and high-angle annular dark-field scanning transmission electron microscope

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1 Japanese Society of Electron Microscopy Journal of Electron Microscopy 50(3): (2001)... Full-length paper Structural study of an Al 73 Ni 22 Fe 5 decagonal quasicrystal by high-angle annular dark-field scanning transmission electron microscopy Koh Saitoh *, Michiyoshi Tanaka and An Pang Tsai 1 Research Institute for Scientific Measurements, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai , Japan and 1 National Institute for Materials Science, Sengen 1-2-1, Tsukuba , Japan * To whom correspondence should be addressed. saitohk@rism.tohoku.ac.jp... Abstract A water-quenched Al 73 Ni 22 Fe 5 decagonal quasicrystal was investigated by the selected-area electron diffraction, convergent-beam electron diffraction and high-angle annular dark-field scanning transmission electron microscope methods. The alloy shows very sharp spots and nearly no diffuse scattering in the diffraction patterns, belongs to centrosymmetric space group P10 5 / mmc and is constructed almost by one type of 2 nm diameter atom cluster having mirror symmetry with a highly quasicrystalline order arrangement. Although a small number of 2 nm atom clusters having fivefold symmetry exists, which are similar to those observed in meltquenched Al 70 Ni 15 Fe 15, the structure of Al 73 Ni 22 Fe 5 is considered to basically be the same as that of water-quenched Al 72 Ni 20 Co 8, which is constructed only by mirror symmetry clusters arranged with a very high quasiperiodicity. The number of valence electrons per atom (e/a) of the present alloy (1.92) is very close to that of Al 72 Ni 20 Co 8 (1.90), but differs from those of phases constructed by only the five-fold symmetry clusters. This implies that these alloys are Hume-Rothery electron compounds, whose structures are determined primarily by e/a value.... Keywords decagonal quasicrystal, high-angle annular dark-field imaging, Z-contrast imaging, convergent-beam electron diffraction... Received 7 November 2000, accepted 3 March Introduction The discovery of the asymmetric cluster of 2 nm diameter in an Al 72 Ni 20 Co 8 decagonal quasicrystal attracted a great interest [1,2], because until then quasicrystalline structures had been considered to be constructed by a quasiperiodic arrangement of a structural motif (atom cluster) which has the same symmetry as its point group symmetry [3 5]. Another remarkable characteristic of Al 72 Ni 20 Co 8 is its very high quality quasicrystalline order. Diffraction patterns of the alloy exhibit very sharp reflection spots and hardly show diffuse scattering, which is frequently seen in many quasicrystals. The high quality quasiperiodic arrangement of the 2 nm atom clusters was observed using conventional high-resolution electron microscope (HREM) [6] and high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) methods [1]. A quasiperiodic arrangement of a single asymmetric decagon tile was first discovered by Gummelt [7]. Steinhardt et al. pointed out that the Gummelt tiling is obtained by the maximization of the density of the decagon tile, which is connected with the neighbouring tiles by two specific overlapping manners [8]. Since the Gummelt tiling was confirmed to agree with the real structure of Al 72 Ni 20 Co 8 [9], the concept of Steinhardt, the density maximization of the energetically stable 2nm diameter atom clusters, appears to have been accepted broadly. On the other hand, an Al 70 Ni 15 Fe 15 decagonal quasicrystal was first found by Tsai et al. [10]. Saito et al. found that the quasicrystal belongs to noncentrosymmetric space group P10m2 by convergent-beam electron diffraction (CBED) [11]. Tsuda et al., using the HREM method, found that the 2 nm atom cluster has five-fold symmetry and discovered inversion domains, in each of which the five-fold symmetry clusters take the same sense [12]. Recently, these results were verified

