Al Sublattice Research Articles

Ab initioCalculations of Elastic Constants of Superalloys

Using the-state-of-the-art ab initio method we have studied elastic constants of alloys potentially interesting for high temperature applications. We have shown that Cr substitutes the Al sublattice in B2 NiAl at concentration up to 40 at. %, but at higher Cr content it prefers the Ni-sublattice. Alloying of NiAl with Cr yields reduced strength but improves the ductility of the alloys. Alloying of NiAl with W which substitutes the Al sublattice, leads to a strong decrease of the shear modulus, and near 50 at. % of W the alloy becomes mechanically unstable as elastic constant C' is negative. This is in agreement with our phonon calculations where we found soft modes along the [110] direction for B2 NiW. According to our calculations in (Ru,Ni)Al alloys the shear modulus is almost constant up to 40 at. % of Ni, at higher Ni concentrations it is drastically reduced. We have shown that the changes in elastic properties of (Ru–X)Al alloys are due to electronic topological transitions.

Optimization Conditions Research of Dual-Alkali FGD Process in a Wet Scrubber

Based on the microscope phase-field model，the evolution of atom morphology, the long range order(lro) parameter and concentration can be gotten, and atomic clustering and ordering during the precipitation process of Ni-Cr-Al alloy could be obtained. The Ni-14at.%Cr-15.5at.%Al alloy is studied and the temperature of precipitation are 973K. It was showed that the ordering of both Al and Cr atoms take place simultaneously during the precipitation process of Ni-Al-Cr alloy, Cr atoms transfer to the boundaries of L12 phases, the domain of rich Cr atoms are formed. At the boundaries of L12 phases, Cr atoms may substitute the Al sublattice, and the D022 phases are formed.

Computer Simulation of Evolution of Ordering Phases and the Boundaries of Ni-Cr-Al Alloy

Based on the microscope phase-field model，the evolution of atom morphology, the long range order(lro) parameter and concentration can be gotten, and atomic clustering and ordering during the precipitation process of Ni-Cr-Al alloy could be obtained. The Ni-14at.%Cr-15.5at.%Al alloy is studied and the temperature of precipitation are 973K. It was showed that the ordering of both Al and Cr atoms take place simultaneously during the precipitation process of Ni-Al-Cr alloy, Cr atoms transfer to the boundaries of L12phases, the domain of rich Cr atoms are formed. At the boundaries of L12phases, Cr atoms may substitute the Al sublattice, and the D022phases are formed.

The effects of the dangling bond on the electronic and magnetic properties of AlN nanoribbon

Though AlN nanoribbon (AlNNR) with H atom termination is a nonmagnetic semiconductor, the dangling bonds cause magnetic semimetal for zigzag edged AlNNR (ZAlNNR) with bare N edge, magnetic metal for ZAlNNR with bare Al edge or bare N and Al edges, magnetic semiconductor for armchair edged AlNNR (AAlNNR) with bare N edge or bare Al edge. Due to the strong coupling of the dangling bonds of dimeric N and Al atoms at the same edge, the AAlNNR with bare N and Al edges is still a nonmagnetic semiconductor. The magnetic moment of ZAlNNR with either bare N edge or bare Al edge is nearly half of the magnetic moment of AAlNNR with either bare N edge or bare Al edge due to its half number of the dangling bonds in per unit cell with respect to AAlNNR cases. The larger difference charge density between the majority spin and minority spin for edge bare N atoms and decaying for N sub-lattices away from edge, as well as the smaller difference charge density for edge bare Al atoms and without decaying for Al sub-lattices away from edge indicate the magnetic moment of both ZAlNNR and AAlNNR with bare N edge is larger than that of them with bare Al edge. The dangling bonds induce both ZAlNNR and AAlNNR with bare N edge a completely (100%) spin polarization at the Fermi level, indicating such structures can be utilized to construct efficient spin-polarized transport devices.

