Aluminum Sublattice Research Articles

Energy Gap Decrease in Cation Multidoped Aluminum Oxide

The electronic structure of metal‐doped α‐Al2O3 with the disordered distribution of multi‐d‐ (Zr, Nb or/and Mo) and s‐ (Mg) impurities in the aluminum sublattice under the variation of their concentrations have been calculated using the coherent potential approximation method. Herein, an energy gap reduction due to appearance of new isolated occupied or empty Zr, Nb, and Mo d bands near the conduction band bottom of undoped α‐Al2O3 is shown. New multi d‐ and s‐doped α‐Al2O3 dielectrics with energy gap values of 7.1, 5.6, and 4.3 eV are found. Four multi‐d‐doped α‐Al2O3 semiconductors , , , and with energy gap values of 0.2–0.4 eV can be considered as promising materials for optoelectronic devices.

Structure and Luminescence Properties of Сu1 – xAl0.25In0.75Se2 (0 < x ≤ 0.20) Copper-Deficient Chalcopyrite Solid Solutions

Сu1 – xAl0.25In0.75Se2 copper-deficient solid solutions with the chalcopyrite structure have been prepared by elemental synthesis from Cu, In, Al, and Se, and their unit-cell parameters have been determined for the first time as functions of x in the range 0 < x ≤ 0.20. It has been shown that the band peaking at 1.320 eV in the 78-K cathodoluminescence spectra of the Сu1 – xAl0.25In0.75Se2 solid solutions is most likely due to CuM and MCu (M = Al, In) antisite defects resulting from exchange of atoms between the copper and indium–aluminum sublattices in the chalcopyrite structure and that the band peaking at 1.245 eV arises from Cu2+ · VCu associates, which are predominant defects.

Superionic State in Alumina Produced by Nonthermal Melting

Density functional–based molecular dynamics reveals a transient superionic state in Al2O3 produced by nonthermal phase transition under extreme electronic excitation. At electronic temperatures above Te ≈ 2.75 eV, the oxygen sublattice exhibits fluid behavior, whereas the aluminum sublattice is in a solid state. This state exists up to Te ≈ 3.25 eV, where Al2O3 turns metallic–superionic; at Te ≈ 3.75 eV, the aluminum sublattice disorders. Quenching the superionic state under pressure &gt;400 GPa freezes it into a metastable mixed amorphous–crystalline phase where the oxygen subsystem is disordered solid, whereas the aluminum one is ordered.

Effect of Impurities on the Formation Energy of Point Defects in the γ-TiAl Alloy

The projector augmented-wave method within density functional theory is used to study the effect of 4d elements and Group IIIA and IVA elements on the energetics of point defect formation and elastic moduli of the γ-TiAl alloy. To calculate the point defect formation energy, the grand canonical formalism was used. It is shown that the formation energy of aluminum vacancies decreases by approximately 1.3 eV with increasing its content in the series of alloys Ti3Al–TiAl–TiAl3, whereas the formation energy of titanium vacancies changes insignificantly. In general, the formation of titanium vacancies in these alloys is preferred to the formation of aluminum vacancies. It has been found that Nb, Mo, Tc, Ru, Rh, and Pd impurities on the aluminum sublattice contribute to an increase in the formation energy of an aluminum vacancies, and Mo and Tc also lead to an increase in the formation energy of titanium vacancies. All 4d impurities, if they substitute for titanium, reduce the formation energy of aluminum vacancies, and Nb and Mo are favorable for increasing the formation energy of titanium vacancies. The influence of impurities on the chemical bond in the γ-TiAl alloy and its elastic moduli and characteristics based on them is discussed.

Structure and magnetic properties of a Ni3(Al, Fe, Cr) single crystal subjected to high-temperature deformation

The structure and magnetic properties of the Ni3(Al, Fe, Cr) single crystal subjected to high-temperature tensile deformation to failure at 850–900°C have been studied. No recrystallized grains and metastable phases were found. The rupture zone of the alloy subjected to deformation (at 900°C) to the highest degree demonstrates the fragmentation accompanied by rotation of atomic layers and changes of the chemical composition in the nickel and aluminum sublattices. Magnetic studies of the alloy have shown the existence of two Curie temperatures for samples cut from the rupture zone. Samples cut away from the rupture zone exhibit no additional magnetic transitions; twines and planar stacking faults in the alloy structure. The alloy deformed to the lower degree of deformation (at 850°C) also demonstrates twins; no ferromagnetic state was found to form.