2 198 J O U R N A L O F E L E C T R O N M I C R O S C O P Y, Vol. 50, No. 3, 2001 by the HAADF-STEM method [13]. Tanaka et al., using the CBED method and the HREM technique, revealed that Al 70 Ni x Fe 30 x alloys with 10 < x < 17.5 form the noncentrosymmetric structure, but those with 17.5 < x < 20 form a centrosymmetric structure [14], in which the 2 nm five-fold symmetry clusters with opposite senses are intermixed. Meltquenched Al 70 Ni x Fe 30 x with 10 < x < 20 always shows diffuse scattering in the diffraction patterns. It is noted that Al- Cu-Co and Al-Ni-Co decagonal quasicrystals composed of fivefold symmetry atom clusters also show the diffuse scattering. Later, Tsuda et al., however, discovered that melt-quenched Al 70 Ni x Fe 30 x with 25 < x < 27 shows almost no diffuse scattering and has nearly no five-fold symmetry clusters in the HREM images [15]. Since all the specimens of the Al-Ni-Fe system mentioned above were prepared by melt quenching without any annealing, they are not of thermodynamically stable phases. Lemmerz et al. found that a thermodynamically stable decagonal phase is formed in a narrow compositional range around Al 71 Ni 24 Fe 5 at about 900 C, which shows nearly no diffuse scattering [16]. No HAADF-STEM images have ever been taken from the alloy. The HAADF image is formed by electrons scattered at high angles of about 100 mrad in a STEM [17]. Because such electrons are incoherent and suffer little dynamical diffraction effects, the image is less sensitive to the thickness variation of the specimen. Since no image formation lens is used, the artefact due to the lens aberration is not introduced. Thus, HAADF images are interpreted more easily than conventional HREM images. Scattered electron intensities at high angles are proportional to the square of the atomic number Z. Thus, the HAADF images of Al-TM (TM: transition metal) alloys are expected to show the TM atom arrangement selectively. We have applied the HAADF technique to the study of decagonal quasicrystals, for the first time, and their approximants, and successfully observed the arrangements of the TM atoms [1,13,18]. In the present paper, we found that a water-quenched Al 73 Ni 22 Fe 5 forms a very high quality decagonal quasicrystal. The symmetry of the 2 nm clusters and their arrangements were examined by the HAADF-STEM method. The space group of the quasicrystal was determined separately by CBED [19]. A relation between the number of valence electrons per atom (e/a) and resultant structures is discussed. Methods An alloy of a nominal composition of Al 73 Ni 22 Fe 5 was melted in an Ar atmosphere, annealed at a temperature of 920 C for 120 h and quenched in cold water. The sample was crushed and dispersed on holey carbon films on copper grids for electron microscopy. HAADF images were taken with an incidence parallel to the decagonal axis. The HAADF study was conducted by using a transmission electron microscope JEM2010F equipped with a scanning unit and an ADF detector, operated at an accelerating voltage of 200 kv. The ADF detector was set to collect electrons scattered at an angular range of mrad. The spatial resolution of the HAADF images was about 0.2 nm. Selected-area electron diffraction and CBED patterns were taken from specimen areas respectively of about 100 nm and 10 nm in diameter using JEM2010 electron microscope operated at 100 kv. Results and discussion Figures 1a 1c show selected-area electron diffraction patterns of Al 73 Ni 22 Fe 5 taken at incidences parallel to the decagonal axis (c axis) and two incidences A and B perpendicular to the c axis, respectively. Figure 1a shows very sharp diffraction spots and very weak diffuse scattering. Spots corresponding to a lattice spacing up to about 3.2 nm are seen. The deviation of reflection spots from the regular positions and shapes, which are caused by linear phason defects *, are not seen. These features indicate that the alloy has a high quasicrystalline order. Figure 1b shows periodic arrays of reflections in the c direction with a periodicity of about 0.4 nm. Diffuse streaks at heights of n / 0.8 nm 1 (n: integer) in the c direction, which are often observed in many decagonal quasicrystals having five-fold symmetry atom-clusters, are extremely weak, indicating a high structural quality. Figure 1c shows a systematic extinction of spots at a height of 1 / 0.4 nm 1 in the c direction. This implies the existence of a c glide plane perpendicular to the b axis. The irregular shapes of the reflection spots are not seen in the incidences perpendicular to the c axis as well as those in Fig. 1a. The features of the electron diffraction patterns described above are very similar to those of water-quenched Al 72 Ni 20 Co 8, implying that Al 73 Ni 22 Fe 5 has a similar structure to that of Al 72 Ni 20 Co 8. It should be noted that the electron diffraction patterns of Al 71.3 Ni 23.4 Fe 5.3 reported by Lemmerz et al. (Figs 3a 3c of [16]) also show the features mentioned above. This implies that Al 