Formation Of Icosahedral Phase Research Articles

Formation and stability of icosahedral phase in Al65Ga5Pd17Mn13 alloy

The formation and characterization are described of a quaternary (pseudo-ternary) icosahedral quasicrystal in Al65Ga5Pd17Mn13 alloy. X-ray diffraction and transmission electron microscopy observations confirmed the formation of icosahedral, B2 type and ξ′ crystalline (orthorhombic structure with unit cell a = 23.5 Å, b = 16.6 Å and c = 12.4 Å) phases in as-cast alloy. The icosahedral phase is formed after annealing at 800°C for 60 h. This is the first report of the formation of an icosahedral phase in an Al–Ga–Pd–Mn quaternary alloy by present technique. The energy dispersive X-ray analysis investigations suggest the presence of Ga (∼5 at.%) in the alloy. Icosahedral Al–Ga–Pd–Mn provides a new opportunity to investigate various characteristics, including surface characteristics. Attempts are made to discuss the micromechanisms for the formation of the quasicrystalline phase in Al–Ga–Pd–Mn alloys.

Effects of Nb on the formation of icosahedral quasicrystalline phase in Ti-rich Ti–Zr–Ni–Cu–Be glassy forming alloys

As-cast (Ti40Zr25Ni8Cu9Be18)100−xNbx (0⩽x⩽5) (Ф3) glassy forming alloys were investigated in order to clarify the role of Nb on the formation of icosahedral quasicrystalline phase (I-phase) in Ti-rich Ti–Zr–Ni–Cu–Be glassy system. It is found that an I-phase is formed in Ti–Zr–Ni–Cu–Be glassy alloy by addition of Nb element; however, the nucleation rate of I-phase increases, whereas the grain growth rate decreases with increasing Nb content. Moreover, with increasing Nb content, the thermal stability against crystallization increases, while the temperature range of stability of the I-phase decreases.

Quasicrystallization of (Zr<sub>65</sub>Al<sub>7.5</sub>Cu<sub>27.5</sub>)<sub>95</sub>Ti<sub>5</sub> Glassy Alloy

Phase transformation in the melt-spun (Zr65Al7.5Cu27.5)95Ti5 glassy alloy was investigated by a combined use of X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was found that the crystallization mode of the Zr65Al7.5Cu27.5 glassy alloy was altered by the addition of Ti, and nanometer scaled icosahedral phase (I-phase) formation was observed in the primary crystallization stage of the (Zr65Al7.5Cu27.5)95Ti5 quaternary glass, which transformed to the stable Zr2Cu-type phase upon further annealing. The crystallization behavior of (Zr65Al7.5Cu27.5)95Ti5 glass was further discussed in terms of the activation energy as compared to that of Zr65Al7.5Cu27.5 alloy.

Formation of icosahedral quasicrystals in (Ag,Au)-based ternary systems

We investigated the phase constitution of rapidly solidified (Ag,Au)-based ternary alloys at various compositions of (M,X)85.7RE14.3 (M=Ag, Au, X=Al,Ga,In, RE=Er,Lu). It is found that an icosahedral phase is formed at an average electrons per atom ratio close to 2.0 as well as at the composition ratio of (M,X)6RE, suggesting that these factors have crucial roles in the stability of the icosahedral phases. It is also found that the trend of the icosahedral phase formation in the Au-based alloys is quite different from that in the Ag-based alloys.

Local structure in quasicrystal-forming Zr-based metallic glasses correlated with a stability of the supercooled liquid state

