasicrystal stability?To solve these problems,the decagonal quasicrystals in the Al-Cr-Fe-(Si)system were studied systematically using electron microscopy methods including selected area electron diffraction and spherical aberration corrected high-angle annular dark field scanning transmission electron microscopy with atomic resolution in this paper.The structural characteristics of the decagonal quasicrystals and some related quasicrystal approximationsin in Al-Cr-Fe-(Si)alloy are revealed,and the answers to some problems in the research field of quasicrystals are given.The main contents are shown as follows:(1)A new quasi-unit-cell was discovered in A174Cr15Fe11 decagonal quasicrystal by using the high-angle annular dark field scanning transmission electron microscopy with atomic level resolution.The structural characteristic of the new quasi-unit-cell was the same as that of Luck decagonal,so it was named Luck quasi-unit-cell by us.The Luck quasi-unit-cell consists of three flattened hexagon tiles and one bowtie tile(D3H+1BT,for short),which is different from the Gummelt decagonal quasi-unit-cell previously used to describe the quasicrystals.This fundamental decagonal unit,with a diameter of approximately 2.0 nm,can be spread over the entire two-dimensional plane through two modes of overlapping H tiles and sharing one edge.The quasi-unit-cell of D3H+1BT has greater flexibility and without strict connecting rules,which might produce rich long-distance quasiperiodic orderings,corresponding to most experimental observations of DQCs.In combination with the Luck quasi-unit-cell found by us and the Gummelt quasi-unit-cell proposed before,the quasicrystals,especially the decagonal quasicrystals can be described by a quasi-unit-cell to a greater extent,thereby the growth and the structure of quasicrystals can be understood better.(2)Studies on the structure research of decagonal quasicrystal in Al-Cr-Fe-Si alloys show that we observed the so far largest tenfold symmetric structural subunit in decagonal Al59Cr21Fe10Si10 scaling up to 5.2 nm.If we do not consider the broken-tenfold symmetry of the individual 2.0 nm decagonal clusters.the diameter of the largest tenfold symmetric structural subunit can be up to 6.2 nm.The large decagon with a diameter of 6.2 nm is composed of 10 tangential 2.0 nm decagon connected to each other,and its interior is a 2.0 nm decagon with perfect tenfold symmetry.The in situ extension of the decagon from small to large provides direct experimental evidence how the self-similarity of the tenfold symmetry of decagonal quasicrystal is realized in the actual crystal structure.(3)Twin boundaries(TBs)of the orthorhombic(3/2,2/1)2 decagonal approximant in the quaternary Al60Cr20Fe10Si10 alloy were investigated,and four types of(101)TBs were found using atomic-resolution high-angle annular dark field scanning transmission electron microscopy.The structural features of these TBs at the atomic level were revealed and the corresponding projected structural models along the pseudo-tenfold axis were proposed.The structural blocks on the TBs contain shield-like tiles,dumbbell-like tiles,star tiles,hexagon tiles,decagon tiles,and these structural blocks arrange along the[1 01]direction with a periodicity of 3.82 nm.These results demonstrate that the diversifications of the TBs of decagonal approximants are closely related to the rich combinations of substructural blocks.Unlike the(100),(201)and(001)twins in the monoclinic Al13TM4 approximants,here only the(101)-type twins were found in the(3/2,2/1)2 approximant.(4)Studies on the stability research of decagonal quasicrystal in Al-Cr-Fe-Si alloys show that a thermodynamically stable quaternary Al59Cr23Fe8Si10 decagonal quasicrystal(named DQC2)has been found in Al-Cr-Fe-Si alloy.Compare with another decagonal quasicrystal(named DQC1)in the same alloy system,we found that DQC1 is metastable at high temperature(e.g.,1000?)and can transfer to DQC2 by annealing.Inversely,the DQC1 cannot be generated by annealing the DQC2 phase.DQC1 and DQC2 are stable at low temperatures(e.g.,500?).The D tiles of both of the DQCs have tenfold and broken-tenfold symmetry.However,95%of all D tiles for DQC2 are broken-tenfold symmetry,with the overlapped D tiles being 52.73%of the total D tiles,and the distribution is random.In contrast,86%of the D tiles in DQC1 have perfect tenfold symmetry,and most of the D tiles of DQC1 locally arrange according to fivefold or tenfold symmetry.The atomic scale structural features of the quasiperiodic plane were revealed based on atomic resolution HAADF-STEM images and the reasons for the stability of the Al59Cr23Fe8Si10 DQC are briefly discussed based on the structural characteristics.We ascribe the stability of DQC2 to three aspects:(1)The lower energy of the 2.0 nm decagonal clusters with broken tenfold symmetry;(2)The random tiling of clusters,which means larger configurational entropy;(3)Decagonal overlap means that decagonal density is higher,which is also conducive to the stability of decagonal quasicrystals.