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.     

Microstructural evolutions and hardness during heat treatment of Al64Cu20Fe12Si4 quasicrystal alloy

The microhardness and microstructural characteristics and subsequent heat treatment of conventionally solidified Al₆₄Cu₂₀Fe₁₂Si₄ quasicrystal were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM) together with energy dispersive spectroscopy (EDS), differential thermal analysis (DTA), and Vickers microhardness tester. XRD analysis indicated that the conventionally solidified samples showed a quasicrystalline icosahedral phase (i-phase) together with cubic β-AlFe, tetragonal θ-Al₂Cu, and monoclinic λ-A₁₃Fe₄ crystal phases. However, the i-phase together with cubic β-AlFe and monoclinic λ-A₁₃Fe₄ phases observed heat threaded samples. As-cast and subsequently heat-treated quasicrystal samples were measured using a microhardness test device. Vickers microindentation tests were carried out on the heat-treated quasicrystal samples with the load ranging from 1 to 500?mN at room temperature. The melting point of the i-phase was determined as 900°C by DTA examinations.     

New stable icosahedral phases in Al-Pd-Ru and Al-Pd-Os systems

We have found stable icosahedral (i) phases in alloys with nominal compositions around Al 72 Pd 17 Ru 11 and Al 72 Pd 17 Os 11 . They display very sharp X-ray diffraction peaks, indicating high structural order as in other Al-based i-phases such as i-(Al-Pd-TM) (transition metal TM = Mn or Re) and i-(Al-Cu-TM) (TM =Fe, Ru or Os). The i phases are formed via peritectic reaction and, upon heating, they transform into a mixture of a liquid and the 1/0 cubic approximant phase. The electrical resistivity of the i phases was measured in a wide temperature range from 14K to above their melting points and a reversible change of the resistivity across the melting point has been observed in situ for the first time.     

Cu-based icosahedral quasicrystal formed in Cu-Ga-Mg-Sc alloys

The first-reported Cu-based icosahedral quasicrystal has been found as an almost single phase in Cu 48 Ga 34 Mg 3 Sc 15 alloy annealed at 1043K for 61h. On the basis of our experiments, this icosahedral quasicrystal is expected to be one of the equilibrium phases in this alloy system. Powder X-ray diffraction and electron diffraction experiments revealed that the quasicrystal exhibits a high degree of structural perfection and has a primitive type quasilattice with a six-dimensional lattice parameter a 6D =0.6938nm. The existence of a Cu 3.7 Ga 2.3 Sc-type structure, which is a bcc structure with diffraction symmetry m3, suggests that the Cu-Ga-Mg-Sc quasicrystal is to be classified into a new structural type to which Cd-based icosahedral quasicrystals and the Zn-Mg-Sc quasicrystal belong. This new type has a characteristic local atomic configuration different from both Mackay-type and Bergman-type quasicrystals.     

Evolution of superlattice order in Al–Mn quasicrystals and its relation to face-centred icosahedral quasicrystals

Abstract

We have established a simple icosahedral to face-centred icosahedral ordering transformation in Al–Mn quasicrystals. This result strongly supports the view that the recently discovered face-centred icosahedral quasicrystal in ternary Al–Cu–Fe and related alloys represents a long-range superstructure of simple icosahedral quasicrystals     

Frank's 'cubic' hexagonal phase: An intermetallic cluster compound as an example

Frank introduced in 1965 the novel idea of projection from a four-dimensional cube to recover the points of a hexagonal lattice with a special c / a ratio of ( 3 - 2 ½. This was called a cubic hexagonal crystal, as there was a similarity to the conventional cubic crystals in that directions were perpendicular to planes with the same Miller-Bravais indices. While a number of crystals in the NiAs-Ni 2 In family have been reported with this special axial ratio, the number of atoms in the unit cell is small. As the first example of an intermetallic cluster compound, we identify µ-Al 4 Mn, µ-Al 4 Cr, Zn-Mg-Sm and a host of related intermetallics featuring a special aggregate of icosahedra as Frank's 'cubic' hexagonal phase or its variant. The metric is generated by the Friauf polyhedra and the icosahedral linkages and leads to a multimetric crystal and several interesting connections with hexagonal quasicrystals, hexagonal phases and derivative orthorhombic and lower-symmetry structures.     

