Formation Of Icosahedral Phase Research Articles

Cast Al-V-Fe Alloys with Nanoscale Particles

The sheets or cylinders of Al-V-Fe alloys were prepared by using a water-cooled copper mould. The mixed structure of nanoscale icosahedral particles embedded on the Al matrix was obtained. The precipitation of icosahedral phase is sensitive to the variation of composition. The volume fraction of icosahedral phase increases with increasing the content of V element and Fe element in the composition range of V2-8% and Fe2-6%, respectively. The cooling rate affects significantly the formation of icosahedral phase. With increasing the thickness from ribbon to 1 mm sheet, the particles size of icosahedral phase in Al-V-Fe alloys increases from 20 nm to 50-60 nm. Further increasing the thickness of sheets, Al 10 V intermetallics may precipitate. Both tensile and compressive yield strength at room temperature were measured. The yield strength of Al-V-Fe alloys is up to 800 MPa. The yield strength of Al-V-Fe alloys improves with increasing the content of V element and Fe element due to the increase of nano-particle of I-phase.

On icosahedral phase formation in mechanically alloyed Al70Cu20Fe10

The formation of icosahedral phase by mechanical alloying of crystalline elemental powders has been investigated. The effect of milling energy on icosahedral phase formation in Al70Cu20Fe10 has been studied using X-ray diffraction and energy dispersive X-ray (EDX) microanalysis. In order to study the alloying process during milling, different starting compositions of Al, Cu and Fe elemental blends have been used. The present study indicates that even though the intermediate phases formed during milling are the same, the icosahedral phase formation is sensitive to the way in which the nominal composition is reached.

Icosahedral phase formation in Ag-substituted Ti–Zr–Ni rapidly-quenched alloys

The formation of icosahedral phases in Ag-substituted Ti–Zr–Ni alloys was investigated by in situ annealing diffraction experiments using synchrotron radiation. The as-quenched (Ti1−xAgx)–Zr–Ni alloys (x=2, 5, 7 and 10at.%) are amorphous and/or in a mixed nanostructured state. Upon heating, the Ti–Zr–Ni–Ag alloys transform to either a crystalline or a quasicrystalline state, depending on the Ag content. Differential scanning calorimetry (DSC) studies were performed to identify the nature and sequence of the phase transitions.

Mössbauer spectroscopy study of Ti–Zr–Ni–Fe icosahedral and approximant phases

Quaternary Ti53Zr21Ni13Fe13 alloys were prepared by melt-spinning using peripheral wheel speeds ω=19.5, 24.2 and 30m/s. The icosahedral (i-) phase formation was studied using high resolution synchrotron radiation powder diffraction. An amorphous phase component is present irrespective of the rapid solidification conditions, its volume fraction decreasing towards lower quenching speed values. The amorphous phase coexists with a b.c.c. rational approximant phase (W-phase). For ω below 20m/s the formation of an i-phase was also noticed. Transmission 57Fe Mössbauer spectroscopy measurements indicate that the number of asymmetrical Fe positions decreases with decreasing ω in agreement with the improved structural perfection of the W-phase and with the formation of an additional i-phase.

Precipitation of icosahedral quasicrystalline phase in Hf65Al7.5Ni10Cu12.5Pd5 metallic glass

An icosahedral quasicrystalline phase was found in a Hf65Al7.5Ni10Cu12.5Pd5 metallic glass annealed in the supercooled liquid region. Upon annealing at high temperature, the quasicrystalline phase was found to decompose to regular crystalline phases, indicating that it is a metastable phase. The present alloy was compared with the previously reported Zr- and Ti-based alloys with the formation of icosahedral quasicrystalline phases. Hf, Zr, and Ti belong to the same 4A column in the element periodical table. Based on the above comparison, conditions in terms of atomic radius and alloy composition which favor the formation of icosahedral quasicrystalline phase in 4A element based alloys, were suggested.