71.3 Ni 23.4 Fe 5.3 has the same structure as that of the present Al 73 Ni 22 Fe 5 alloy. Figures 2a 2c show CBED patterns taken at the same incidences as those of Figs 1a 1c, respectively. Figure 2a shows ten-fold rotation symmetry and two types of mirror symmetry parallel to the c axis as indicated by m, resulting total symmetry 10mm. Figure 2b shows two types of mirror symmetry parallel and perpendicular to the c axis as indicated by m, resulting total symmetry 2mm. Figure 2c shows two mirror symmetries parallel and perpendicular to the c axis, resulting total symmetry 2mm. On the basis of the projection approximation of CBED patterns [18], possible point groups are limited to 10 / mmm and The CBED patterns in Fig. 2d, taken at the same incidence as in Fig. 2c, include higher-order Laue zone (HOLZ) reflections, which provide three-dimensional information of the structure. The pattern formed by HOLZ reflections also shows the two mirror symmetries. Thus, *A phason defect is a lattice defect of quasicrystals, which destroys a longrange quasiperiodic order. The phason defect is caused by a few connection ways of the atom-cluster with different intercluster distances.

3 K. Saitoh et al. HAADF-STEM study of decagonal Al73Ni22Fe5 199 the point group was determined to be 10 / mmm. Figure 2e shows a CBED pattern taken at an incidence slightly tilted from direction A indicated in Fig. 1a to the c direction. Dark lines (G-M lines) [20] are seen in the odd-order reflection disks as indicated by the arrowheads. These lines indicate the existence of 105 screw axes and/or c-glide planes perpendicular to direction B. Other than the extinction of the reflections due to c-glide planes, no systematic extinctions are seen in the selected-area diffraction patterns. Thus, the lattice type is determined to be P. Therefore, space group of the present alloy is determined to be P105 / mmc, which is the same as that of Al72Ni20Co8. Figure 3a shows a HAADF-STEM image of Al73Ni22Fe5 taken at an incidence along the c axis. In this image, the bright dots correspond to transition metal atom columns. Al columns are not seen clearly in the image, because the scattering power is relatively weak and the resolution of the image is not high enough. A specific atom cluster with a 2 nm diameter is seen as indicated by the black circles. The atom cluster shows three bright dots around the centre as indicated by the three arrows, ten bright dots surrounding the three dots and ten elongated dots surrounding the ten dots. These features are very similar to those of 2 nm atom clusters of Al72Ni20Co8, which have mirror symmetry. Thus, the cluster can also be decomposed into two types of small clusters with 0.4 nm radii (P and S clusters), as indicated at the cluster located at around the centre of the figure. The decomposition manner is the same as that of Al72Ni20Co8 [1]. We, therefore, conclude that the cluster has the same structure as that of Al72Ni20Co8. We refer to the cluster as cluster m hereafter. Another type of cluster is seen as indicated by the white circles. The cluster shows a bright pentagon at the centre instead of the three dots in cluster m. The feature of ten dots surrounding the pentagon and ten elongated dots surrounding the ten dots is similar to that of cluster m. The entire image of the cluster is very similar to that of melt-quenched Al70Ni15Fe15. Thus, the cluster is considered to have the same structure as that of melt-quenched Al70Ni15Fe15 [13]. We refer to the cluster as cluster f hereafter. About 10% of the 2 nm atom clusters belongs to cluster f. We already know that in the Al-Ni-Fe and Al-Ni-Co systems, decagonal alloys composed of only cluster f have many phason defects and always show strong diffuse streaks perpendicular to the c axis at heights of n / 0.8 nm 1 in the c direction. The diffuse streaks may be caused by irregular structures in cluster f, which have a 0.8 nm periodicity in the c direction and are randomly distributed in the quasiperiodic direction [21,22]. It can be considered that Fig. 1 Selected-area electron diffraction patterns of water quenched Al73Ni22Fe5 taken at (a) an incidence along the decagonal direction, (b) an incidence along direction A and (c) an incidence along direction B, where directions A and B are indicated in (a), which are perpendicular to the decagonal axis. Reflections corresponding to a lattice spacing of about 0.2 nm in the decagonal direction are indicated by black arrowheads in (b) and (c).

