Approximant Crystalline Phases Research Articles

Features of the formation of a quasi-crystalline phase in Al-Cu-Fe cast alloys

The phase transformations in Al-Cu-Fe cast alloys in the region of existence of the ico-phase are characterized via X-ray diffraction, X-ray fluorescence, and scanning electron microscopy. It is established that the production of multiphase structures consists of a mixture of quasi-crystalline and approximant phases, the amount and morphology of which depend on both the shield composition and crystallization mode. The optimum annealing time for the cast samples at 800°C has been determined, resulting in complete transformation of approximant crystalline phases into a quasi-crystalline ico-phase.

Quasicrystals and atomic clusters

Historically, two principal approaches exist to determine the quasicrystal atomic structures: one is derived directly from diffraction experiments, using cut and projection of higher dimensional space-representation techniques; the other one takes use of the similarity between so-called approximant crystalline phases, observed to co-exist with quasicrystals. The known structure of these phases is shown to be the starting point of building a quasicrystal model in terms of constitutive polyatomic clusters of icosahedral symmetry. It requires to apply inflation properties of quasiperiodicity, and decoration of elementary building tiles. One example is detailed and discussed, in terms of a two-cluster model, in the case of AlLiCu. Historiquement, il existe deux approches principales pour déterminer la structure atomique dʼun quasicristal : lʼune découle directement, par des techniques de coupe et de projection de représentations dans des espaces de dimensionnalité supérieure à 3, des expériences de diffraction ; lʼautre utilise la similarité avec des cristaux approximants qui coexistent avec les quasicristaux. Leurs structures, connues, peuvent être le point de départ de la construction de celle dʼun quasicristal, en fonction dʼamas polyatomiques de symétrie icosaédrique constitutifs, et en appliquant des règles dʼinflation propres à la quasipériodicité, et une décoration atomique des briques élémentaires. Un exemple est donné, puis discuté, pour AlLiCu, en fonction dʼun modèle à deux amas.

Microstructure evolution of high strength AlFeVTi nanoquasicrystalline alloys at elevated temperature

Abstract The stability of a series of Al 92 Fe 3 V 3 Ti 2 nanoquasicrystalline alloys produced with decreasing cooling rates was investigated. As-cast microstructure in rapidly solidified ribbons varied from a mixture of 5 nm granular amorphous phase and Al grains at the highest cooling rates, to a composite microstructure consisting of a dispersion of 200–300 nm diameter quasicrystals and intermetallic phases at the lowest cooling rate. In situ monitoring of phase transformations in thick samples, during annealing up to 873 K, was done using high-energy diffraction studies at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The nanoquasicrystalline phase was found to be stable even in low quench rate alloys up to temperatures of 420 °C, decomposing into more stable intermetallic phases through intermediate stages at which approximant crystalline phases are formed.

X-ray magnetic circular dichroism in quasicrystals

The spectra of X-ray magnetic circular dichroism (XMCD) and extended X-ray absorption fine structure (EXAFS) were measured in the ferromagnetic quasicrystals (a stable decagonal phase of Al40Mn25Fe15Ge20, a unstrained icosahedral phase of Al50Pd15Mn20B15, a strained icosahedral phase of Al42.5Pd7.5Mn30B20) and the orthorhombic approximant crystalline phase of Al30Mn40B30. In Al40Mn25Fe15Ge20, the intensity of XMCD at Mn K-edge and Fe K-edge were −7×10−5 and −5×10−5, respectively, which are normalized by the edge jump of the normal absorption spectra. The values suggest that Mn and Fe in this alloy contribute to the magnetism to the same degree. From the result of EXAFS, the nearest neighbors of Mn atoms and Fe atoms were analyzed to be at the same distance. The environment around Mn and Fe were found not so similar except for the nearest neighbor distance. In Al50Pd15Mn20B15, Al42.5Pd7.5Mn30B20 and Al30Mn40B30 alloys, the intensity of XMCD around the Mn K-edge were about −3×10−4. The magnetic moments of Mn atoms in these alloys are roughly estimated to be 0.8μB. The difference of a magnetic structure was detected between the quasicrystals and the approximant. The result of EXAFS analysis indicates that the distances from Mn to its first nearest neighbor and its second nearest neighbor are almost equal. The existence of phason strain field have no appreciable influence on the moment of Mn.