Use of electron channelling contrast imaging to assess near-surface mechanical damage

The crystal structure of a cubic phase in the system Zn-Mg-Er has been solved by a combination of high-resolution electron microscopy and X-ray powder diffraction. This phase is considered to be related to that of the quasicrystalline phase. The structure consists of 448 atoms in the unit cell with lattice constant of a ₀ = 20.20Å and the space group is F43m. Important structural elements in the cubic structure are interpenetrating icosahedral units around Zn and Mg atoms and Frank-Kasper polyhedra around the Mg atoms. No giant icosahedral atomic cluster, such as the 136-atom Bergman cluster, was found in the stucture.     

Sign determination of the 57 Fe electric-field gradient in Al-based crystals and icosahedral quasicrystals

Abstract

The icosahedral quasicrystals i-AIMn, isomorphically substituted by 28 at.% Fe or by a mixture of (CrFe) atoms, have been studied for the first time by in-field Mössbauer spectroscopy in order to determine the sign and asymmetry parameter of the dominant electric-field gradient (EFG) term. In addition, the orthorhombic o-Al(MnFe) and cubic α- and hexagonal β-Al(MnFe)Si crystalline phases have been studied. We show that the previous Mössbauer results are inadequate for determining whether there are two sites in the quasicrystalline structure in the ratio of the golden number. Our results for i-Al(MnFe) show that the dominant EFG is negative, with an asymmetry parameter of about 0·6. For i-Al(CrFe), essentially no deviations are found from the model of Czjzek or the Gaussian isotropic model. One crystalline phase, the hexagonal β phase, is found to have a very similar quadrupole effect to that found in i-Al(MnFe). In addition it is found that this phase undergoes a change which is at least partly of magnetic origin.     

Formation and hydrogen adsorption properties of Ti-Hf-Ni quasicrystals and crystal approximants

The formation of a high-order rational approximant (RA) phase in rapidly quenched Ti-Hf-Ni alloys and the hydrogen absorption properties of that phase are reported. Electron diffraction patterns show systematic shifts of the diffraction spots from their expected positions for the icosahedral phase (i-phase); the diffraction patterns are consistent with those expected for a 3/2 RA phase RA-(Ti-Hf-Ni). Based on differential scanning calorimetry studies, RA-(Ti-Hf-Ni) is metastable, transforming between 350 and 500°C to a quasicrystal with strong phason disorder. This crystallizes at 620°C to a Ti 2 Nitype phase. RA-(Ti-Hf-Ni) readily absorbs hydrogen, up to 1.2 hydrogen atoms per metal atom ([H][M] =1.2). Pressure-composition isotherm studies for gasphase loading show that the pressure plateau for RA-(Ti-Hf-Ni) is similar to that observed for the i-(Ti-Zr-Ni) phase, although it occurs at as lightly higher pressure and extends over a smaller range of hydrogen concentrations. Unlike i-(Ti-Zr-Ni), no irreversible hydride phase forms with hydrogen loading at 250°C, suggesting that RA-(Ti-Hf-Ni) may have superior cycling properties, of interest for hydrogen storage applications.     

Nanoprecipitates of icosahedral phase in quasicrystal-strengthened Mg-Zn-Y alloys

Extensive microstructural studies have been performed with respect to the formation of the icosahedral quasicrystalline phase and its relationship to other phases in Mg 95 Zn 4.2 Y 0.8 alloy. The icosahedral phase forms as an intergranular eutectic phase as well as precipitates in the matrix. The precipitates are nanosized (typically 50 nm) with a definite orientation relationship with the matrix, sharply faceted on twofold planes which are on the basal and prismatic planes of the matrix. The detailed crystallographic relationship with the matrix is described. The icosahedral phase is occasionally found to coexist with the cubic W-Zn 3 Mg 2 Y 3 phase with a definite crystallographic relationship.     