4 200 J O U R N A L O F E L E C T R O N M I C R O S C O P Y, Vol. 50, No. 3, 2001 Fig. 2 CBED patterns of Al73Ni22Fe5 taken at (a) an incidence along the decagonal direction, (b) an incidence along direction A indicated in Fig. 1a, (c and d) an incidence along direction B indicated in Fig. 1a, and (e) an incidence slightly tilted from direction A to the decagonal direction. (a) Ten-fold and two mirror symmetries (10mm) are seen. (b) Two mirror symmetries parallel and perpendicular to the decagonal direction (2mm) are seen. (c) Two mirror symmetries parallel and perpendicular to the decagonal direction (2mm) are seen. (d) The HOLZ reflections show two mirror symmetries parallel and perpendicular to the decagonal direction (2mm). (e) Dark lines are seen in the odd-order reflection discs along the c*-axis, indicating the existence of a 105 screw axis and/or a c-glide plane.

5 K. Saitoh et al. HAADF-STEM study of decagonal Al73Ni22Fe5 201 Fig. 3 HAADF images of water-quenched Al73Ni22Fe5. The bright dots correspond to transition metal atom columns. (a) A specific atom cluster with 2 nm diameter indicated by the black circle shows three bright dots at the centre, having only mirror symmetry. The mirror symmetry cluster can be decomposed into P and S clusters as indicated at the cluster located at around the centre. Another type of 2 nm atom cluster indicated by the white circles show a bright pentagon at the centre, having five-fold symmetry. The five-fold symmetry cluster is the minor part in Al73Ni22Fe5. (b) HAADF image superposed by a pentagon Penrose pattern. All the vertices of the pentagon Penrose pattern fit very well with the positions of the 2 nm atom clusters, indicating a highly perfect quasiperiodicity. The five-fold symmetry clusters are also located at the vertices of the Penrose pattern, indicating its structural similarity to the mirror symmetry cluster. such an irregular structure is induced by a stress due to a phason defect, because Sun et al. found that an Al-Ni-Ru decagonal quasicrystal composed only of cluster f, which is arranged with a high quasiperiodic order, shows almost no diffuse streaks in the diffraction patterns [23]. Further analysis will be necessary to discuss more details. Figure 3b shows the same HAADF image as Fig. 3a over an extended area, to which a pentagon Penrose pattern is super-

6 202 J O U R N A L O F E L E C T R O N M I C R O S C O P Y, Vol. 50, No. 3, 2001 posed. It is seen that all the vertices of the pentagon Penrose pattern agree very well with the centres of the 2 nm atom clusters, indicating that the 2 nm clusters arrange with a very high quasiperiodicity. The small disagreement of the vertices and the centres are due to the distortion of the image caused by the instability of the specimen stage and the inaccuracy of probe scanning. We should note that the senses of cluster m also agree well with the local senses of the vertices of the pentagon Penrose pattern. That is, at the vertices where three lines are terminated, the three dots of clusters m are always located inside the corners formed by the lines, as indicated by the arrows at atom cluster A. At the vertices where four lines are terminated, the three dots are located inside the three wide corners, as indicated by the arrows at atom cluster B. As is mentioned above, the 2 nm cluster is decomposed into two types of basic clusters with 0.4 nm radii, or clusters P and S [1]. The good agreement of the arrangement of cluster m with the pentagon Penrose pattern indicates that the arrangement of clusters P and S agrees with the rhombic Penrose pattern because the pentagon Penrose tiling is uniquely related to the rhombic Penrose tiling. That is, clusters P and S are located at the vertices of the rhombic Penrose pattern, as those of Al 72 Ni 20 Co 8 (Fig. 3a of [1]). The agreement of the arrangement with Gummelt s decagon tiling is also confirmed because this tiling is uniquely related to the pentagon Penrose tiling as well. From the close similarity of both the symmetry of the atom cluster and its high quality quasiperiodic arrangement of Al 73 Ni 22 Fe 5 and Al 72 Ni 20 Co 8, the structure of Al 73 Ni 22 Fe 5 is concluded to basically be the same as that of Al 72 Ni 20 Co 8, and must be described by the basic clusters of Al 13 Fe 4 approximant [1]. Lemmerz et al. reported that the high quality decagonal phase is formed in a very narrow composition range of about 1 at.