Effect of Fe ternary addition on ductility of NiAl intermetallic alloy

Mechanical and physical properties of NiAl intermetallics with B2 structure are of great technological interest for high temperature applications. However, NiAl is brittle at low temperature, which limits the practical applications in industry. Recently, it was reported that an addition of Fe on the Al sublattice results in an unusual solid softening effect in NiAl. The mechanism of the solid softening effect remains unclear. In this paper, the solid softening effects of ternary addition of Fe in NiAl were investigated by both experimental work and theoretical calculations. The results show that addition of 6–10 at.% Fe on the Al sublattice enhances the ductility of NiAl markedly. The compressive plasticity increases from 0.75 % in Ni60Al40 to 19.5 % in Ni50Al40Fe10 and the compressive fracture strength from 900 MPa to 1180 MPa. In contrast, the yield strength decreases from 873 MPa in Ni60Al40 to 473 MPa in Ni50Al40Fe10. Furthermore, elastic and slip properties of Ni(Al,Fe) intermetallic alloys were calculated by first-principles and the solid softening effect in NiAl was discussed with the theoretical results.

Microscopic phase-field modeling of atomic anti-site behaviors in precipitation progress of Ni 3(AlFe)

The effects of temperature on atomic anti-site behaviors in L1 2-Ni 3(AlFe) phases were studied using microscopic phase-field dynamic model in precipitation progress of Ni 75Al 20Fe 5 alloy. The results show that with the increase of temperature, the formation of Ni Al and Al Ni anti-sites is much easier in Ni 3(AlFe), and Ni and Al anti-site atoms show clearly stronger temperature-dependent than Fe anti-site atoms. The evolution progress of anti-site atoms is completed at the initial growth stage of L1 2-Ni 3(AlFe) phases. The site occupation probabilities of Ni atoms on the sublattice A (Ni Ni, face centers sites of FCC), and Al and Fe atoms on the sublattice B (Al Al and Fe Al, corners sites of FCC) all present the degressive tendency with the temperature increasing. Fe atoms mainly prefer to occupy the Al sublattice at the whole temperature range.

Magnetism-driven dislocation dissociation, cross slip, and mobility in NiAl

We have examined the interplay between magnetism and dislocation core properties in NiAl alloys using quantum mechanics/molecular mechanics simulations. We observe the magnetism-driven site preference of Fe and Co impurities at the dislocation core. When occupying the Ni sublattice, Fe and Co impurities are found to induce spontaneous dislocation cross slip. When occupying the Al sublattice, the impurities render the originally undissociated dislocation to split into two partials. Finally we observe the magnetism-driven dislocation mobility when Fe impurity occupies the Al sublattice, which has also been discovered experimentally. The magnetic interaction between the impurities and the host atoms is responsible for the significant changes in the dislocation core structure and mobility.

Structure-Induced Covalent Bonding in Al-Li Compounds

Formation mechanism of a deep pseudogap in the electronic density of states of the Al-Li Bergman and Zintl compounds is discussed with an emphasis on the differences among isostructural Al-Mg compounds. Since Li scatters electrons very weakly in comparison with Al and Mg, the potential landscape for electrons in Al-Li compounds is not that of the entire close-packed structure but that of the Al sublattice, which is a rather porous network like the diamond lattice. The porous network structure realized by the chemical decoration of close-packed structures enhances the covalent nature of electronic structures, hence the deep pseudogap in the electronic density of states. A concept of structure-induced covalent bonding in a network realized by the chemical decoration of close-packed structures may provide a novel picture in the electronic structures of complex intermetallic compounds.

Effects of alloying elements on elastic properties of Ni 3Al by first-principles calculations

The effects of alloying elements (Cr, Hf, Pt, Y and Zr) on the elastic constants of Ni 3Al have been investigated using first-principles calculations within the generalized gradient approximation. The results are compared with the theoretical and experimental data in the literature and analyzed based on volume changes, electron density and pure alloying element's properties. The calculated enthalpies of formation are used to determine the site preference. It is found that the shear modulus decreases with the additions of alloying elements due to the expansion of the equilibrium volume, wherein Y has the largest effect. It is also observed that the bulk modulus of Ni 3Al alloys is strongly related to the total molar volume and electron density, and that the solute atom with high bulk modulus tends to give high bulk modulus of Ni 3Al. Based on the calculated results, the change rates of elastic constants due to each alloying element in Ni 3Al have been obtained. Our calculations show that Hf prefers the Al sublattice with a negative change rate of the Young's modulus of Ni 3Al, agreeing well with measurements. The predicted Young's modulus of IC 218 alloy shows a good agreement with available experimental data.

