Formation Of Icosahedral Phase Research Articles

Effect of icosahedral phase formation on the stress corrosion cracking (SCC) behaviors of the as-cast Mg-8%Li (in wt.%) based alloys

Through exploring the stress corrosion cracking (SCC) behaviors of the as-cast Mg-8%Li and Mg-8%Li-6%Zn-1.2%Y alloys in a 0.1 M NaCl solution, it revealed that the SCC susceptibility index (ISCC) of the Mg-8%Li alloy was 47%, whilst the ISCC of the Mg-8%Li-6%Zn-1.2%Y alloy was 68%. Surface, cross-sectional and fractography observations indicated that for the Mg-8%Li alloy, the α-Mg/β-Li interfaces acted as the preferential crack initiation sites and propagation paths during the SCC process. With regard to the Mg-8%Li-6%Zn-1.2%Y alloy, the crack initiation sites included the I-phase and the interfaces of I-phase/β-Li and α-Mg/β-Li, and the preferential propagation paths were the I-phase/β-Li and α-Mg/β-Li interfaces. Moreover, the SCC of the two alloys was concerned with hydrogen embrittlement (HE) mechanism.

Peculiarities in structure formation and corrosion of cast quasicrystalline Al63Cu25Fe12 and Al63Co24Cu13 alloys in sodium chloride aqueous solution

In this work the structure and corrosion behavior of quasicrystalline cast Al63Cu25Fe12 and Al63Co24Cu13 alloys in 5-% sodium chloride solution (рН 6.9–7.1) were investigated. The alloys were cooled at 5 К/s. The structure of the samples was studied by methods of quantitative metallography, X-ray analysis, and scanning electron microscopy. Corrosion properties were determined by the potentiodynamic method. The made investigations confirm the formation of stable quasicrystalline icosahedral (y) and decagonal (D) phases in the structure of Al63Cu25Fe12 and Al63Co24Cu13 alloys correspondingly. In 5-% sodium chloride solution, the investigated alloys corrode under electrochemical mechanisms with oxygen depolarization. Compared with Al63Cu25Fe12 alloy, Al63Co24Cu13 alloy has a less negative value of free corrosion potential (–0.43 V and–0.66 V, respectively), and its electrochemical passivity region extends due to the inhibition of anodic processes. A corrosion current density, calculated from (E,lgi)-curve, for Al63Co24Cu13 alloy amounts to 0.18 mА/сm2 and for Al63Cu25Fe12 alloy – to 0.20 mА/сm2. The lower corrosion resistance of Al63Cu25Fe12 alloy may be explained by the presence of iron-containing phases in its structure. Based on obtained results, the Al63Co24Cu13 alloy was recommended as a coating material for rocket-and-space equipment working in a marine climate.

OXIDATION INFLUENCE ON THE QUASICRYSTALLINE PHASES OF ALLOYS Al62.5Cu25Fe12.5 AND Al65Ni15Co20

In this article we will cover a study of the formation of icosahedral and decagonal phases two quasicrystals Al 62.5 Cu 25 Fe 12.5 and Al 65 Ni 15 Co 20 oxidation of influence in this alloy. For this purpose research used the diffraction of X-ray, scanning electron microscopy/energy dispersive spectroscopy, differential scanning calorimetry and thermogravimetric analysis. The results displayed found aspects of morphological structural as well as the surface of the two compositions of quasicrystals, these were prepared and obtained in electric arc furnaces and induction and arc. Oxidation of Al 62.5 Cu 25 Fe 12.5 alloy, intermetallic phases presented with combinations of alloying elements and above 675°C it was observed that the crystalline phase is stable. In icosahedral phase oxidation of aluminum forms a dense layer on the passivating outer most surface of the quasicrystal which causes depletion in both copper and iron. In Al 65 Ni 15 Co 20 nominal composition of oxygen interaction occurs on the surface of symmetry 10 times plane perpendicular vector .The formation of a thin film of aluminum oxide having well-ordered hexagonal structure and with the opposition area decagonal phase with the lateral size of approximately 35A .

