Precipitate assemblies formed on dislocation loops in aluminium-silver-copper alloys

The precipitation microstructure of the γ′ (AlAg₂) intermetallic phase has been examined in aluminium-silver-copper alloys. The microstructure developed in an Al-0.90at.%Ag-90at.Cu alloy was significantly different from that reported for binary Al-Ag alloys. The orientation relationship between the matrix and precipitate was unchanged; however, the γ′ phase formed assemblies with a two-dimensional, open arrangement of precipitates. Each such assembly contained two variants of the γ′ phase alternately arranged to form a faceted elliptical unit. The θ′ (Al₂Cu) phase formed on these assemblies after further ageing. Each assembly was formed via repeated precipitation of the γ′ phase on dissociated segments of a single dislocation loop. This faceted elliptical assembly has not been previously reported for the γ′ precipitate. The difference between the precipitation behaviour of the γ′ phase in Al-Ag and Al-Ag-Cu alloys was attributed to copper modifying the as-quenched defect structure of the matrix. The formation of faceted elliptical γ′ phase assemblies clarifies earlier observations on the precipitate number density and mechanical properties of aluminium-silver-copper alloys.     

Novel charge density wave transition in crystals of R5Ir4Si10

We review the observation of novel charge density wave (CDW) transitions in ternary R₅Ir₄Si₁₀ compounds. A high quality single crystal of Lu₅Ir₄Si₁₀ shows the formation of a commensurate CDW along c-axis below 80 K in the (h, 0, l) plane that coexists with BCS type superconductivity below 3.9 K. However, in a single crystal of Er₅Ir₄Si₁₀, one observes the development of a 1D-incommensurate CDW at 155 K, which then locks into a purely commensurate state below 55 K. The well-localized Er³ moments are antiferromagnetically ordered below 2.8 K which results in the coexistence of strongly coupled CDW with local moment antiferromagnetism in Er₅Ir₄Si₁₀. Unlike conventional CDW systems, extremely sharp transition (width ∼ 1.5 K) in all bulk properties along with huge heat capacity anomalies in these compounds makes this CDW transition an interesting one.     

Structural,electronic and magnetic properties of C59Ir,C58Ir2, and C69Ir heterofullerene nano-cages: first principles study

We studied the structural, electronic, and magnetic properties of C₅₉Ir, C₅₈Ir_2, and C₆₉Ir heterofullerenes by employing density functional theory and the generalized gradient approximation. There are six distinct isomers of C₅₈Ir₂ with high probability to form stable structures. The most stable structure of the C₆₉Ir heterofullerene was investigated by comparing the iridium binding energies at the different atomic sites on the D5h C₇₀ cage. There is a strong hybridization between the atomic orbitals of the iridium and those of the carbon atoms, leading to the spin quenching of the iridium atoms in the most stable C₅₈Ir₂ heterofullerene.     

The charge-density-wave transition in Lu5Ir4Si10 under uniaxial pressure

Abstract

An uniaxial pressure cell for low temperature use is described in detail. Then we present data of the electrical resistance of single crystals of Lu₅Ir₄Si₁₀, which is known to show a charge-density-wave transition around 83 K and to become superconducting near 3.8 K, both phenomena being anticorrelated under pressure. Since the CDW in Lu₅Ir₄Si₁₀ is a quasi one-dimensional phenomenon because of a chain-like structure, it responds to uniaxial pressure in a specific way.     

Three-dimensional size and orientation of the precipitates in AZ91 magnesium alloys measured by TEM techniques

Knowledge of the microscopic structure, including three-dimensional (3-D) size and orientation of the precipitates, is essential to fully understand the mechanical properties of the magnesium alloys and designing the alloys with better performance. Analytical TEM with high spatial resolution offers the simultaneous measurements of 3-D size, structure, orientation, composition of the precipitates from one typical sample along an established crystallographic axis. Besides popular Burgers orientation relationship (OR), other ORs such as Pitsch--Schrader OR, Crawley OR, Potter OR and a new OR with the form of [0001]_α 1.0° from [311]_γ and (11\bar 20)_α 2.0° from (03\bar 3)_γ between the magnesium matrix and the precipitate γ -Mg₁₇Al₁₂ are identified by TEM imaging and diffraction techniques. As a case study, the thicknesses of the individual precipitates with Burgers OR are further measured to be 100--200~nm through both electron energy-loss spectroscopy and x-ray energy dispersive spectroscopy combining differential x-ray absorption and extrapolation, which are in agreement with the overall 3-D size statistic distribution results obtained through analysing various samples along various directions. Furthermore, the fabricated wedge-shape structure provides a platform on which to study the dependence of the interfacial strain on the variation of the thickness.     