Nanoscale icosahedral quasicrystalline phase formation in a rapidly solidified Zr80Pt20 binary alloy

It is found that an icosahedral quasicrystalline phase is directly formed in a Zr80Pt20 binary alloy during rapid solidification from the melt. The size of the icosahedral particles lies in the diameter range below 10 nm, and the particles are distributed homogeneously. The formation of the nanoscale icosahedral phase indicates that the icosahedral short-range order exists in the melted state of Zr–Pt binary alloy. The strong chemical affinity between Zr and Pt contributes to the restraint of the long-range rearrangement of constitutional elements to form a stable crystalline phase, which is the important factor of the stabilization of an icosahedral phase.

Formation of icosahedral quasicrystalline phase in Zr70Ni10M20(M=Pd, Au, Pt) ternary metallic glasses

An icosahedral quasicrystalline phase was confirmed as a primary precipitation phase in melt-spun Zr70Ni10M20 (M=Pd, Au and Pt) ternary metallic glasses with two-stage crystallization process. The onset temperature of the transformation from amorphous to icosahedral phase are 687 K for Pd, 754 K for Au and 783 K for Pt at a heating rate of 0.67 K s−1. The size of the icosahedral particles is in the range of 5–20 nm. The second crystallization reaction results in the formation of Zr2Ni+Zr2Pd and Zr2Ni+ZrPt phases in the Zr–Ni–Pd and Zr–Ni–Pt alloys, respectively, and Zr3Au phase in the Zr–Ni–Au alloy through a single exothermic reaction. The formation of nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the glassy state.

Nano-icosahedral quasicrystalline phase formation from a supercooled liquid state in Zr–Fe–Ni ternary metallic glass

It was found that an icosahedral quasicrystalline phase was formed as a primary precipitation phase from the supercooled liquid region of the melt-spun Zr70Fe20Ni10 ternary metallic glass. The precipitation of an icosahedral phase takes place at 673 K at the heating rate of 0.67 K s−1. The precipitated icosahedral particle has a nearly spherical morphology and a fine grain size in the diameter range of 5–10 nm. The second crystallization reaction proceeds through a broad exothermic peak and results in the formation of Zr2Ni and Zr2Fe phases. The formation of nanoscale icosahedral phase by the addition of Fe as well as noble metals such as Pd, Au, and Pt indicates the possibility of existence of an icosahedral short-range order in the various Zr-based metallic glasses.

Icosahedral phase formation by the primary crystallization of a Zr-Cu-Pd metallic glass

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan(Received January 24, 2000)(Accepted March 7, 2000)Keywords: Metallic glasses; Quasicrystal; Melt spinning; Phase transformations; TEMIntroductionZr-based bulk metallic glasses, which exhibit a large supercooled liquid regime,DT

Icosahedral quasicrystal formation in Ti[sbnd]Zr[sbnd]based alloys and a new classification technique

Until recently, icosahedral phase (i-phase) formation was studied primarily in Al-transition metal alloys. The Al-based i-phases generally fall into one of two classes: those believed to be based on the Pauling triacontahedron, fundamental to the Bergman 1/1 phase, and those based on the double-shell Mackay icosahedra found in the 1/1α-(Al-Mn-Si) phase. Notable Bergman-type quasicrystals include i-(Al Li-Cu) and i-(Al-Mg-Zn); i-(Al-Mn-Si) forms the best known Mackay-type i-phase. The large number of Ti-based i-phases now known, and the differences in their diffraction features raise the question of their fundamental structural units. To address this partially, results of X-ray and electron microscopy studies of Ti-Zr-Ni alloys, where Ni is replaced by Fe and Co, are reported. The character of the i-phases varies smoothly from the Ti-Zr-Ni quasicrystals, which probably are Bergman-type i-phases, to the Ti-Zr-Fe quasicrystals, which probably are Mackay types. A new classification method for icosahedral quasicrystals based on the ratio of the quasilattice constant a q to the average atomic separation , computed from the measured density, is introduced and applied to both Al- and Ti-based quasicrystals. On the basis of this scheme, most Ti-based i-phases, including the Ti-3d transition metal-Si-O phases and Ti Zr Fe, form a third group, different from the Al-based Mackay and Bergman groups. Ti-Zr-Ni and Ti-Zr-Co quasicrystals fall into the same class as the Bergman-type Al-based i-phases.

