Atom Clusters In A 2/1 Cubic Approximant Phase For Understanding The Structures Of Icosahedral Phases

1998 ◽  
Vol 553 ◽  
Author(s):  
K. HIRAGA
Keyword(s):  
Icosahedral Phase ◽  
The Body ◽  
Cubic Phase ◽  
Icosahedral Quasicrystal ◽  
Icosahedral Symmetry ◽  
Atom Cluster ◽  
Atom Clusters ◽  
Simple Cubic ◽  
Icosahedral Phases ◽  
Rotational Direction

AbstractTwo types of atom clusters found in the β-(A1PdMnSi) cubic phase, referred to as a 2/1 crystalline approximant, with a composition of approximately Al70Pd23Mn6Si1 which is near to the composition Al72Pd20Mn8 of the icosahedral phase, are discussed in detail for understanding the structure of the Al-Pd-Mn icosahedral phase. A large dodecahedral atom cluster located at the body-centered position can be divided into 19 atom shells with approximately icosahedral symmetry, and a dodecahedron of the 12th shell internally touches the surface of the cubic unit cell with a lattice constant of 2.0211 nm. At each vertex of the dodecahedron, a small icosahedral atom cluster consisting of 12 Al atoms surrounding a central Pd atom is located. The dodecahedron is connected to each other by edge-sharing, namely by sharing two small icosahedral atom clusters, along the twofold rotational direction, and forms a simple-cubic packing of the atom cluster in the β-(AlPdMnSi) cubic phase. Another atom cluster located at the origin fills up gaps of the simple-cubic packing of the large dodecahedral atom cluster. By using the dodecahedral and bridging atom clusters, the structure of the Al-Pd-Mn icosahedral quasicrystal is discussed.

Microstructural characterization of rapidly solidified Al–Fe–Si, Al–V–Si, and Al–Fe–V–Si alloys

2001 ◽  
Vol 16 (7) ◽  
pp. 2103-2117 ◽  
Author(s):  
A. K. Srivastava ◽  
S. Ranganathan
Keyword(s):  
Elevated Temperatures ◽  
Microstructural Characterization ◽  
Icosahedral Phase ◽  
Silicide Phase ◽  
Icosahedral Quasicrystal ◽  
Icosahedral Symmetry ◽  
Scanning Calorimetry ◽  
Lower Temperature ◽  
Rapidly Solidified ◽  
Microscopy Techniques

The present study of rapidly solidified melt-spun Al80Fe14Si6 Al80V14Si6, and Al80Fe10V4Si6 alloys by electron microscopy techniques, x-ray diffractometry, and differential scanning calorimetry leads to a number of microstructural results. Coexistence of a micro-quasicrystalline state of an icosahedral phase with monoclinic θ–Al13Fe4 and hexagonal β–Al6V in Al–Fe–Si and Al–V–Si alloys, respectively, is reported. Also, the growth morphology of the icosahedral phase surrounded by a crystalline ring was investigated in an Al–Fe–V–Si alloy. The crystalline ring has the particles of the cubic α–Al12(Fe,V)3Si silicide phase. Evidence of irrational twinning of cubic crystals, giving rise to a symmetry not deviating much from icosahedral symmetry was found in this alloy. In all the three alloys crystalline intermetallics were elucidated in the context of rational approximants of an icosahedral quasicrystal. It was noticed that while the icosahedral phase in Al–Fe–Si and Al–V–Si alloys transforms to crystalline intermetallics at about the same temperature (approximately 610 K), the transformation of icosahedral phase in Al–Fe–V–Si alloy occurred at a relatively lower temperature (540 K). The origin of different metastable microstructures and their stability at elevated temperatures, in these alloys, are compared and discussed.

On the new structural phases in Al65Cu20Cr15 quasicrystalline alloy

1995 ◽  
Vol 10 (8) ◽  
pp. 1905-1912 ◽  
Author(s):  
Varsha Khare ◽  
N.P. Lalla ◽  
R.S. Tiwari ◽  
O.N. Srivastava
Keyword(s):  
Metastable Phase ◽  
Structural Characteristics ◽  
Icosahedral Phase ◽  
Cubic Phase ◽  
Face Centered Cubic ◽  
Body Centered Cubic ◽  
Structural Variants ◽  
Simple Cubic ◽  
Face Centered ◽  
Specific Orientation