HRTEM and HAADF-STEM tomography investigation of the heterogeneously formed S (Al2CuMg) precipitates in Al–Cu–Mg alloy

The distribution of variants and three-dimensional (3D) configurations of the heterogeneously formed S (Al₂CuMg) precipitates at dislocations, grain boundaries and the Al₂₀Cu₂Mn₃ dispersoid/Al interfaces were studied in this research. By means of high resolution transmission electron microscopy, we systematically investigated the orientation relationships (ORs) between these heterogeneously formed S precipitates and the Al matrix, and further unraveled that the preferred orientation of S variants at grain boundaries and at dispersoid/Al interfaces are respectively associated with the OR between the precipitate habit plane and the grain boundary plane, and the OR between the precipitate habit plane and the interface plane. The inherent characteristic of the crystal structure of the S phase, i.e. the symmetry of the pentagonal subunit, was considered to be the fundamental factor determining the preference of the variant pair. By using high angle annular dark field scanning transmission electron microscopy tomography, we successively obtained the 3D reconstruction of the S precipitates at these defects. Both the morphology of an individual S precipitate and the overall configuration of the S precipitates nucleated at these defects can be clearly observed without misunderstandings induced by the overlap and projection effects of the conventional two-dimensional methods.     

Role of spin–orbit coupling and electron correlation in the electronic structure of a 5d pyrochlore,Y2Ir2O7

Spin–orbit coupling in 5d transition metal oxides such as Ir oxides is expected to be strong due to large atomic number of Ir and electron correlation strength will be weak due to large radial extension of the 5d orbitals. Hence, various anomalous electronic properties often observed in these systems are attributed to large spin–orbit interaction strength. Employing first principles approaches, we studied the electronic structure of Y ₂Ir₂O₇, which is insulating and exhibits ferromagnetic phase below 150 K. The calculated results reveal breakdown of both the above paradigms. The role of spin–orbit interaction is found to be marginal in determining the insulating ground state of Y ₂Ir₂O₇. A large electron correlation strength is required to derive the experimental bulk spectrum.     

Local Moment of Ir in Fe,Co and Ni Hosts Probed by Ir L 2,3 Edge X-Ray Magnetic Circular Dichroism

The formation of an induced 5d magnetic moment on Ir in Fe₉₇Ir₃, Co₉₅Ir₅ and Ni₉₅Ir₅ alloys has been investigated by X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectra (XAS) measurements at Ir L _2,3 edges. Using a sum rule which relates the integrals of these spectra with the ground state expectation value of the orbital angular momentum 〈L _Z 〉 of the probed atom, the orbital moment m _orb of Ir could be determined as −0.071(2) μ _B in an Fe host, −0.067(2) μ _B in a Co host and −0.041(1) μ _B in a Ni host. The spin magnetic moment m _spin of Ir is also found to be the maximal in Fe and the minimal in Ni. The total moment of Ir is found to be approximately 1/5 of total moment of Fe, 2/13 of the total moment of Co, and 1/4 of the total moment of Ni. This revised version was published online in September 2006 with corrections to the Cover Date.     

X-ray magnetic circular dichroism at IrL2,3 edges in Fe100−x Ir x and Co100−x Ir x alloys: Magnetism of 5d electronic states

The formation of induced 5d magnetic moment on Ir in Fe_100−x Ir _x (x=3, 10 and 17) and Co_100−x Ir _x (x=5, 17, 25 and 32) alloys has been investigated by X-ray magnetic circular dichroism (XMCD) at Ir L_2,3 absorption edges. Sum rule analysis of the XMCD data show that the orbital moment of Ir is in the range of −0.071(2)μB to −0.030(1)μB in Fe-Ir alloys and −0.067(2)μB to 0.024(1)μB in Co-Ir alloys. We find that the total moment of Ir in Fe-Ir alloys is approximately 1/5 of the total 3d moment on Fe at all the three compositions. In contrast, the total moment on Ir in Co-Ir alloys varies between 1/6 to 1/16 of the 3d moment on cobalt. The observed trends of Ir moments and the role of interatomic exchange interactions in 5d moment formation are discussed.     

TEM characterization of yttrium silicide layers synthesized by ion implantation

The structure of Si implanted with high doses of yttrium has been investigated by varying implantation doses and energies. As implantation doses increase into the low 10¹⁷ cm^–2 range, silicide precipitates form. The precipitates are thin and long and lie parallel to {111} planes in the Si matrix. As dopant concentrations increase, the precipitates themselves become more equiaxed, aspect ratios decrease, and precipitates densities increase until the precipitates coalesce to form a continuous buried layer of yttrium silicide within the Si matrix. The layer thickness is relatively uneven. As implant doses increase to 4×10¹⁷ cm^–2, the layer thicknesses become more uniform although there are still defects present. As the implant doses increase further, the precipitate bands on either side of the continuous layer decrease due to gettering of yttrium to the layer. As the energy of the implant is increased, the appearance of the sample is similar to that of the lower energy implants except that the layer is buried deeper in the Si matrix. Observations of the silicide are consistent with its having the AlB₂ structure with ordered vacancies on the Si sublattice.Address from July 1, 1992: Arizona State University, Tempe, AZ     

Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al–Si-based alloys

The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al–5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al–5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}_α-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al₅FeSi intermetallic was also observed, depending on subsequent controlled cooling.     