Microstructure and Mechanical Properties of Rapidly Solidified Al–Cr–Ce–M (M=Transition Metal) Alloys Containing High Volume Fraction of the Icosahedral Phase

Coexistent icosahedral (I) and fcc-Al phase alloys exhibiting good bending ductility and high tensile strength ( f ) were found to form in rapidly solidified Al 95.5+x Cr 3 Co 1.5 Ce x (x≤3 at%) and Al 94.5-x Cr 3 Co 1.5 Ce 1 M x (M=Ti, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo or Hf) alloys. The particle size and interparticle spacing of the I-phase decrease down to about 40 and 10 nm, respectively, with increasing Ce content, accompanying the increase in the volume fraction of the I-phase. This microstructural change is interpreted to result from the increase in the quenching effect with increasing Ce content. The highest f value is 1340 MPa for the Al 94.5 Cr 3 Co 1.5 Ce 1 alloy and 1330 MPa for the Al 93.5 Cr 3 Co 1.5 Ce 1 Ti 1 alloy. Even after annealing for 900 s at 673 K, the high f values of 830 and 680 MPa are obtained for the Zr- and Fe-containing penternary alloys, respectively. The high f values for these alloys are due to the high thermal stability of the I-phase against the grain growth and coalescence. The achievement of the high f over 600 MPa and the high structural stability allows us to expect that high-strength bulk alloys by utilizing the I-phase as a strengthening medium can be prepared by extrusion at elevated temperatures. Furthermore, it was found from the high resolution TEM and EDX analyses that the Al base I-phase containing 7.2 at%Cr, 1.6%Cr and 2.6%Ce had a disordered structure on a short-range less than 1 nm. This is presumably because of the introduction of a high density of interface between I-phase and phason-induced approximant crystalline phase. The formation of the disordered structure is thought to be the reason for the good ductility which enables the achievement of high f

Mechanical properties and deformation behaviour of large Al 70Pd 20Mn 10 single quasi-crystals

An icosahedral single quasi-crystal of Al 70Pd 20Mn 10 was found to be deformed in the temperature range above 1000 K, accompanying the generation of a number of deformation markings. The deformation appears to occur inhomogeneously along the deformation bands which lie on the fivefold plane and the region in the deformation bands is identified to have deviated alloy compositions and approximant crystalline structures. As the test temperature increases, the density of the deformation markings increases accompanying the decrease in their spacing. Superplasticity is also obtained at temperatures above 0.98 T m, presumably because of the sliding at the interface between icosahedral and approximant crystalline phases. Furthermore, a number of dislocations are observed in the icosahedral phase deformed at temperatures above 1000 K and the dislocation density is much higher near the interface. The Burgers vector was analysed to have the twofold direction for the dislocations on the fivefold plane. It is therefore concluded that the icosahedral single quasi-crystal can be deformed via dislocations along the interface between icosahedral and approximant crystalline phases at temperatures above 1000 K.

Ultrahigh Strength of Rapidly Solidified Al<SUB>96−<I>x</I></SUB>Cr<SUB>3</SUB>Ce<SUB>1</SUB>Co<I><SUB>x</SUB></I> (<I>x</I>=1, 1.5 and 2%) Alloys Containing an Icosahedral Phase as a Main Component

An extremely high tensile strength (σf) of 1340 MPa combined with good bending ductility was obtained as a mixed structure consisting of nanoscale icosahedral (I) particles with a size of about 40 nm surrounded by a thin fcc-Al layer with a thickness of about 2 nm in rapidly solidified Al99−x−yCrxCe1Coy alloys containing 0 to 5%Cr and 0 to 1.5%Co. Furthermore, each I-particle has a random orientation and contains a high density of phason defects and subnanoscale approximant crystalline regions. The replacement of Co by Mn, Fe, Ni or Cu causes the decrease in σf through the increases in the particle size of the I-phase and the width of the Al phase as well as the decrease in the density of phason defects in the I-phase. The unique mixed structure is formed through the process of the precipitation of the primary I-phase, followed by the solidification of the remaining liquid as an Al phase. The good ductility for the alloys containing the I-phase as a main constitute is presumably due to the ease of the sliding along the interface between I- and approximant crystalline phases as well as the existence of the Al phase which surrounds the I-phase. The high σf value is interpreted by the effective contribution of the I-particles as a strengthening phase because of the achievement of a good ductile nature for the mixed phase alloys.

Electron diffraction studies of the decagonal phase and its crystal approximants in AlNiCo alloys

Structures of alloys with compositions Al 70Ni 17Co 13, Al 70Ni 15Co 15 have been studied by electron diffraction after being subjected to various heat treatments. A new type of decagonal phase has been found. The phase can be regarded as having the structure in which a superlattice order is introduced into that of the decagonal phase reported so far. Four types of approximant crystalline phases have been identified, which have often been observed to form (micro-)twins. It has been found that the amount of the approximant phases is larger in the sample annealed at a high temperature of 900°C than in those annealed at 800 or 600°C.