The quasicrystalline (qc) alloy Al65Cu20Cr15, unlike its Ru- and Fe-bearing counterparts like Al65Cu20Ru15 and Al65Cu20Fe15, is a metastable phase. This qc alloy has been shown to possess several structural variants and curious structural characteristics. We have investigated the qc alloy Al65Cu20Cr15 with special reference to the possible occurrence of new structural variants. TEM exploration of the as-quenched qc alloy has indeed revealed the existence of several new phases. These are (i) body-centered cubic (bcc) (a = 12.60 Å, disordered) and simple cubic (s.c.) (a = 12.60 Å, ordered), which are the 1/1 approximants of the primitive icosahedral phase (i phase); (ii) a twice order-induced modulated cubic phase (bcc, a = 25.20 Å) which has been shown to correspond to 1/1 approximant of the ordered i phase [i.e., face-centered icosahedral (FCI)]; and (iii) real crystalline bcc (a = 8.90 Å) and face-centered cubic (fcc) (a = 17.98 Å) phases possessing a specific orientation relationship with the icosahedral matrix phase. Tentative structural models showing the interrelationships between the bcc/fcc phases have been outlined.

Electrical Resistivity of an Al-Re-Si Cubic Approximant Phase and Role of Local Environment in Electronic Transport

2000 ◽  
Vol 643 ◽  
Author(s):  
Ryuji Tamura ◽  
Takayuki Asao ◽  
Mutsuhiro Tamura ◽  
Shin Takeuchi
Keyword(s):  
Electrical Resistivity ◽  
Electronic Transport ◽  
Electronic States ◽  
Local Environment ◽  
Negative Temperature ◽  
Icosahedral Phase ◽  
High Resistivity ◽  
Icosahedral Quasicrystal ◽  
Icosahedral Phases

AbstractIn order to gain an insight into the role of the local atomic environment in the electronic transport of the icosahedral quasicrystal, the electrical resistivity of α-AlReSi, which is the (1/1,1/1,1/1) approximant of the icosahedral phase, has been investigated. Very high resistivity and its pronounced negative temperature dependence have been observed, indicating that the electronic states of the 1/1 cubic approximant are quite similar to those of icosahedral phases. In order to further elucidate which structural entity is responsible for such anomalous transport, a comparison of the electrical resistivity between (1/1,1/1,1/1) and (1/0,1/0,1/0) approximants has been made. The typical transport behavior of icosahedral phases which is also seen in 1/1 and higher-order approximants was not observed in any of the studied 1/0 cubic approximants. The result can be regarded as an implication that the intercluster distance between the TM clusters plays a significant role in the confinement of electronic states.

Rapid Thermal Transformation of A-15 Crystal Structure Metallic Thin Films

1995 ◽  
Vol 387 ◽  
Author(s):  
M. J. O'Keefe ◽  
C. L. Cerny
Keyword(s):  
Crystal Structure ◽  
Thin Films ◽  
Thermal Annealing ◽  
Rapid Thermal Annealing ◽  
Physical Vapor Deposition ◽  
Thermal Transformation ◽  
The Body ◽  
Cubic Phase ◽  
Body Centered Cubic ◽  
Body Center Cubic

AbstractPhysical vapor deposition of Group VI elements (Cr, Mo, W) can lead to the formation of a metastable A-15 crystal structure under certain processing conditions. Typically, a thermally induced transformation of the metastable A-15 structure into the equilibrium body centered cubic structure has been accomplished by conventional furnace annealing at T/Tm ≈ 0.3 from tens of minutes to several hours. In this study we report on the use of rapid thermal annealing to transform sputter deposited A- 15 crystal structure tungsten and chromium thin films into body centered cubic films within the same temperature range but at times on the order of one minute. The minimum annealing times and temperatures required for complete transformation of the A-15 phase into the BCC phase varied from sample to sample, indicating that the transformation was dependent on the film characteristics. The electrical resistivity of A-15 Cr and W films was measured before and after rapid thermal annealing and was found to significantly decrease after transformation into the body center cubic phase.

Textures of the Hot-Forged Ti6Al4V Alloy

2013 ◽  
Vol 203-204 ◽  
pp. 111-114
Author(s):  
Adam Bunsch ◽  
Wiktoria Ratuszek ◽  
Małgorzata Witkowska ◽  
Joanna Kowalska ◽  
Aneta Łukaszek-Sołek
Keyword(s):  
Deformation Temperature ◽  
Phase State ◽  
Hexagonal Phase ◽  
Ti6al4v Alloy ◽  
The Body ◽  
Cubic Phase ◽  
Body Centered Cubic ◽  
Two Phase ◽  
Different Temperatures

This paper presents the results of the texture investigation in the hexagonal phase and the body-centered cubic  phase of the Ti6Al4V alloy hot-deformed by forging. Forging was performed at two different temperatures on the occurrence of the single  and in the two-phase  +  state. It was found that after deformation both  and  phases are textured and their textures strongly depends on deformation temperature.