The partial substitution of Y with Gd on microstructures and mechanical properties of as-cast and as-extruded Mg-10Zn-6Y-0.5Zr alloy

Mg-10Zn-6Y-0.5Zr (wt%) alloy and Mg-10Zn-3Y-3Gd-0.5Zr (wt%) alloy with medium RE content have been fabricated by conventional casting and indirect extrusion. The as-extruded Mg-10Zn-6Y-0.5Zr (wt%) alloy shows a bimodal microstructure containing dynamic recrystallized (DRXed) grains with average grain size of 2μm and deformed regions. In addition to the fragmented W phase particle bands distributing along the extrusion direction, large amount of nano W phase and small amount of nano β2′ phase are precipitated in the matrix. While the as-extruded Mg-10Zn-3Y-3Gd-0.5Zr (wt%) alloy exhibits completely dynamic recrystallized microstructure with average grain size of 3μm. The broken W phase particles are distributed homogeneously in the matrix. Icosahedral quasicrystal phase layer is observed at the surface of some W phase particles in the as-extruded Mg-10Zn-3Y-3Gd-0.5Zr alloy. The as-extruded Mg-10Zn-6Y-0.5Zr (wt%) alloy exhibits yield strength of 341MPa, ultimate tensile strength of 368MPa and elongation to failure of 6.8%. While the as-extruded Mg-10Zn-3Y-3Gd-0.5Zr (wt%) alloy shows lower yield strength of 247MPa, ultimate tensile strength of 330MPa and higher elongation to failure of 19.8%. The higher strength in the as-extruded Mg-10Zn-6Y-0.5Zr (wt%) alloy is mainly attributed to higher area fraction of unDRXed regions with strong texture and the precipitation of nano W and β2′ phase. The fully recrystallized microstructure with weak texture and the formation of icosahedral quasicrystal phases with good coherent bond with α-Mg contribute to the good ductility of the as-extruded Mg-10Zn-3Y-3Gd-0.5Zr alloy.

Al62Cu25Fe12 and Quasicrystalline Phases and Their Influence on Oxidation

In this article, we will cover a study of the formation of icosahedral and decagonal phases two quasicrystals Al 62 Cu 25 Fe 12 and Al 65 Ni 15 Co 20 oxidation of influence in this alloy. For this purpose, research used the diffraction of X-ray, scanning electron microscopy/energy dispersive spectroscopy, differential scanning calorimetry and thermogravimetric analysis. The results displayed found aspects of morphological structural as well as the surface of the two compositions of quasicrystals, these were prepared and obtained in electric arc furnaces and induction and arc. Oxidation of Al 62 Cu 25 Fe 12 alloy, intermetallic phases presented with combinations of alloying elements and above 675 °C it was observed that the crystalline phase is stable. In icosahedral phase oxidation of aluminum forms a dense layer on the passivating outer most surface of the quasicrystal which causes depletion in both copper and iron. In Al 65 Ni 15 Co 20 nominal composition of oxygen interaction occurs on the surface of symmetry 10 times plane perpendicular vector. The formation of a thin film of aluminum oxide having well-ordered hexagonal structure and with the opposition area decagonal phase with the lateral size of approximately 35A. DOI: http://dx.doi.org/10.17807/orbital.v9i1.872

Icosahedral Quasicrystal Layer Observed on λ Phase in Al-Cu-Fe Alloy

Icosahedral quasicrystals display irregular shape if it is embedded in bulk material. If it has free surface, it has well-defined facets, reflecting its unique 5-, 3-, and 2-fold rotational symmetries. In this study, an Al-Cu-Fe alloy with nominal composition of Al65Cu20Fe15 was prepared by arc melting and the microstructure was studied by using Scanning Electron Microscope, Energy Dispersive X-ray Spectroscopy, and Electron Back Scattering Diffraction (EBSD). On the surface of λ crystalline phase, an extra layer is found. EBSD from this layer revealed 5-, 3-, and 2-fold rotational symmetries, demonstrating the icosahedral quasicrystalline structure. Further, it has been found that the icosahedral quasicrystalline extra layer and the λ substrate have orientation relationship revealed by the coincidences of Kikuchi bands and poles on the EBSD patterns. This report is important to future studies regarding the formation of icosahedral quasicrystalline phase and thin film preparation related to icosahedral quasicrystalline phase.

Phase transitions in zirconium–palladium alloys

A statistical theory of zirconium–palladium alloys has been developed with allowance for the formation of amorphous, icosahedral, and crystalline phases. The free energies are calculated. The phase diagram near the stoichiometric composition of the Zr2Pd alloy has been constructed. The structural phase transitions are studied. The possibility of formation of various spin orders on the sites of zirconium and palladium atoms is found. The temperature dependence of the heat capacity of the alloy is constructed, and the heat capacity jumps at the Curie points are estimated. The theoretical results are compared to experimental data.