First-principles study on the site preference of Dy in B2 NiAl

Abstract The site preference behavior of Dy in B2 NiAl was investigated by a combination of the first-principles and statistical–mechanical Wagner–Schottky method. At T = 0 K, the site occupation of Dy in B2 NiAl is highly stoichiometric-dependent, whereas, Dy showing a consistent preference for the Al sublattices in three types of B2 NiAl at temperatures higher than 300 K. The density of states (DOS) and Mulliken bond population calculations show that the interactions between Al (s, p) electrons and Ni (d) electrons are much stronger in the Dy-doped NiAl than those in pure NiAl, which could be responsible for the improvement of mechanical properties of the Dy-doped NiAl.

Crystal structure and magnetic properties of YbZn 8.5Al 2.5 compound

Details of the crystal structure of new aluminide YbZn 8.5Al 2.5 (BaHg 11-type structure, SG Pm 3 ¯ m, a = 0.845202(6) nm, R I = 0.071, R P = 0.083) have been investigated by X-ray methods. Basic characterization by magnetic methods and heat capacity measurement reveals a lack of magnetic order. The Curie behaviour of susceptibility with reduced effective moments can be explained by low concentration of Yb ions in the 4 f 13 state. It can be understood considering a statistical occupancy of the Zn–Al sublattice, forming different types of environments of Yb.

Substitution behavior of Si in Al3Ti (D022): a first-principles study

A first-principles total-energy plane-wave pseudopotential method based on densityfunctional theory has been used to investigate the site preference of Si inAl3Ti. The site preferenceparameter S has beencalculated and the value of S decreases on increasing Si concentration, but is always greater than 1 whilethe Si concentration is lower than 25 at.%, indicating Si has a strong sitepreference for the Al sublattice. The physical origin of the site preference of Si inAl3Ti was studied by calculating the densities of states (DOSs) of theAl3Ti–Si systems. The heat of formation and the DOS of the position of theFermi energy level were also studied to analyze the site preference of Si inAl3Ti.

Electron energy losses by channelling-enhanced microanalysis in a γ′-strengthened Ni-base alloy with Ti additions

The addition of Ti to a γ′-strengthened Ni-base model alloy shows a strong preference of Ti for the γ′-phase. Ti strongly influences the diffusion behaviour of constituents of the Ni 3Al (γ′) phase, although the mechanism is uncertain. Electron energy loss channelling experiments were therefore performed to determine the electronic structure in the vicinity of the Ti atom. The experimental results showed an increase in the Ti L 3/L 2 ratio when probing the Ni sublattice planes compared to the Al sublattice planes, indicating a strong charge transfer from the Ti to the Ni atoms. This finding was supported by first-principles calculations, indicating an increase in electron density in the vicinity of the Ti atom and a strengthening of the chemical bonds.

Antisite Defects of the L12 Structure Determined by the Phase Field Microelasticity Model

A phase field microelasticity simulation is performed to examine the antisite defect of L12-Ni3Al in Ni75 Al5.3 V19.7 ternary alloy. Combinimg strain energy with the phase field model leads to an atom configuration change as time proceeds. For the Ni sublattice, the antisite defect AlNi, the equilibrium occupancy probability (OP) of which declines, precedes NiNi and VNi in reaching equilibrium; subsequently, NiNi and VNi present a phenomenon of symmetrical rise and decline individually. Similarly, for the Al sublattice, the antisite defect NiAl, the OP of which eventually rises, takes fewer time steps than AlAi and VAl to attain equilibrium. Thereafter, AlAl rises while VAl declines symmetrically at the axes of the NiAlMAI curve. Furthermore, the OP for the Al sublattice is much more sensitive to strain energy than that for the Ni sublattice.

Site-preferences and local spin-polarization of transition metal solute atoms in B2 type Ni–Al alloys

In the framework of the Local Spin Density Approximation we study the electronic structure, site preference energies and magnetism in B2 Ni 50(Al 37.5Ni 12.5) alloys doped with 3d-transition metals using a Coherent Potential Approximation for treating effects of substitutional disorder. We find that the lattice expands also for non-magnetic Ti and V substitutions and thus this effect cannot entirely be related to magnetism as it was conjectured earlier. Except Co all TM atoms are found to prefer the Al sublattice. For the magnetic substitutions (Cr, Fe, Mn) we also find that the lattice expands independently on the state of magnetic order – only local atomic spin-polarization is important. An effective magnetic coupling between substituted atoms has been calculated and it is shown that it does not scale with the magnitude of the local moments.

