Electronic Microscopy Study Of Quasicrystal Approximants And Defects In Al-Cr-Fe-Si Alloys

Al-Cr-Fe-Si alloy system is a very special decagonal quasicrystal forming system,and its structure characteristics are completely different from the widely studied Al-Co-Ni and other decagonal quasicrystal systems.Studies have pointed out that there are many novel structures such as new type of decagonal quasicrystal,a variety of quasicrystal approximants,novel quasicrystal-related solid state with both periodic and quasi-periodic in Al-Cr-Fe-Si alloys.However,the detailed study on the giant approximants of quasicrystal and the related structural defects in the alloys,especially the atomic-scale structural characteristics,is still very limited.Systematic study of quasicrystal approximants is not only helpful to understand the corresponding quasi-phase structure,but also instructive to the design of new materials.The purpose of this thesis is to explore the atomic-scale structural characteristics of the quasicrystal approximants and related structural defects in the Al-Cr-Fe-Si alloy.The structure of the approximants,as well as the connection and application of the basic structural building units are systematically studied using electron microscopy methods including selected area electron diffraction,spherical aberration corrected high-angle annular dark field-scanning transmission electron microscopy.In addition,two novel structural defects related to the approximants with larger unit cell are studied,and the structural characteristics and formation mechanism are further analyzed.Moreover,two types of one-dimensional quasicrystals with the periodic ratio along one of the two-fold axes of the corresponding decagonal quasicrystal of golden ratio τ were observed,and the atomic-scale structural characteristics of one-dimensional quasicrystals were revealed for the first time.Studies on the quasicrystal approximants in Al-Cr-Fe-Si alloys show that(3/2,5/3)Fibonacci approximant and its various structural variants,whose basic structural building block is fat hexagon composed of six decagonal units,exist in the samples with the composition of Al60Cr20Fe10Si10 and Al62Cr17.5Fe11Si9.5.The lattice parameters of(3/2,5/3)phase are:a=3.11 nm,b=1.23 nm,c=5.89 nm,and the phase composition is about Al62.16Cr17.34Fe11.12Si9.38.Two types of(2/1,5/3)approximants possessing the same lattice parameters(a=1.91 nm,b=1.23 nm,c=5.87 nm)but different crystal structures were observed in Al60Cr20Fe10Si10 alloy.Their structural projections along the[010]direction are both composed of hexagon,star and decagon structural units,but the star structural units of the first(2/1,5/3)phase have two opposite orientations,while in the second(2/1,5/3)phase have only one orientation.Studies of the basic structural units in the approximants of Al-Cr-Fe-Si quasicrystals show that the basic structural units are mainly hexagon,boat,star,and decagon,and the lengths of their sides are measured about 0.62 nm.The shield-like tile is composed of two hexagon structural units with an angle of 72° and one corresponding star structural unit,with a total of ten orientations.There are 31 types of connections between two shield-like tiles,which can be classified into two categories:sharing edges and overlapping regions.The application of the shield-like tiles is helpful to analyze the complex structures related to quasicrystals and to predict new phases that may appear in the alloys.Besides,one decagon unit with perfect tenfold symmetry and five types of decagon units with broken tenfold symmetry were observed.According to the atomic structure characteristics of different decagonal units,it can be inferred that decagons of different structures can be transformed from one to the other under certain conditions.The investigation of structural defects in Al-Cr-Fe-Si quasicrystal approximants show that the special 180° domain structure related to the(3/2,2/1)1 approximant comes from shield-like tiles with opposite orientations on both sides of the domain boundary,rather than the inversed unit cells as in traditional crystals.The displacement of the sequence of hexagon units and the inversed star structural units on both sides of the domain boundary play a key role in the reverse orientation of the shield-like tiles on both sides of the domain boundary and the formation of 180°domains.The structural characteristics of the special plane defects related to the(3/2,5/3)phase is that three ordering domains composed of fat hexagon structural blocks are separated by phason plane defects formed by different types of phason defects.Phason defects come from the displacement of the distribution plane of the decagon structural units between adjacent domains along a certain direction of about 1.12 nm,and will not cause lattice distortion.The research of one-dimensional quasicrystals in Al62Cr17.5Fe11Si9.5 alloy suggests that the periods of the two types of one-dimensional quasicrystal along one of the two-fold axes of the corresponding decagonal quasicrystal is 2.99 nm and 1.85 nm,respectively.The decagon structural units distributed along five directions with an angle of 72°are the main structural units of these two one-dimensional quasicrystals.The distribution of decagon structural units distribution planes of the one-dimensional quasicrystal with a period of 2.99 nm exhibits an evident periodic pattern of long and short units(LSLSLS…,the ratio of the long distance(L)and short distance(S)is the golden ratio τ)along the direction perpendicular to p2,while the distribution of decagon distribution planes of the one-dimensional quasi-crystal with a period of 1.85 nm exhibits only the long units along the direction perpendicular to p2.There is no periodicity in the other four directions for these two one-dimensional quasicrystals.The phason strain analysis and the period of the decagon units distribution planes in the high-angle annular dark field-scanning transmission electron micrographs indicate that the period ratio of the two one-dimensional quasicrystals is the golden ratio τ,rather than the ratio of adjacent numbers of Fibonacci series indicated in literatures.It is generally believed that the one-dimensional quasicrystals are intermediate states between quasicrystal approximants and two-dimensional quasicrystals,studies of the atomic-scale structural characteristics of the one-dimensional quasicrystal is important for in-depth understanding of the formation of the quasicrystal quasi-periodic structure,and the structural transition between the quasicrystal and the quasicrystal approximant.