Energy Localization in the Ordered Condensed Systems: A 3 B Alloys With L12 Superstructure

Energy localization in the ordered condensed systems is analyzed using А3В alloys with L12 superstructure. Using molecular dynamics, possible localization of phonon vibration energy in the aluminum sublattice and generation of a localized mode oscillating at a frequency belonging to the phonon spectrum energy gap and not representing a breather is considered for the ordered Pt3Al and Ni3Al alloys with L12 superstructure, which possess translational symmetry. Manifestations of anharmonicity in the atomic interactions and the relevant features of energy redistribution between sublattices of the ordered alloys are investigated. It is found out that in the ordered alloys of an A3B stoichiometry with L12 superstructure, whose phonon spectrum has a forbidden energy gap, vibrational energy can localize on the sublattice of B atoms. The alloys, found in a state of thermodynamic equilibrium at a certain temperature, are characterized by a local effect of spontaneous energy localization (the effect of energy redistribution between the lattices).

Combined atom probe tomography and first-principles calculations for studying atomistic interactions between tungsten and tantalum in nickel-based alloys

We investigate the partitioning behavior of tungsten to the γ(face-centered cubic) and γ′(L12) phases in Ni-based alloys employing atom probe tomography (APT), first-principles calculations and computational thermodynamics. Several APT studies of Ni-based alloys indicate that the partitioning of tungsten atoms to the γ′(L12) phase is reversed in favor of the γ phase due to tantalum atom additions. First-principles calculations of substitutional formation energies at 0K indicate that tungsten and tantalum atoms share the aluminum sublattice sites of the γ′(L12) phase, and that tantalum has a larger tendency to partition to the γ′(L12) phase than does W. We also calculate the binding energies of W–W and Ta–W dimers in the γ(fcc) and γ′(L12) phases, respectively, and use these values in a quantitative model to demonstrate that interatomic interactions between tungsten and tantalum atoms play a significant role in the partitioning reversal of tungsten.

A Methodology for Determination of γ′ Site Occupancies in Nickel Superalloys Using Atom Probe Tomography and X-ray Diffraction

A methodology for determining the preferred site occupancy of various alloying elements within ordered γ′ precipitates was developed and applied to Rene88 samples. The method utilized atom probe tomography and X-ray diffraction techniques with controlled monochromated synchrotron beams to determine element positions. Samples were solutionized at 1423 K (1150 °C) for 30 minutes and cooled at 24 K/min with subsequent aging at 1033 K (760 °C). The synchrotron X-ray diffraction results indicate that niobium prefers to reside on the aluminum sublattice site of the γ′ phase. Additionally, the results indicate that chromium prefers the nickel sublattice sites, while cobalt is likely to occupy both the aluminum and nickel sublattice sites. The X-ray results on the chromium occupancy disagree with atom probe results from the same alloy that indicate that chromium prefers the aluminum sublattice sites.

Site occupancy of chromium in the γ′-Ni3Al phase of nickel-based superalloys: a combined 3D atom probe and first-principles study

Transition-metal dopants play a critical role in the high-temperature mechanical strength and corrosion resistance of nickel-based superalloys. In this article, the site occupancy behavior of chromium in γ′-Ni3Al has been investigated by combining three-dimensional (3D) atom probe and high-resolution transmission electron microscopy characterizations with ab initio density functional theory (DFT) calculations. The 3D atom probe data show a clear preference of chromium on the aluminum sublattice over the nickel sublattice in Rene88 super alloys. First-principles DFT total-energy calculations were performed to understand the site occupancy of chromium in the L12 structured γ-Ni3Al. The obtained chromium site preference energies have been compared using the anti-site and vacancy-based substitution formation mechanism, as well as using the standard defect formation formalism. It was found that chromium prefers aluminum site, consistent with the 3D atom probe result. In addition, interaction energies between two chromium atoms have also been determined from first-principles calculations. Our results show that chromium atoms prefer to be close by on either nickel or aluminum sublattices or on a nickel–aluminum mixed lattice, suggesting a potential tendency of chromium segregation in the γ′ phase.

Structural disorder in sapphire induced by 90.3 MeV xenon ions

In our previous work [15], we have evidenced, using RBS-C, two effects in the aluminium sublattice of sapphire irradiated with 90.3 MeV xenon ions: a partial disorder creation that saturates at ∼40% followed above a threshold fluence by a highly disordered layer appearing behind the surface. In this work, by RBS-C analysis of the oxygen sublattice, we have observed only one regime of partial disorder creation that saturates at ∼60% in tracks of cross-section double of that found for the aluminium sublattice. Complementary analysis by X-ray diffraction shows that the lattice strain increases with the fluence until a maximum is reached about 7.5 × 10 12 ions/cm 2. For higher fluences, strain decreases first indicating a little stress relaxation in the material and tends afterwards, to remain constant. This stress relaxation is found to be related to the aluminium sublattice high disorder.