Icosahedral phase formation in an Al-Cu-Fe quasicrystal

The formation of the icosahedral phase in an Al70Cu20Fe10 quasicrystal and the structural transition from the crystal praphase to the quasicrystalline phase are simulated and studied by means of X-ray absorption near edge structure spectroscopy. Al-Cu-Fe samples are fabricated by means of powder metallurgy. The ω phase of the praphase crystal forms following exposure at 500°C for 20 min; the quasicrystalline phase forms after exposure at 800°C for 2 h. The model of the structural transition from the crystal to quasicrystal is a diffusion displacement of aluminum atoms around iron atoms with the formation of an icosahedron with the iron atom at the center. The immediate environment of copper atoms consists mainly of aluminum and copper atoms, and no iron atoms are observed.

The Effect of Heat Treatment on Morphology and Phase Composition of Grain Boundary Phases in Mg-Zn-Y-Nd-Zr

The Mg-Zn-Y system attracts interest of researchers due to the formation of quasicrystalline particles which are believed to improve mechanical properties of Mg-based alloys. In the Mg-Zn-Y-Nd-Zr alloy (WE43 modified by addition of Zn) studied here the formation of icosahedral phase (I-phase) with quasicrystalline structure competes with cubic W-phase. Grain boundary phases in Mg-Zn-Y-Nd-Zr alloys subjected to various heat treatments were characterized. It was found that the portion of the I-phase in Mg-Zn-Y-Nd-Zr alloy can be increased by a suitable heat treatment. Moreover, the solidification process is influenced by the cooling rate resulting in a striking difference in the morphology of grain boundary phases.

Formation of Icosahedral Phase in Bulk Glass Forming Ti-Zr-Be-Cu-Ni Alloy

Formation of an icosahedral phase in the bulk glass forming alloy during crystallization from amorphous phase and solidification from melt is investigated. The icosahedral phase with a size of 10 to 15 nm forms as a thermodynamically stable phase at intermediate temperature during the transformation from amorphous to crystalline phases such as Laves and -(Ti-Zr) phases, indicating that the existence of the icosahedral cluster in the undercooled liquid. On the other hand, the icosahedral phase forms as a primary solidification phase even though the Laves phase is stable at high temperature, which is can be explained based on the high nucleation rate of icosahedral phase relative to that of competing crystalline Laves phase due to lower interfacial energy between icosahedral and liquid phases.

Synthesis of quasicrystalline film of Al–Ga–Pd–Mn alloy

Present work describes the synthesis of Al (Ga)–Pd–Mn quasicrystalline films via flash evaporation followed by annealing. The icosahedral quasicrystal of Al65Ga5Pd17Mn13 alloy has been used as a precursor material. An amorphous phase with very fine icosahedral grain was found in the as-deposited films of ~1.5μm thickness. After annealing at 300°C for 120h, the film completely transformed into an icosahedral phase showing micron size islands, these are precipitated on the surface of film. The X-ray diffraction and transmission electron microscopy confirms the formation of icosahedral phase. The energy dispersive X-ray analysis reveals that the final stoichiometry is maintained in the film. The formation of icosahedral Al–Ga–Pd–Mn film provides opportunities to investigate the useful properties of quasicrystals including the surface characteristics, which may well enhance the scientific interest in applied aspect of quasicrystals.

Formation of Al67Cu23Fe10 quasicrystals by microwave heating

AbstractThe present work was carried out to compare the formation of single icosahedral phase during conventional heating and microwave processing. Al67Cu23Fe10 alloy powder was synthesized from high purity (99.9%) powder elements by mechanical alloying. Differential scanning calorimetry combined with in-situ synchrotron energy dispersive X-ray diffraction was used to identify the main solid state reactions and the phase evolution of the powders. Inductive microwave processing in the magnetic field anti-node was performed to obtain the quasicrystalline phase in only a few seconds. Due to the rapid cooling of the sample it was possible to stabilize the icosahedral phase against its competing quasicrystalline approximants. Laboratory X-ray diffraction analysis was used to characterise the atomic structure of the specimen and scanning electron microscopy was used to characterise the microstructure after the microwave processing.