Nanoquasicrystallization and the local structure of several Zr-based metallic glasses were investigated in correlation with a stability of the supercooled liquid state. Single icosahedral quasicrystalline phase (I-phase) formation is confirmed in the Zr70Al10Ni20 ternary and Zr70Cu30 binary metallic glasses with a high stability of the supercooled liquid state by the addition of a small amount of a noble metal such as 0.5–1at.% Pd or Au. Since the local structure does not change by nanoquasicrystallization as well as by the addition of 1at.% Pd in the Zr70Al10Ni20 ternary metallic glass on the basis of an extended X-ray absorption fine structure (EXAFS) study, we propose an icosahedral local structure in the quasicrystal-forming metallic glasses. No I-phase formation is observed in the Zr70Ni30 binary alloy by the addition of 1at.% Pd or Au, in which the stability of the supercooled liquid state is lower than those in the Zr70Cu30, Zr65Al7.5Cu27.5 and Zr70Al10Ni20 alloys. Since the tetragonal Zr2Ni-like local structure is identified in the Zr70Ni30 amorphous alloy, it is concluded that a strong correlation exists between the stability of the supercooled liquid state and the icosahedral local structure in the glassy state in Zr-based metallic glasses.

Formation of Icosahedral Phase in an Al93Fe3Cr2Ti2 Bulk Alloy.

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Formation of icosahedral phase in Zr 65Al 7.5Ni 10Cu 12.5Ag 5 metallic glass

Two Zr 65Al 7.5Ni 10Cu 12.5Ag 5 amorphous specimens were prepared from the mother ingots with different solidification microstructures. Observation by high-resolution electron microscopy (HREM) revealed that there existed numerous nano-sized short-range order regions (SRORs) in the as-cast specimens although their amorphous nature had been verified by X-ray diffraction (XRD). The average size of the SRORs decreased significantly as the solidification microstructure of the mother ingot became finer. The transformation from amorphous to icosahedral phase (I-phase) in the Zr 65Al 7.5Ni 10Cu 12.5Ag 5 metallic glass was systematically investigated by differential scanning calorimetry (DSC) under isochronal and isothermal conditions. The results indicated that the transformation kinetics slightly changed, and that the formation of I-phase was triggered by the growth of those SRORs as the pre-existing embryos. It was strongly suggested that the atomic configuration of the unit structure in the metallic glasses showed an icosahedron-like one.

Formation of glassy and icosahedral phases in as-cast (Zr9Ni4)75(Al1−Ti )25 alloys

The icosahedral atomic cluster has been recognized as the building block of icosahedral quasicrystals (I-phase) and many bulk metallic glasses (BMGs). With this in mind, a series of (Zr 9 Ni 4 ) 75 (Al 1− x Ti x ) 25 alloys were designed, and ribbons and rods of 3 mm in diameter were prepared. Structural analysis of these bulk samples revealed a pronounced alloying effect of Al and Ti on the formation of BMG and I-phase. BMGs were formed at low Ti compositions ( x ⩽ 0.20). As the Ti concentration increased, an I-phase began to form in the glassy matrix, and a nearly monolithic I-phase alloy was obtained in the Al-free alloy. The alloying effect is discussed by considering the different roles of Al and Ti in the stabilization of local icosahedral atomic structures.

Effect of Mn on the formation of Mg-based spherical icosahedral quasicrystal phase

Three-dimensional spherical icosahedral quasicrystalline phase (I-phase) has been obtained by introducing Mn into Mg–Zn–Y alloy under normal casting conditions. Due to the addition of Mn, Mg 45Zn 47Y 5Mn 3 primary I-phase,which is thermodynamically stable and homogeneously distributed, has been generated instead of decahedral quasicrystalline phase during the solidification process; morphology of primary I-phase in the solidification microstructure changed from petal-like one (60–80 μm) to spherical one (≤20 μm). When the Mn content reached 2 wt.%, volume fraction of Mg 45Zn 47Y 5Mn 3 spherical I-phase in as-cast Mg–Zn–Y–Mn master alloy was about 30–40%. Meanwhile, primary I-phase with highest spheroidization ratio and highest circular degree can be obtained; brittleness and hardness of as-cast Mg–Zn–Y–Mn master alloy reduced while density of as-cast Mg–Zn–Y–Mn master alloy increased. By analyzing spheroidization mechanism of Mg-based I-phase, spheroidization theory of Mg-based quasicrystals could be enriched a lot and a new way will be provided for the industrialization of Mg-based quasicrystals.