Atomic structure of a stripe phase onAl2O3∕Ni3Al(111)revealed by scanning force microscopy

Received 26 March 2007; revised manuscript received 3 July 2007; published 30 July 2007 The most remarkable feature of the ultrathin aluminum oxide film grown on Ni3Al111 as reported in the literature is its surface reconstruction resulting in a dot structure with a large rhombic surface unit cell. Here, we demonstrate that this is the reconstruction of the dominant phase of the oxide film, while 5%‐20% of the surface area may be covered by another reconstruction that is characterized by zigzag features arranged in parallel stripes. When investigated with scanning tunneling microscopy, this stripe phase appears to be very different from the dominant phase; however, highest resolution dynamic scanning force microscopy operated in the noncontact mode reveals great similarity of the atomic structures. Both phases consist of a modulated hexagonal lattice with 0.51 nm periodicity resembling the aluminum sublattice of the Ni3Al111 substrate.

Site Preference Occupation of Ni and V in Fe3Al-Based Alloys

Nonempirical study of the site preference occupation for Ni and V substituting in Fe3Al has been carried out in the framework of the coherent potential approximation. Obtained values of total energies show in a full agreement with experiments that Ni atoms in the equilibrium configuration occupy the iron sub-lattice for alloying with 5 at % of Ni in the Fe3Al-based alloy. Calculations of alloys with the V-doped iron aluminide in the D03 phase show differences in bonding and site occupation preferences in comparison with Ni doping. V atoms occupy aluminum sublattice.

Anisotropic X-ray diffraction peak broadening and twinning in diaspore-derived corundum

Corundum produced by dehydration of diaspore at temperatures between 400 and 1000 °C was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The broadening of XRD peaks and the size of corundum twin domains are related to the dehydration temperature. At low dehydration temperatures (<450 °C) as well as at high temperatures (>700 °C) large twin domains are obtained which yield sharp XRD peaks for all reflections. Dehydration between 450 and 600 °C leads to twin domains smaller than 10 nm. In this temperature range, significant peak broadening is observed, however, only for reflections, which structure factors are dominated by the aluminum sublattice and which are not common to both twin variants of corundum. The experimental results show that XRD peak broadening is only caused by fine twinning. Porosity in corundum as a reason for XRD peak broadening is excluded: the lamellar pore system is well ordered in corundum produced at low temperatures, however narrow peaks are observed. The reasons for the development of twin domains with differing sizes at different dehydration temperatures are discussed in detail.

Magnetism-induced solid solution effects in intermetallic Ni 60Al 40

In this study, experimental work on solute effects in an intermetallic alloy, NiAl, has been complemented by first-principles quantum mechanical calculations to investigate the effect of iron and cobalt solutes on lattice parameter and hardening. Ternary additions of iron and cobalt with similar atomic sizes were added to replace nickel in NiAl containing 40% Al. Cobalt solutes did not affect the lattice parameter or the hardening behavior of NiAl alloys. Iron solutes, on the other hand, substantially expanded the lattice, resulting in unusual solid solution softening. These results could not be explained from a consideration of the size mismatch based on the Goldschmidt radii for iron and cobalt atoms. From the contrasting behavior of the iron and cobalt solutes, magnetic interactions induced by iron atoms located on the aluminum sublattice have been identified as a new factor responsible for the unusual large lattice dilation and resultant solid solution softening in these intermetallic alloys.

Atom Probe Field Ion Microscopy Investigations on Antiphase Boundaries and Super Dislocations in NiAl Alloyed with Chromium

Atom probe field ion microscopy (APFIM) investigations on antiphase boundaries and super dislocations in B2 ordered NiAl alloys containing 2 at% Cr have been performed. An a/2(111){123} antiphase boundary was detected in the nickel-rich Ni 50 Al 48 Cr 2 alloy which possessed segregation of Cr atoms in higher local concentration. Cr atoms show a strong site preference for the aluminium sublattice of NiAl. Therefore, Cr atoms are substituting for Al atoms. The Gibbsian interfacial excess is calculated. Super dislocations have been imaged without any significant Cr enrichment. No dissociation into partial dislocations was detected due to the high antiphase boundary energy. The results are discussed in view of literature data.