Structural, tribological and resistivity studies of Ga substituted (Al71−xGax)Pd21Mn8 icosahedral and other intermetallic phases

We report the formation of icosahedral phase in Al71−xGaxPd21Mn8 (x=0, 4, 6, 8, and 10at.%) by adopting powder metallurgical route. Such a variation in Ga substitution in the alloy composition gave rise to a nearly single phase face centred icosahedral phase for x=6. All other Ga substituted compositions displayed formation of intermetallic phases in addition to the icosahedral quasicrystals. The 6d hypercubic lattice parameter for the Ga substituted icosahedral phase is higher than that of Al71Pd21Mn8 composition. The change in the lattice parameter is non-monotonic. Such a change in lattice parameter has been explained based on the mixed valence states of Ga on Al site. The electrical resistivity of x=6 sample is comparable to those of earlier studies pertaining to flux grown icosahedral composition of Al(Ga)PdMn alloy. Further, x=6 sample has displayed highest value of static friction coefficient. This has been understood in terms of the shift of Fermi level from the pseudo-gap location owing to mixed valence states of Ga.

Structural evolutions of the mechanically alloyed Al70Cu20Fe10 powders

Elemental mixtures of Al, Cu, Fe powders with the nominal composition of Al70Cu20Fe10 were mechanically alloyed in a planetary ball mill for 80 h. Subsequent annealing of the as-milled powders were performed at 600–800°C temperature range for 4 h. Structural characteristics of the mechanically alloyed Al70Cu20Fe10 powders with the milling time and the heat treatment were investigated by X-ray diffraction (XRD), differential scanning calorimeter (DSC) and differential thermal analysis (DTA). Mechanical alloying of the Al70Cu20Fe10 did not result in the formation of icosahedral quasicrystalline phase (i-phase) and a long time milling resulted in the formation of β-Al(Cu,Fe) solid solution phase (β-phase). The i-phase was observed only for short-time milled powders after heat treatment above 600°C. The β-phase was one of the major phases in the Al70Cu20Fe10 alloy. The w-Al7Cu2Fe1 phase (w-phase) was obtained only after heat treatment of the short-time milled and unmilled samples. The present investigation indicated that a suitable technique to obtain a large amount of quasicrystalline powders is to use a combination of short-time milling and subsequent annealing.

Formation of quasicrystalline phase in Al70−x Ga x Pd17Mn13 alloys

In the present investigation, the formation and stability of icosahedral phase in Al70− x Ga x Pd17Mn13 alloys has been explored using X-ray diffraction, scanning, transmission electron microscopy and energy dispersive X-ray analysis. Cast alloys and melt-spun ribbons with x = 2.5, 5, 7.5, 10, 12.5, 15 and 20 have been investigated. In both cases, the alloys up to 5 at% Ga exhibit the formation of pure icosahedral phase. However, for x ≥5 at% Ga content, the cast alloy exhibits the formation of multiphase material, consisting of an icosahedral phase along with AlPd-type B2 and ξ′ crystalline (orthorhombic structure with unit cell a = 23.5 Å, b = 16.6 Å and c = 12.4 Å) phases. In the case of the melt spun ribbon for x = 5 at% Ga, only an icosahedral phase has been found, but for 15 > x > 5 at% Ga, an icosahedral phase is the majority phase with AlPd-type B2 phase being the minority component. For x = 15 at% Ga, a Al3Pd2-type hexagonal phase together with a small amount of quasicrystalline phase is formed. However, for x = 20, only a hexagonal Al3Pd2 phase results.

Formation of nano quasicrystalline and crystalline phases by mechanical alloying

In the present work, the formation of nano quasicrystalline icosahedral phase in Al 86Cr 14, Al 84Fe 16 and Al 62.5Cu 25Fe 12.5 alloys has been investigated by mechanical alloying. Mixtures of quasicrystalline and related crystalline phases have been observed under various milling conditions. The X-ray diffraction, differential thermal analysis and electrical resistivity techniques have been used for characterization and physical property measurements. The particle size was calculated by X-ray profile using Williamson–Hall plot method and it was found to be 25–50 nm size.