On the nature of quasicrystal phase transitions in AlCuFe alloys

In this investigation, a systematic study on the effects of the composition and annealed treatments in the formation of the AlCuFe icosahedral phase is carried out. The experimental results obtained from 30 different compositions under normal solidification methods and rapid solidification techniques show that the main driving force to the formation of the icosahedral phase is a cubic phase known in the past as the β phase. The β (AlCuFe) phase has a crystalline structure similar to the compound Al50Fe50. However, its lattice parameter is different from the parameter corresponding to the binary compound Al50Fe50, and it varies depending on the amount of copper. This gives rise to a cubic solid solution, the so-called β phase. The studied compositions were in the ranges of: Al(73–55 at.%), Cu(20–25 at.%) and Fe(10–15 at.%). Different structural and chemical characterization techniques were employed such as: X-ray diffractometry, scanning electron microscope (SEM) with EDS attachments, transmission electron microscopy (TEM) and differential thermal analysis (DTA) observations.

Quasicrystalline and related phases in titanium based alloy systems

Amongst the non-aluminium based quasicrystalline alloys, investigations of titanium containing alloys in regard to the occurrence and stability of quasicrystalline phases have aroused considerable interest in recent years. Employing X-ray and TEM techniques a systematic investigation of the influence of substitution for Fe by Si and Ni on the stability of icosahedral phase in rapidly quenched Fe-Ti alloy has been carried out. The occurrence of metastable phases including commensurately modulated phase and the decagonal phase in Fe-Ti-Si system have been found. In addition, the occurrence ofstructural disorder manifested by arcs of diffuse scattering in diffraction patterns and anisotropy in the shape of idffraction spots has also been pointed out. It has been observed that 6 at.% of Si in Fe-Ti-Si system results in the formation of single-phase icosahedral quasicrystals. We have shown that contrary to some earlier reported results on Ti 2Ni, Ti 2Ni(Si) and Ti 56Ni 23Fe 5Si 16 alloys do not possess the icosahedral phase. In Ti 68Fe 26− x Ni x Si 6 alloy system, icosahedral phase formation ability is limited to the value of x < 9. The occurrence of icosahedral phase in these alloy systems has been analysed in terms of e/a ratio.

Icosahedral-phase formation and stability in Ti Zr Co alloys

We present the first report of icosahedral phase i-phase formation in rapidly quenched alloys of Ti Zr Co . Electron diffraction patterns of i TiZrCo 53 27 20 contain features, such as anisotropic spot shapes and arcs of diffuse scattering, that are characteristic of the disordered icosahedral phases found in Ti 3d transition metal Si O and Ti Zr Fe alloys. The features are less prominent than in those alloys, however, suggesting that this i-phase may have structural order intermediate between those strongly disordered i-phases and more ordered Ti Zr Ni i-phases, showing none of these features. The quasilattice constant for a i TiZrCo, 0 51 nm, is close to that of i TiZrNi . The i-phase in rapidly q quenched Ti Zr Co alloys is deeply metastable and transforms exothermically o to the hexagonal Laves phase about 500 C. The Laves phase transforms to the o T bcc solid solution phase b-Ti and the Ti Ni-type fcc structure for 630 C. A 2 reversible transformation between the b-Ti and the hexagonal solid solution phase a-Ti is observed on temperature cycling; the Ti Ni-type fcc phase is 2 stable over the entire temperature range.

Disorder in as-quenched Al-TM ternary icosahedral phases

Models for Al-TM (TM is transition metal) quasicrystals consisting of networks of icosahedral units have the merit to be specific about the transition metal surrounding. Experimental arguments are brought for the importance of local stable Al12TM clusters in the formation of disorder-free icosahedral phases.