Raman scattering of indium-richAlxIn1−xN: Unexpected two-mode behavior ofA1(LO)

${\text{Al}}_{x}{\text{In}}_{1\ensuremath{-}x}\text{N}$ films $(0.03\ensuremath{\le}x\ensuremath{\le}0.80)$, particularly indium-rich AlInN films, are studied by Raman scattering. We clearly observe a two-mode behavior of ${\text{A}}_{1}(\text{LO})$ phonon and prove that the previous theoretical prediction and experimental attribution of InN-like ${\text{A}}_{1}(\text{LO})$ to ${E}_{2}^{\text{H}}$ mode are incorrect. Using the modified random-element isodisplacement model of Chang and Mitra, the information about AlInN lattice vibration is extracted. We believe that these results imply a strong positive force interaction between In and Al sublattices in AlInN. The strong interaction makes AlInN different with AlGaN in ${\text{A}}_{1}(\text{LO})$ phonon mode behavior.

The partitioning and site preference of rhenium or ruthenium in model nickel-based superalloys: An atom-probe tomographic and first-principles study

The partitioning behavior and sublattice site preference of Re or Ru in the Ni3Al (L12) γ′- precipitates of model Ni–Al–Cr alloys are investigated by atom-probe tomography (APT) and first-principles calculations. Rhenium and Ru are experimentally observed to partition to the γ(fcc)-phase, which is consistent with the smaller values of the γ-matrix Re and Ru substitutional formation energies determined by first-principles calculations. APT measurements of the γ′-precipitate composition indicate that Re and Ru occupy the Al sublattice sites of the Ni3Al (L12) phase. The preferential site substitution of Re and Ru at Al sublattice sites is confirmed by first-principles calculations.

Microscopic phase-field simulation of Cr atom substitution character during formation of Ll2 and DO22 phases in Ni-Cr-Al alloy

The simulations of Cr atom substitution character during the formation of L1(subscript 2) and DO22 phases in Ni-Cr-Al alloy were performed at 873 K based on microscopic phase-field model. It is found that the substitution of Cr is affected by Cr and Al contents and limits of occupation probabilities of Cr atom in L1(subscript 2) phase are present. The precipitate is single L1(subscript 2) phase when the component is less than the limit, Cr atoms substitute the Al sublattices in L1(subscript 2) phase, and both of atoms Al and Cr occupy the β-sites and complex phases Ni3(Al(subscript 1-x)Cr(subscript x)) are formed; Cr atoms enter Ni sites when Al and Cr contents exceed the limit, and substitute β-sites or both of α- and β-sites. The DO22 phase is formed at the boundary of L1(subscript 2) phase.

First-principles study of site occupancy of dilute 3d, 4d and 5d transition metal solutes in L1 0 TiAl

Using a statistical-mechanical Wagner–Schottky model parametrized by first-principles density-functional (DFT-GGA) calculations on 32-atom supercells, we predict the lattice site occupancy of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition-metal elements in L1 0 TiAl intermetallic compound as a function of both alloy composition and temperature. The effects of local atomic relaxations, anisotropic lattice distortions, as well as magnetism on point defect energetics are fully taken into account. Our calculations show that, at all alloy compositions and temperatures, Zr and Hf consistently show a preference for the Ti sublattice, while Co, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt and Au consistently show a preference for the Al sublattice. In contrast, the site preference of V, Cr, Mn, Fe, Ni, Cu, Nb, Mo, Tc, Ta and W strongly depend on both alloy stoichiometry and temperature. Our calculated results compare favorably with the existing theoretical and experimental studies in the literature.

Chromium and tantalum site substitution patterns in Ni3Al(L12) γ′-precipitates

The site substitution behavior of Cr and Ta in the Ni3Al(L12)-type γ′-precipitates of a Ni–Al–Cr–Ta alloy is investigated by atom-probe tomography (APT) and first-principles calculations. Measurements of the γ′-phase composition by APT suggest that Al, Cr, and Ta share the Al sublattice sites of the γ′-precipitates. The calculated substitutional energies of the solute atoms at the Ni and Al sublattice sites indicate that Ta has a strong preference for the Al sites, while Cr has a weak Al site preference. Furthermore, Ta is shown to replace Cr at the Al sublattice sites of the γ′-precipitates, altering the elemental phase partitioning behavior of the Ni–Al–Cr–Ta alloy.