Icosahedral quasicrystalline Fe10Cu20Al70 by electrothermal explosion: Structural and magnetic properties

A systematic study on the formation and stability of the icosahedral quasicrystalline (IQC) Al70Cu20Fe10 alloy has been carried out using electrothermal explosion (ETE) reaction as a preparation method. The influence of added elements on the formation of icosahedral phase has been investigated by Mossbauer spectroscopy, XRD, and field emission scanning electron microscopy (FESEM). Combined ball milling and ETE reaction (BM-ETE) of Al63Cu25Fe12 composition resulted in the formation of the ICQ Fe10Cu20Al70 phase with a face-centered structure. In all cases, an Al(Cu, Fe) solid solution and β-Fe3Al have been detected along with the ICQ phase. The icosahedral phase appears to form in a peritectoid-type reaction between Fe3Al and Al2Cu. A mechanism for formation of the IQC phase in the Al-Cu-Fe system was suggested. VSM measurements showed the ferromagnetic behavior of the alloy. The Mossbauer measurements in transmission and XRD data showed that the reaction product contained 65% Fe10Cu20Al70 with an admixture of β-Fe3Al and Al2Cu.

Spray Forming of Bulk Ultrafine-Grained Al-Fe-Cr-Ti

An Al-2.7Fe-1.9Cr-1.8Ti alloy has been spray formed in bulk and the microstructure and properties compared with those of similar alloys produced by casting, powder aomization (PA), and mechanical alloying (MA) routes. In PA and MA routes, a nanoscale metastable icosahedral phase is usually formed and is known to confer high tensile strength. Unlike previous studies of the spray forming of similar Al-based metastable phase containing alloys that were restricted to small billets with high porosity, standard spray forming conditions were used here to produce a ~98 pct dense 19-kg billet that was hot isostatically pressed (“HIPed”), forged, and/or extruded. The microstructure has been investigated at all stages of processing using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and synchrotron X-ray diffraction (XRD) at the Diamond Light Source. Consistent with the relatively low cooling rate in spray forming under standard conditions, the microstructure showed no compelling evidence for the formation of metastable icosahedral phases. Nonetheless, after downstream processing, the spray-formed mechanical properties as a function of temperature were very similar to both PA rapid solidification (RS) materials and those made by MA. These aspects have been rationalized in terms of the typical phases, defects, and residual strains produced in each process route.

Quasicrystalline phase formation during heat treatment in mechanically alloyed Al65Cu20Fe15 alloy

Quasicrystalline phase formation during heat treatment in the mechanically alloyed Al65Cu20Fe15 powders was studied by X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The mechanical alloying was performed at speed of 300 rev min–1 for times up to 70 h. It was found that mechanical alloying of the Al65Cu20Fe15 powders did not result in the quasicrystalline (QC) icosahedral phase (i-phase) formation. The long time milling resulted in the formation of a cubic Al (Cu,Fe) solid solution phase (β-phase). The cubic Al (Cu,Fe) solid solution identified as β-phase was observed to be present as one of the major phases in the Al65Cu20Fe15 alloy. The formation of the quasicrystalline icosahedral phase (i-phase) was only observed for short time milled powders after additional annealing at temperature above 500°C. The present investigation also showed that a tetragonal Al2Cu phase (θ-phase) forms with short time milling. The tetragonal Al7Cu2Fe1 phase (w-phase) was observed after heat treatment of the short time milled and unmilled powders. The present investigation indicated that an effective process to prepare the quasicrystalline materials was using a combination of short time milling and subsequent annealing.

Precipitation of Icosahedral Phase in Zr-Ni-Nb-Cu-Al Metallic Glasses

The crystallization behaviors of Zr-Ni-Nb-Cu-Al metallic glasses are greatly influenced by the substitution of Ni for Zr. For the glassy Zr71� xNixNb3Cu16Al10 (x ¼ 7, 9, 11 at%) alloys, the onset temperature of crystallization Tx increases from 717 to 748 K with increasing Ni content from 7 at% to 11 at%. The primary crystallization reaction of the studied glassy alloys corresponds to the formation of icosahedral phase from amorphous matrix. The precipitation of quasicrystalline phase with a large volume fraction in the Zr64Ni7Nb3Cu16Al10 alloy indicates that the decrease of Ni content promotes the formation of icosahedral phase. Johnson-Mehl-Avrami analysis of isothermal transformation data suggests that the icosahedral phase precipitation proceeds by a diffusion-controlled growth mode with nearly constant nucleation rate. Furthermore, the Zr71� xNixNb3Cu16Al10 (x ¼ 7, 9, 11 at%) alloys possess high glass-forming ability, and the critical diameter for glass formation reaching 6 mm, 9 mm and 9 mm by copper-mould casting method, respectively. [doi:10.2320/matertrans.M2009072]