Deposition dynamics and chemical properties of size-selected Ir clusters on TiO2

We report a study of Ir_n/TiO₂ samples prepared by size and energy-selected deposition of Ir_n⁺ (n=1, 2, 5, 10, 15) on rutile TiO₂(1 1 0) at room temperatures. The Ir clusters are found to be formally in the zero oxidation state, and there are no significant shifts in Ir 4f binding energy with cluster size. Over a wide range of impact energies, both Ir XPS intensity and peak position are constant, indicating constant sticking coefficient, and no impact-driven redox chemistry. Low energy ion scattering spectroscopy (ISS) suggests that the deposited Ir clusters remain largely intact, neither fragmenting nor agglomerating, and retaining 3-D structures for the larger sizes. For impact energies above 10 eV/atom, comparison of ISS and XPS data show that the Ir clusters are penetrating into the TiO₂ surface, with the extent of penetration increasing with both per atom energy and cluster size. Temperature programmed desorption (TPD) of CO is used to further characterize the deposited Ir_n. This system shows pronounced substrate-mediated adsorption (SMA) in low CO exposures, with strong dependence on cluster size. ISS and sputtering experiments indicate that CO adsorbed via SMA is bound differently than CO adsorbed in high dose experiments. In experiments with sequential C¹⁶O and C¹⁸O doses, facile C¹⁶O → C¹⁸O exchange is observed for Ir₅ and larger clusters, but not for Ir₂. The peak CO desorption temperature is found to decrease with cluster size. The cycle of CO adsorption and heating comprising a TPD experiment have a dramatic effect on the sample morphology, leading to encapsulation of Ir by a thin TiO_x layer.     

Ab initio study of 5d-shells Ir substitution for Fe-based SmOFe1−xIrxAs

The lattice and electronic properties for 5d-shells Ir substituted Fe-based superconductor SmOFe_1−xIr_xAs (x=0,0.2,0.25,0.3) are investigated based on the density functional theory (DFT) with a spin generalized gradient approximation SGGA+U method. The electronic density of states (DOS) of SmOFe_1−xIr_xAs is studied and well compared with the results of experimental X-ray photoemission spectroscopy (XPS). The calculation indicated that iridium substitution at the Fe site induced a modification of the FeAs₄ tetrahedron and suppressed the magnetic ordering corresponding to the Fe-3d, which may be the main cause of inducing superconductivity in Ir-doped SmOFeAs system.     

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al–Cu–Mg alloy

The hardening precipitation of an Al–Cu–Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier–Preston–Bagaryatskii (GPB) zones type or to the so-called S-Al₂CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces.     

Effect of Zr concentration on the mechanical and thermodynamic properties of NbIr3 intermetallic compounds from theoretical estimations

ABSTRACT

The iridium is an important metal which has excellent resistance to corrosion at high temperature. L1₂ intermetallic compounds i.e. Ir₃Nb and Ir₃Zr, with similar lattice parameters are ideal for working at high temperature. They are fully soluble due to their low lattice misfit. A first-principle investigation into the effect of doping Zr with different concentrations on the electronic structure, mechanical and thermodynamic properties of NbIr₃ has been studied to prompt the development of novel high-temperature materials. Nine Zr_xNb_8?xIr₂₄ compounds are carefully considered. The results show that adding Zr into these compounds can strengthen their structural stability and ductility. Nevertheless, it reduces the elastic modulus and elastic stiffness. Simultaneously, with the increase of Zr content, the thermodynamic properties of these compounds decrease. It is also found that the changes of elastic modulus are mainly attributed to the variations of bonds in these compounds.     

Kern-Isomerie bei77Ir192 und77Ir194

Theβ- andγ-radiation of Ir¹⁹² and Ir¹⁹⁴, produced by slow neutron irradiation, has been studied with scintillation spectrometers. A new isomeric activity with a (47±2) second-half life is found to be Ir^194m , decaying by a 130 kev-transition to Ir¹⁹⁴ and byβ-emission to excited states of Pt¹⁹⁴. Gamma rays of energy (130±4), (323±7) and (625±20) kev were found. An upper limit for the conversion coefficient of the isomeric transition is given, which shows in connection with energy-half life-relations, that the 130 kev gamma ray is anE 3 transition.     