% around Al 71.3 Ni 23.4 Fe 5.3 [16]. This fact may not be consistent with the entropy stabilization scheme [24], because the composition of Al 71.3 Ni 23.4 Fe 5.3 is not so suitable for the gain of entropy, which is clearly less than that of Al 71.3 Ni Fe 14.35, where it is assumed that the main contribution to the entropy is due to a chemical disorder between Ni and Fe [24,25]. A plausible explanation for the narrow composition range of the formation of the quasicrystal with the mirror symmetry clusters at a composition Al 73 Ni 22 Fe 5 is the Hume-Rothery mechanism for the stabilization of the phase. Using the number of holes per atom (Ni: 0.61 and Fe: 2.66) given by Raynor [26], the e/a values of Al 73 Ni 22 Fe 5 and Al 71.3 Ni 23.4 Fe 5.3 are calculated to be 1.92 and 1.88, respectively. The high quality quasicrystal of water-quenched Al 72 Ni 20 Co 8 has an e/a value of Such e/a values may stabilize the high quality decagonal quasicrystalline structure by lowering the Fermi energy, i.e. these alloys are considered to be Hume-Rothery electron compounds. On the other hand, melt-quenched Al 70 Ni x Fe 30 x with 10 < x < 25 and Al 70 Ni x Co 30 x with 10 < x < 20, which are composed of cluster f only, have e/a values within a range of 1.51 to The clear discrepancy of the e/a values between the phase composed of clusters m and that of cluster f also implies that these quasicrystals are Hume-Rothery electron compounds, whose structures are determined primarily by the e/a values. It is seen that the bright pentagon of cluster f is more intense than the three bright dots at the centre of cluster m. The difference in the intensity at the dots of the clusters may be caused by the difference in the number of TM atoms. A quantitative analysis of the HAADF image is necessary to discuss the TM composition at the dots more precisely. It should be noted that cluster f is located at the vertices position of the pentagon Penrose lattice as well as clusters m. This indicates that cluster f appears not to destroy the Penrose tiling of the neighbouring clusters m. Thus, the structure of cluster f is not very different from that of cluster m, except the central part of the cluster. It is noted that, recently, we observed the structural change between clusters m and f through an in situ highresolution electron microscope observation [Saitoh, in preparation]. This also supports that both the structures of clusters m and f are close. Concluding remarks It was found that a high quality quasicrystalline phase is formed in the Al-Ni-Fe system as well as in the Al-Ni-Co system. The structure of Al 73 Ni 22 Fe 5 is basically the same as that of water-quenched Al 72 Ni 20 Co 8. Al 73 Ni 22 Fe 5, however, has a small amount of structural imperfections of the atom clusters, or cluster f, which is considered to have a similar structure to that of cluster m, except for its central part. We are planning to prepare an Al-Ni-Fe sample with an e/a value of 1.90 to examine whether cluster f is excluded in the sample and to confirm the alloys to be a Hume-Rothery electron compound. To obtain further evidence of Hume-Rothery compounds, the ALCHEMI technique [25] will be applied to reveal the chemical disorder between Ni and Fe in the Al-Ni-Fe alloys. Acknowledgements The authors are thankful to Dr Kawasaki and Mr Ibe of JEOL Ltd for experimental assistance using JEM2010F. They also thank Mr F. Sato for his skilful maintenance of the electron microscopes. A. P. Tsai is grateful for the support of the Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation. The present studies were partly supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. 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