Modulated Structures, Nanocrystals and Quasicrystals in Mg-Zn-Y Alloys

2000 ◽  
Vol 643 ◽  
Author(s):  
Anandh Subramaniam ◽  
S. Ranganathan
Keyword(s):  
Cast Alloy ◽  
Icosahedral Phase ◽  
Cubic Phase ◽  
Nominal Composition ◽  
Modulated Structures

AbstractThe formation of the icosahedral phase, BCC (Body centred cubic) phase and nanocrystals are seen in the as-cast alloy with nominal composition of Mg4Zn94Y2. FCC (face centred cubic) phase and modulated structures are formed in the alloys with higher Y content (10% and 25% Y respectively). These phases are analysed keeping in view their relation to the quasicrystals of the Mg-Zn-Y system.

scholarly journals Quasi-periodic crystals – a paradigm shift in crystallography

2014 ◽  
Vol 70 (a1) ◽  
pp. C2-C2
Author(s):  
Dan Shechtman
Keyword(s):  
Modern Science ◽  
Icosahedral Phase ◽  
Icosahedral Symmetry ◽  
Atomic Order ◽  
X Ray Diffraction ◽  
Theoretical Understanding ◽  
Vast Number ◽  
X Ray ◽  
Definition Of ◽  
Periodic Materials

Crystallography has been one of the mature sciences. Over the years, the modern science of crystallography that started by experimenting with x-ray diffraction from crystals in 1912, has developed a major paradigm – that all crystals are ordered and periodic. Indeed, this was the basis for the definition of "crystal" in textbooks of crystallography and x-ray diffraction. Based upon a vast number of experimental data, constantly improving research tools, and deepening theoretical understanding of the structure of crystalline materials no revolution was anticipated in our understanding the atomic order of solids. However, such revolution did happen with the discovery of the Icosahedral phase, the first quasi-periodic crystal (QC) in 1982, and its announcement in 1984 [1, 2]. QCs are ordered materials, but their atomic order is quasiperiodic rather than periodic, enabling formation of crystal symmetries, such as icosahedral symmetry, which cannot exist in periodic materials. The discovery created deep cracks in this paradigm, but the acceptance by the crystallographers' community of the new class of ordered crystals did not happen in one day. In fact it took almost a decade for QC order to be accepted by most crystallographers. The official stamp of approval came in a form of a new definition of "Crystal" by the International Union of Crystallographers. The paradigm that all crystals are periodic has thus been changed. It is clear now that although most crystals are ordered and periodic, a good number of them are ordered and quasi-periodic. While believers and nonbelievers were debating, a large volume of experimental and theoretical studies was published, a result of a relentless effort of many groups around the world. Quasi-periodic materials have developed into an exciting interdisciplinary science. This talk will outline the discovery of QCs and describe the important role of electron microscopy as an enabling discovery tool.

scholarly journals Phase transition induced by an external field in a three-dimensional isotropic Heisenberg antiferromagnet

2018 ◽  
Vol 32 (32) ◽  
pp. 1850390
Author(s):  
Minos A. Neto ◽  
J. Roberto Viana ◽  
Octavio D. R. Salmon ◽  
E. Bublitz Filho ◽  
José Ricardo de Sousa
Keyword(s):  
Magnetic Field ◽  
Mean Field ◽  
Three Dimensional ◽  
Effective Field ◽  
The Body ◽  
Bravais Lattice ◽  
Body Centered Cubic ◽  
Cubic Lattice ◽  
Cubic Lattices ◽  
Simple Cubic

The critical frontier of the isotropic antiferromagnetic Heisenberg model in a magnetic field along the z-axis has been studied by mean-field and effective-field renormalization group calculations. These methods, abbreviated as MFRG and EFRG, are based on the comparison of two clusters of different sizes, each of them trying to mimic a specific Bravais lattice. The frontier line in the plane of temperature versus magnetic field was obtained for the simple cubic and the body-centered cubic lattices. Spin clusters with sizes N = 1, 2, 4 were used so as to implement MFRG-12, EFRG-12 and EFRG-24 numerical equations. For the simple cubic lattice, the MFRG frontier exhibits a notorious re-entrant behavior. This problem is improved by the EFRG technique. However, both methods agree at lower fields. For the body-centered cubic lattice, the MFRG method did not work. As in the cubic lattice, all the EFRG results agree at lower fields. Nevertheless, the EFRG-12 approach gave no solution for very low temperatures. Comparisons with other methods have been discussed.

The crystal stucture of a gamma-(Al-Cu-Ru-Si) cubic phase: A new approximant phase for the Al-Cu-Ru icosahedral quasicrystal

1998 ◽  
Vol 77 (3) ◽  
pp. 165-171 ◽  
Author(s):  
Kazumasa Sugiyama ◽  
Takeshi Kato ◽  
Kaichi Saito ◽  
Kenji Hiraga
Keyword(s):  
Cubic Phase ◽  
Icosahedral Quasicrystal
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