Stoichiometric selectivity in Mackay icosahedra

Abstract

We present a discussion of the effect of chemical substitution on the stability of Mackay icosahedra in Al-Mn alloys based on a microscopic model of the total energy of metallic alloys. In particular, we demonstrate why the addition of Si stabilizes both the crystalline α-(Al-Mn-Si) structure and the icosahedral phase of Al-Mn-Si. We predict specific A1 sites which are favourable for Si substitution.     

Crystal structure of a hexagonal phase and its relation to a quasicrystalline phase in Zn-Mg-Y alloy

The crystal structure of a Zn-Mg-Y hexagonal phase, considered to be related to that of the quasicrystalline phase, has been determined by single-crystal X-ray diffraction. The atomic model, refined to a final R value of 0.027, has the composition Zn65.22Mg27.92Y6.86 and 92 atoms in a unit cell with lattice constants a = 14.579(2) A and c = 8.687(1) A and the space group P63/mmc. The Y atoms are situated at two different sites; one is fully occupied by Y atoms, while the other is shared with Mg atoms. The structure can be characterized by a layer structure stacked along the c axis, and also viewed as a columnar structure composed of fused Friauf polyhedras sharing hexagonal rings. The similarity of this hexagonal phase to the MgZn2 phase is shown. The structural relationship between this hexagonal phase and the icosahedral phase is also discussed.     

Icosahedral quasicrystal in annealed ZnMgSc alloys

A new icosahedral quasicrystal has been found in Zn 85-x Mg x Sc 15 ( x = 3,4,5) alloys annealed at 922K for 5-100h. Electron diffraction and powder X-ray diffraction experiments indicate that this quasicrystal has a primitive icosahedral quasilattice (P type) and a six-dimensional lattice parameter a 6D =0.7115nm. The stoichiometric composition of the icosahedral quasicrystal was estimated to be close to Zn 80 Mg 5 Sc 15, since the icosahedral quasicrystal coexists in the x = 5 samples with small amounts of a MgZn 2 -type Laves phase and a Zn 17 Sc 3 -type cubic phase with lattice parameter a =1.3854nm. The latter is interpreted to be a 1/1-type approximant crystal of the icosahedral quasicrystal. The atomic cluster included in the Zn 17 Sc 3 -type cubic phase indicates a structural similarity between the Zn-Mg-Sc quasicrystal and the Cd-Yb (and Cd-Ca) icosahedral quasicrystals recently reported.     

Evidence for structural disorder in the icosahedral phase

Abstract

We report transmission electron microscopy results on quasicrystalline samples of Al–25 wt% Mn and Al-38 wt% Mn-5 wt% Si melt-spun flakes. The selected-area diffraction patterns (SADPs) from ‘off-axis’ orientations show not only sharp diffraction maxima but also a diffuse ring, invariant of sample thickness, indicating that it does not arise from amorphous surface oxide. Over-exposed SADPs taken from axial orientations also showed the weak diffuse ring. We suggest that our results indicate the presence of disordered material within the icosahedral phase. Models for the icosahedral phase which involve an assembly of icosahedral clusters inherently contain interstitial voids, which may contain disordered material. Thus our results support these models rather than those derived from space-filling filings, or multiple twinning.     

Determination of atomic structure of phason planes in the ξ′-Al–Pd–Mn phase

The atomic structures of specific types of linear defects (phason lines) and planar defects (phason planes) in the complex metallic alloy phase ξ′-Al–Pd–Mn have been determined by high-resolution electron microscopy (HREM) and theoretical HREM simulation. The results show that a representational atomic structural model for phason planes can be constructed by introducing a shift between two parts of the perfect crystalline structure using a translation vector of r ?=?(1/2) a ?+?(1/2τ) c . This typical phason plane is normally parallel to the (001) plane of the ξ′-Al–Pd–Mn phase and consists of phason lines, which are arranged side-by-side with their linear direction parallel to the [010] axis. HREM simulations, based on the structural model for both edge-on and inclined types of phason lines, agree well with the experimental results. Taking into account the fact that the structural difference between various curved phason planes arises from the variation in the arrangement of individual phason lines, the atomic structures of the edge-on and inclined phason lines can be used to explain the various curved phason planes frequently observed in the ξ′-Al–Pd–Mn phase.     