Paramagnetic hyperfine structure of193Ir in K2IrCl6 diluted into K2PtCl6

The paramagnetic hyperfine splitting of Ir⁴⁺ (5d⁵) in K₂IrCl₆ diluted into diamagnetic K₂PtCl₆ has been observed at 4.2 K and Ir∶Pt ratios of 1∶10 and 1∶25. In the latter case a narrow paramagnetic pattern with a hyperfine coupling constant ofA=−13.1(2) mm/s was observed, but both samples also exhibit a single Ir⁴⁺ line typical for fast relaxation, either because of macroscopic inhomogeneities in the Ir distribution or because part of the Ir spins are still coupled to nearest Ir neighbours.     

Surface oxides of Ir(111) prepared by gas-phase oxygen atoms

The Ir(111) surface is oxidized with gas-phase oxygen atoms under vacuum condition to achieve an oxidation level beyond its saturation coverage for chemisorption. Two surface oxides, rutile IrO₂ of (100) domain and corundum Ir₂O₃ of (001) domain, have been grown at 550 K with different oxygen exposure of 3.6 × 10⁵ L and 7.2 × 10⁵ L respectively. The temperature programmed desorption (TPD) experiment of rutile IrO₂(100) shows its desorption curve (at 4 K s^? 1) peaks at 750 K, followed by a long tail of less pronounced desorption features. On the other hand, TPD of corundum Ir₂O₃(001) displays a symmetric trace, peaking at 880 K. Carbon monoxide titration experiments show that adsorbed CO reduces corundum Ir₂O₃(001) at 400 K, but CO does not adsorb on rutile IrO₂(100) and no reduction reaction occurs. Evidently, among the two surface oxides, corundum Ir₂O₃(001) involves in catalysis of carbon monoxide oxidation, while rutile IrO₂(100) does not. The formation of two surface oxides is also compared, we conclude that the atom arrangement favors Ir₂O₃(001) at the oxide/metal interface.     

Characterisation of precipitate phases in magnesium alloys using electron microdiffraction

Recent results of the characterisation of the structure, morphology and orientation of fine-scale, strengthening precipitate phases in selected magnesium alloys using transmission electron microscopy and microdiffraction are reviewed. The strengthening precipitate phases in Mg–Y–Nd alloys, aged to maximum hardness at 250°C, have been found to include two metastable precipitate phases β′ and β₁, and the equilibrium precipitate β. The β′ phase has a globular form, a base-centred orthorhombic structure (potential point group of mmm), and an orientation relationship such that (100)_β′//(1 10)_α, [001]_β′//[0001]_α. The β₁ phase has an f.c.c. structure (space group and an orientation relationship that may be described by (100)_β1//[0001]_α, and forms as plates parallel to The β phase has an f.c.c. structure (space group ) and also forms as plates on with an orientation relationship with the matrix phase that is identical to that observed for β₁ phase. Precipitates in Mg–Al alloys, aged isothermally at 200°C, invariably have the b.c.c. structure of the equilibrium precipitate phase β (Mg₁₇Al₁₂). Three orientation relationships have been observed between β and the matrix phase. Most precipitates have an irrational orientation relationship that approximates to the Burger's relationship, (001)_β//(0001)_α, and a faceted lath morphology with habit plane parallel to (0001)_α. A minor fraction of precipitates posses an orientation relationship that is of the form

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and have a prismatic rod morphology. The long axes of these rods are parallel to [0001]_α, and their faceted surfaces are parallel to A few precipitates are observed to have an orientation relationship such that

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and a rod shape, with their long axes apparently inclined with respect to [0001]_α.     

HAADF-STEM study of β′-type precipitates in an over-aged Al–Mg–Si–Ag alloy

Coarse, rod-shaped precipitates growing along ?100?Al directions in an Al–1.0?wt% Mg₂Si alloy with 0.5?wt% Ag additions were investigated by high-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All investigated precipitates had complex structures, being composed of domains separated by anti-phase resembling boundaries. The domains consist of a modified hexagonal β′-type structure that contains a considerable amount of Ag. Based on HAADF-STEM images, an average atomic model with space group P-62?m (189) and composition Al₃Mg₃Si₂Ag is proposed, having Al incorporation and Ag replacing certain Si atomic columns. Co-existence with the Ag-free β′-Mg₉Si₅ phase has been observed for some precipitates. The boundaries may be described as full or half units of the orthorhombic U2-AlMgSi precipitate phase. The HAADF-STEM images indicate partial replacements of Al atoms by Ag, in both the β′-type domains and the U2-type boundaries. Ag enrichment of the Al matrix near the precipitate/Al interface was observed for all the investigated precipitates