Site-distributions of Fe alloying additions to B2-ordered NiAl

The site-distributions of Fe alloying additions to B2-ordered NiAl alloys have been measured by ALCHEMI (atom location by channeling-enhanced microanalysis). Twelve alloy compositions were examined, of stoichiometries Ni50−xAl50Fex (Ni-deficient), Ni50Al50−xFex (Al-deficient), and Ni50−x/2Al50−x/2Fex (intermediate), with four different alloying levels, x=0.25, 2, 5, and 10. The data indicate that Fe tends to act as a buffer between the two sublattices, preferentially occupying the site of the stoichiometrically deficient host element. However, the Fe alloying addition was found to partition between the two sublattices to some extent in all alloys, so that ‘Ni’-site vacancies or Ni anti-site defects are required to maintain the site balance. The observed trends in the site-distributions suggest that the range of compositions reported by Darolia et al. (Darolia R, Lahrman DF, Field RD. Scripta Metall Mater 1992;26:1007). to exhibit enhanced ductility may coincide with an inversion in the site-substitution behavior, where more Ni than Fe occupies the aluminum sublattice.

Doping of sapphire single crystals with 111 In and 111 Cd detected by perturbed angular correlation

The electric hyperfine interaction of ion beam implanted 111In and 111Cd probe atoms in sapphire (Al2O3) single crystals has been investigated using perturbed angular correlation spectroscopy. For both probe atoms the same distinctive electric field gradient was found, indicating that nearly all the implanted probe atoms form a stable substitutional configuration in the temperature range between 77 K and 873 K on the aluminum sublattice. A comparative study between 111In and 111Cd-measurements points to a dynamic interaction initiated by the electron-capture of 111In(EC)111Cd similar to In2O3 and La2O3. Size and orientation of the EFG are discussed in comparison to experimental results in Cr2O3 single crystals.

Relationship between the dehydration temperature of Boehmite and the imperfection structure of γ-alumina

The relationship between the imperfection structure of γ-alumina formed by dehydration of Boehmite and the dehydration temperatures is established by analysing the integral width of X-ray reflections. Characterization of the imperfection structure is by calculation of average sizes of the aluminum and oxygen sublattices, and the lattice strains. Two types of stacking faults are introduced to explain the imperfection structure. One type of stacking fault appears at low dehydration temperatures and has more influence on the aluminum sublattice, the other formed at higher dehydration temperatures affects the oxygen sublattice.

Oxygen incorporation in aluminum nitride via extended defects: Part II. Structure of curved inversion domain boundaries and defect formation

Three distinct morphologies of curved (curved, facetted, and corrugated) inversion domain boundaries (IDB's), observed in aluminum nitride, have been investigated using conventional transmission electron microscopy, convergent beam electron diffraction, high-resolution transmission electron microscopy, analytical electron microscopy, and atomistic computer simulations. The interfacial structure and chemistry of the curved and facetted defects have been studied, and based upon the experimental evidence, a single model has been proposed for the curved IDB which is consistent with all three observed morphologies. The interface model comprises a continuous nitrogen sublattice, with the aluminum sublattice being displaced across a {1011} plane, and having a displacement vector R = 0.23〈0001〉. This displacement translates the aluminum sublattice from upwardly pointing to downwardly pointing tetrahedral sites, or vice versa, in the wurtzite structure. The measured value of the displacement vector is between 0.05〈0001〉 and 0.43〈0001〉; the variation is believed to be due to local changes in chemistry. This is supported by atomistic calculations which indicate that the interface is most stable when both aluminum vacancies and oxygen ions are present at the interface, and that the interface energy is independent of displacement vector in the range of 0.05〈0001〉 to 0.35〈0001〉. The curved IDB's form as a result of nonstoichiometry within the crystal. The choice of curved IDB morphology is believed to be controlled by local changes in chemistry, nonstoichiometry at the interface, and proximity to other planar IDB's (the last reason is explained in Part III). A number of possible formation mechanisms are discussed for both planar and curved IDB's. The Burgers vector for the dislocation present at the intersection of the planar and curved IDB's was determined to be b = 1/3〈1010〉 + t〈0001〉, where tmeas = 0.157 and tcalc = 0.164.

Structure of the Σ=5 (310) interface in NiAl

The atomic structure of the Σ=5 [001] (310) grain boundary in NiAl was examined by high resolution electron microscopy and multislice image simulation. As in most other intermetallic compounds, the grain boundaries in NiAl are intrinsically brittle at low temperatures. Although there have been few studies on this alloy, the energies of NiAl grain boundaries have been calculated using embedded atom potentials for both stoichiometric and non-stoichiometric structures. These studies are consistent with the results of Bradley and Taylor, which indicate that nickel-rich compositions result from nickel antisite defects on the aluminum sublattice, while aluminum-rich compositions produce constitutional vacancies on the nickel sublattice, and with recent field ion microscopy results on nickel-rich alloys.The Σ=5 grain boundary was prepared by diffusion bonding at 1000 °C. A JEOL 4000EX was used for HREM imaging and the NUMIS multislice simulation program was used to simulated images. Analysis of these images considered the effects of grain boundary expansion, rigid body displacements along the boundary, grain boundary stoichiometry, and point defects at the boundary.