Extended energy-loss fine structure analysis of 3d transition metals using L ionization edges

The extended energy-loss fine structure (EXELFS) in electron-energy-loss spectroscopy has been compared experimentally with the extended X-ray absorption fine structure (EXAFS), for determining local structure around 3d transition metals. Since the EXELFS spectrum is acquired in an analytical transmission electron microscope, the probing beam can be focused to offer a lateral spatial resolution in the nanometre range, which is several orders of magnitude better than that of X-rays. Also, the microscope allows the area of interest to be imaged and analysed by other methods. However, difficulties in the analysis of EXELFS of 3d transition metals arise from the overlap of the edges in the accessible L series. EXELFS of L ionization edges of crystalline Ni and Cu were examined as test specimens. The overlapped L₁,L₂ and L₃ edges were separated and compared with the K-edge EXAFS of the same samples. The first- and second-nearest-neighbour distances from EXELFS are in agreement with those measured from EXAFS and X-ray diffraction. As a structural probe, however, the accuracy of quantitative analysis of EXAFS is still superior since the L-edge EXELFS decays more rapidly than K-edge EXAFS.     

Effects of ball milling and annealing of an alloy in the Al-Fe-Cu system: Implications for phase equilibria

An icosahedral quasicrystalline alloy in the Al-Fe-Cu system has been mechanically milled in a high-energy ball mill (Szegvari attritor) for 1, 3, 6 and 10 h. Samples were characterized by X-ray diffraction and transmission electron microscopy. The evolution of nanosize crystallites of the disordered B2 phase (bcc; a = 0.29 nm), coexisting with either the parent icosahedral phase or an amorphous phase, occurs during milling. Isothermal heat treatment of milled powder at various temperatures (200, 500, 600, 700, 800 and 850°C) leads in all cases, except at 200°C, to the transformation from disordered B2 and amorphous phases to an ordered B2 phase with a high degree of long-range ordering. The maximum degree of superlattice ordering was found after isothermal treatment at 800^oC. The implications of these results are discussed with reference to phase equilibria existing between crystalline and quasicrystalline phases in the Al-Fe-Cu system.     

Arrangement of atomic clusters in a 2/1 cubic approximant in the Al-Zn-Mg alloy system

An arrangement of the so-called Bergman clusters of atoms in a 2/1 cubic crystalline approximant phase, which is closely related to Frank-Kasper-type icosahedral quasicrystals, in an Al-Zn-Mg alloy system is discussed in detail. The 2/1 cubic structure has eight Bergman clusters in its unit cell and the positions of the clusters can be understood as twelvefold sites in the 2/1 cubic approximant of the three-dimensional Penrose lattice. The atomic clusters are located at vertices of two types of rhombohedron, which are formed by six rhombic planes with an edge length of 3 1/2 a 2 /( 2 + 1) 1/2 and a shorter diagonal of 2 a 2 /( 2 + 1) 1/2 ( is the golden ratio and a is a quasilattice constant), which correspond to half a body diagonal and an edge length respectively of the 1/1 cubic approximant unit cell. The present results provide an important key for understanding the arrangements of atomic clusters in icosahedral quasicrystals.     

Effect of Nb on nanoquasicrystalline Al-based alloys

Nanoquasicrystalline Al-based alloys, containing icosahedral particles in an α-Al matrix, exhibit high strength at elevated temperature. The metastability of the quasicrystals can limit the use of these alloys. In the present work, the microstructural evolution of Al₉₃(Fe₃Cr₂)₇ and Al₉₃Fe₃Cr₂Nb₂ (at%) alloys was studied using heat treatments and structural characterization by XRD, TEM and STEM-EDX analysis. It was observed that the Nb is dissolved in the Al–Fe–Cr icosahedral phase. This provides higher thermal stability, retaining the fine nanoquasicrystalline microstructure for longer times at high temperature.