Icosahedral and related phase formation in rapidly quenched Ti-Zr-Fe alloys

Abstract We report the formation of the icosahderal phase (i-phase) in some rapidly quenched Tix, Zry, Fez, (45 ≤ × ≤ 70; 5 ≤ y ≤ 25; 20 ≤ z ≤ 30) alloys. Transmission electron microscopy (TEM) studies show that unlike the Ti-Zr-Ni i-phase, i(TiZrFe) is strongly disordered, showing strong spot anisotropy and intense localized diffuse scattering. These features, and a smaller quasilattice constant for i(TiZrFe) (a 0 = 4.85Å) than for i(TiZrNi) (a 0 = 5.12), indicate that i(TiZrFe) is more closely related to the icosahedral phases found in Ti-3d transition metal-Si alloys. TEM studies also reveal many related crystalline phases dominated by local polytetrahedral order, including α(TiZrFe), a bcc phase that is a 1/1 approximant to the i-phase, the λ-phase, an orthorhombic phase that appears to be an approximant to a decagonal phase, a Laves phase of the hexagonal MgZn2-type, and a Ti2Ni-type fcc phase. Evidence for ordering in the α(TiZrFe) and bcc solid solution phases is presented.

The role of oxygen in the formation of titanium-based icosahedral phases and crystal approximants

Abstract The first report of an oxygen-stabilized quasicrystal is presented. Oxygen is shown to be essential for the formation of the icosahedral and 1/1 approximant phases in Ti-Mn-Si and Ti-Cr-Si alloys. Recent X-ray and neutron diffraction structural studies by us of the 1/1 approximant phase reveal specific sites for the oxygen in the Mackay icosahedral clusters. On the basis of these data, possible explanations for the role of oxygen in quasicrystal formation are considered. These include firstly the creation of a nucleation barrier for competing crystalline phases, secondly the stabilization of the fundamental Mackay icosahedral cluster and thirdly an adjustment of the electron-to-atom ratio to shift the Fermi surface into a minimum in the density of states.

An electron diffraction study of quasicrystals in Ti-37 at% Mn and Ti-24 at% Mn-13 at% Fe alloys

Electron diffraction studies were carried out to establish the icosahedral phase formation in rapidly quenched Ti-37 at% Mn and Ti-24 at% Mn-13 at% Fe alloys. Distortions in the diffraction spots and diffuse intensities in the diffraction patterns were investigated. The existence of a rational approximant structure and a decagonal like phase are also reported.

Amorphous and quasi-crystalline phases in rapidly solidified MgAlZn alloys

The formation of amorphous and icosahedral phases and approximants has been identified in rapidly solidified MgAlZn alloys. The icosahedral phase is located at the composition range between those of the amorphous phase and approximants. Mg 40Al 40Zn 20 is a cubic 1 1 approximant in the equilibrium state and a cubic 5 3 rational approximant in the rapidly solidified state. Rapidly solidified Mg 50Al 30Zn 20 and Mg 50Al 20Zn 30 have a disordered icosahedral structure with a high density of random phason strain. The valence concentration e/ a plays an important role in the selections of rapidly solidified structure between the amorphous and the icosahedral phases and of phason strains between isotropic and anisotropic.

Structural and Mossbauer effect studies of quasicrystalline Al-Pd-Cr-Fe alloys

Structural studies of conventionally solidified Al80-2xPd20CrxFex alloys have shown that the formation of a well ordered icosahedral phase occurs for a composition where the free-electron Fermi sphere is coincident with the first pseudo-Brillouin zone boundary. This condition indicates the importance of the Hume-Rothery stabilization criterion for icosahedral phase formation in this system. Mossbauer effect studies show the similarity of the local structural order in stable Al72Pd20Cr4Fe4 to that found in stable quasicrystalline Al-Cu-Fe alloys and indicate a higher degree of structural order compared with that found in metastable icosahedral alloys prepared by non-equilibrium processing techniques.

Icosahedral phase formation in Al-Mn alloys produced by means of simultaneous vapour deposition and ion implantation

Icosahedral phase formation in Al-Mn alloys produced by means of simultaneous vapour deposition and ion implantation