Formation condition of the icosahedral phase in rapidly quenched Ag–In–RE (rare earth) alloys

Formation of icosahedral (i) phases in rapidly quenched Ag–In–RE (rare earth) (RE = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) ternary alloys is investigated. It is found that the relative magnitude of the quasilattice constant is well explained by the atomic radius of the RE elements for Ag–In–RE alloys with trivalent RE elements, and the formation of the i phase is governed by the atomic radius of the RE element. Moreover, the i phase is found to be formed in a wide composition range for the Ag/In atomic ratio but in a very narrow range for the RE concentration, which suggests that the RE concentration plays a crucial role in the formation, i.e. the stability, of the i phase rather than the average electrons-per-atom ratio. Furthermore, it is suggested that the structural change caused by the variation of the Ag/In concentration ratio in the i phase cannot be understood as a consequence of simple mutual replacement between Ag and In.

Formation and magnetic properties of mechanically alloyed Al 65Cu 20Fe 15 quasicrystal

The formation of icosahedral phase by mechanical alloying of crystalline elemental powder of Al, Cu and Fe has been investigated. The effect of milling time on the formation of icosahedral phase of nominal composition of Al 65Cu 20Fe 15 has been studied using the X-ray diffraction technique. Further studies have been carried out by scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDAX), particle size, magnetisation and ferromagnetic resonance studies. All these studies indicate that the icosahedral alloy shows soft ferromagnetic behaviour.

Formation of amorphous and icosahedral phases in Ti–Zr–Hf–LTM (LTM=Ni, Pd or Pt) alloys

A nanoscale icosahedral (I) phase was formed in the melt-spun $Ti_{40}Zr_{20}Hf_{20}(LTM)_{20}$ (LTM = Ni, Pd, or Pt) alloys. However, in Pd-bearing alloy it is presumed to be a distorted I-phase or a high-order approximation. I phase, $Hf_{2}Pt$ and \beta-(Ti, Zr, Hf, Ni) solid solution phases are formed in the arc-melted $Ti_{40}Zr_{20}Hf_{20}Pt_{20}$ in got.

Analysis of the crystallization of Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass using electrical resistivity measurement

Abstract The crystallization behavior of the Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 bulk metallic glass was investigated using the isothermal electrical resistivity measurements at 390 °C in the super-cooled liquid region. The crystallization process is completed through two sequential phase transformations. The first phase transformation was the formation of icosahedral phases, which is followed by the formation of intermetallic compounds.

Crystallographic orientations and interfaces of icosahedral quasicrystalline phase growing on cubic W phase in Mg–Zn–Y alloys

Formation of icosahedral quasicrystalline phase on the W cubic phase in α-Mg matrix has been studied in a Mg 97Zn 2.5Y 0.5 alloy. The orientation relationship of the W phase with the matrix is determined to be [0 0 1] W∥ [ 1 ¯ 0 0 ] Mg , (1 1 0) W∥(0 0 1) Mg, equivalent to Burgers orientation relationship. The icosahedral phase forms curved interfaces with the W phase, but planar interfaces on five-fold and two-fold planes with the matrix. The icosahedral phase shows its unique orientation relationship to the W phase, which results in two orientation relationships WOR1 and WOR2 with the matrix. WOR1 is a slight deviation of the orientation relation, OR1, of icosahedral phase precipitates in the matrix. In WOR2, one of the five-fold axes occur along the hexagonal axis. The resulting morphology and the interface structures are described here. It is shown that WOR1 and WOR2 are related to each other by twinning of the icosahedral phase.

X-ray absorption studies of Al–Mn system

X-ray K-absorption studies have been carried out on Al–Mn system comprising Al90Mn10, Al88Mn12, Al86Mn14, Al84Mn16, Al82Mn18 and Al81Mn19 samples. These samples have been characterized by x-ray diffraction (XRD) studies which support the formation of icosahedral phase. The x-ray K-absorption discontinuities in case of the Al–Mn system have been found to be shifted towards the high energy side with respect to the K-absorption edge of pure metal Mn, which indicates the behaviour of manganese atom as the cation. The x-ray absorption chemical shifts suggest a covalent bonding. The studies also indicate that with the increase in Mn concentration chemical shifts also increase.We have attempted to correlate the values of the electron to atom ratio (e/a) with the chemical shifts (ΔE). The nature of relationship between e/a and ΔE is quite similar to that between ΔE and Mn concentration. Further, we have estimated the density of states at the Fermi level D(EF) for these samples and plotted them against chemical shift values. Extrapolation from this graph between D(EF) and ΔE indicates a D(EF) value for Al metal which is quite near to the actual value for Al metal. These studies establish the existence of a pseudo gap in the case of presently investigated Al–Mn samples. The present studies also support our assumption of assigning the chemical shift values to the Fermi level in the case of Al–Mn system under present investigation.