Formation of Quasicrystals and Approximant Crystals by Mechanical Alloying in Mg–Al–Zn Alloy System

The formation of quasicrystals and related approximant crystalline phases is studied in the Mg-Al-Zn alloy system by applying mechanical alloying technique. This technique is advantageous over the conventional melt-spinning because of the ease in sample preparation involving only the solid-state reaction. The quasicrystal is formed over a wide composition range. Those formed along the Bergman line corresponding to Mg 39.5 , (Al 1-x Zn x ) 60.5 are found to be metastable and to transform into stable (1/1-1/1-1/1) Frank-Kasper phase by evolving an exothermic peak in the DSC run. Its stability is found to increase with decreasing Al content

ＭＡにより得られたＭａ‐Ａｌ‐Ｚｎ系準結晶と近似結晶

The formation of quasicrystals and related approximant crystalline phases is studied in the Mg-Al-Zn alloy system by applying mechanical alloying technique. This technique is advantageous over the conventional melt-spinning because of the ease in sample preparation involving only the solid-state reaction. The quasicrystal is formed over a wide composition range. Those formed along the Bergman line corresponding to Mg39.5(All-xZnx)60.5 are found to be metastable and to transform into stable (1/1-1/1-1/1) Frank-Kasper phase by evolving the exothermic peak in the DSC run. Its stability is found to increase with decreasing Al content. On the other hand, those formed in the Mg-rich and 12-15 at.%Al region off the Bergman line are found to transform into the (2/1-2/1-2/1) approximant phase at temperatures exceeding 400°C, which accompanies the endothermic peak in the DSC run. The peritectic reaction is suggested to be involved. The most stable quasicrystal obtained in this experiment is Mg47.5Al15.0Zn37.5

An icosahedral quasicrystal and its cubic approximant in the Al—Pd—Mn system

Abstract Structural changes after heat treatment of an icosahedral phase of melt-spun Al—Pd—Mn alloys have been studied by X-ray powder diffraction and transmission electron microscopy. Around the compositions Al70Pd20Mn10 and Al70Pd23Mn7, no phase transition occurs on annealing, and the icosahedral phase is stable up to the melting point. However, for the Al70Pd26Mn4 sample, the icosahedral phase transforms into a 2/1 cubic approximant crystalline phase at 750°C. A decagonal quasicrystal is formed for a melt-spun Al75Pd20Mn5 alloy.

Structure of a high-order icosahedral approximant phase in Mg–Ga–Al–Zn system

Abstract The melt-spun Mg39·5Ga16·4 A14·1Zn40·0 ribbon consists entirely of micrograins of an icosahedral phase (I-phase). The calorimetric and X-ray diffraction studies have shown that the I-phase undergoes an exothermic transformation to an approximant crystalline phase at 630 K on heating at a rate of 20 K min−1. Detailed structural analysis of the crystalline phase has revealed that the phase corresponds to a 3/2–2/1–2/1 rational approximant of I-phase, which has the orthorhombic C-type Bravais lattive with large cell parameters, a = 36·93 ± 0·06 A, b = 22·83 ± 0·04 A and c = 22·96 ± 0·04 A.

Research of quasicrystalline state in TiNiV system

Abstract The large unit cell c-Ti2Ni phase (Fd3m, a= 1.1278 nm) is shown to be a valuable approximant crystalline phase by the study of its local order in direct space and the distribution of intense peaks in reciprocal space. After annealing of amorphous melt-spun TiNiV ribbons, c-Ti2Ni is obtained together with a small volume fraction of another phase, for which only a few peaks on X-Ray diffraction patterns are visible. Only a 6D indexation scheme was found to be suitable to explain these peaks. However, we do not observe this phase by TEM.

Annealing effects on X-ray diffraction peaks of melt-spun AlCuFe quasicrystals

Structural changes of melt-spun AlCuFe icosahedral phase after heat treatments have been studied by X-ray powder diffraction. The widths of the diffraction peaks for the icosahedral phase in Al 65Cu 20Fe 15 are much reduced by annealing at temperatures above 600 °C and become comparable to the peak width of the f.c.c. Al standard sample for the sample annealed at 820 °C. In contrast, in the case of Al 65Cu 22Fe 13, the peaks are observed to distort remarkably by annealing around 600 °C, which is attributable to the generation of a rhombohedral-type approximant crystalline phase.