Influences of hydrostatic pressure during casting and Pd contenton as-cast phase in Zr-Al-Ni-Cu-Pd bulk alloys

The influences of sample diameter (D), Pd content (x), and hydrostatic pressure (P) in a chamber during casting on the structure of as cast Zr65Al7.5Ni10Cu17.5−xPdx(x=10,17.5at.%) bulk alloys were investigated. Zr65Al7.5Ni10Cu7.5Pd10 and Zr65Al7.5Ni10Pd17.5 alloys (D=3mm) cast in a vacuum chamber (P∼4.0×10−3Pa) were mainly of the tetragonal-Zr2Ni equilibrium phase and nanosize icosahedral primary phase, respectively, while the same alloys cast in inert argon gas at atmospheric pressure (P∼0.1MPa) were of the single glassy phase. Due to the higher cooling rate obtained by decreasing the sample diameter (D=2mm) even in the vacuum chamber, the Zr65Al7.5Ni10Pd17.5 alloy was still of the icosahedral phase, while the Zr65Al7.5Ni10Cu7.5Pd10 alloy froze into a single glassy phase. These results indicate that the temperature- and time- transformation curves for the icosahedral and subsequent equilibrium phase formations in the alloy system shifts to a shorter time side with decreasing P, and the pressure sensitivity of the icosahedral phase formation increases with x.

Formation of icosahedral–hexagonal H phase nano-composites in Mg–Zn–Y alloys

Abstract A reaction is identified in formation of icosahedral phase in dilute Mg–Zn–Y alloys, involving a hexagonal phase H-Zn3MgY (a = 0.918, c = 0.95 nm). The crystallographic relationship and the interface between the H and the icosahedral phase are described. Formation of an icosahedral–H phase nano-composite in a Mg93Zn6Y alloy is shown.

Nanoscale Icosahedral Phase Formation in Cu-Based Bulk Glass Formers

Nanoscale Icosahedral Phase Formation in Cu-Based Bulk Glass Formers

Synthesis of icosahedral phase during cold working and annealing of elemental multilayers in the Al–Cu–Fe ternary system

The icosahedral phase formation and associated structural evolutions in the Al–Cu–Fe ternary system have been examined through the repeated cold rolling and folding (R&F) process of elemental foils with a target composition of Al 62.5 Cu 25 Fe 12.5 (at.%). Upon early increments of the rolling and folding cycles, the elemental Fe foil was dispersed through the refined Al and Cu layers. When the rolling and folding step reached 40 cycles the Al 2 Cu phase has been observed, however, no ternary phases were identified even upon additional R&F cycles. The stable icosahedral phase (i-phase) could be obtained after isothermal annealing of the R&F processed specimens at 650 and 750 °C. The phase evolution during repeated cold R&F process of the Al–Cu–Fe system and upon heat-treatments are discussed in terms of the formation and dissolution of the Al 2 Cu and Al 7 Cu 2 Fe phases.

Devitrification of Zr-Ni-Al-Cu-Ti(Nb,Ta) glassy alloys

The devitrification behavior and phase formation in Zr 65–55Ni 10Al 7.5Cu 7.5Ti 5–10(Nb,Ta) 5–10 metallic glass have been studied by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. It has been found that mutual addition of Ti and Ta/Nb benefits the formation of nanoscale icosahedral phase in the glassy phase on heating and the oxygen content in the alloy makes significant influence on the devitrification behavior of these alloys. At the same time in Nb-bearing alloys and alloys containing 5 at.% Ta icosahedral phase was found to coexist with crystalline one.