First-Principles Calculations of Defect Structures in B2 AlCo and GaCo

First-principles electronic structure calculations have been performed for defect structures in nonstoichiometric B2 AlCo and GaCo. To determine the type of constitutional defects, the compositional dependence of the energy of formation and lattice parameter was obtained by calculations employing supercells of various sizes (16 and 32 atoms) as well as special quasirandom structures (SQSs) developed for random pseudobinary A_1?x B _x C with compositions x = 0.25 and 0.5. According to the results, Co vacancies are the constitutional point defects in the Al-rich side of both B2 AlCo and B2 GaCo, while Co vacancies present the minimum energy for the Ga-rich side. For the Co-rich side of both B2 AlCo and B2 GaCo, the Co antisite is the most stable defect. To investigate the thermal defect concentrations at finite temperature, we adopted the Wagner–Schottky model using enthalpies of formation of point defects obtained from the SQS approach. The present results suggest that the predominant thermal defects in AlCo are of complex type whereas for GaCo they are of interbranche Co type. The results of these calculations show agreement with available theoretical and experimental data.     

Vertical section phase diagrams of La−Fe−B ternary system

The vertical sections of the La?Fe?B system were investigated using electron probe microanalysis and differential thermal analysis. Based on the microstructures and phase compositions of the as-cast and equilibrium alloys, together with their heat flow?temperature curves, phase diagrams for three vertical sections were drawn: La_xFe₈₂B_y (x+y=18), La_xFe₇₀B_y (x+y=30) and La_xFe₅₃B_y (x+y=47), where x and y represent mass fraction of La and B, respectively, % . Additionally, according to the phase diagrams, the compound La₂Fe₁₄B was identified as a stable phase at high temperatures. It was found to be stable between 926.2 and 792.6 °C; at low temperatures, however, it decomposed into α-La, α-Fe and LaFe₄B₄, according to the reaction La₂Fe₁₄B→α-Fe+α-La+LaFe₄B₄.     

Half-metallic full-Heusler compound Ti2NiAl: A first-principles study

Full-Heusler compound Ti₂NiAl with Hg₂CuTi-type structure is demonstrated to be a new half-meltal ferromagnet by first-principles calculations. The compound has a complete (100%) spin polarization around the Fermi level in the total density of state. The band structure calculations show that the majority spin is strongly metallic, while the minority spin shows an insulating behavior. The compound has a total magnetic moment of −3.0 μ_B per formula on first-principles calculations which complies well with the Slater-Pauling (SP) rule. Though having different atomic surroundings, the profiles of atom-projected density of states of Ti(A) and Ti(B) are similar. The half-metallic character is retained when the lattice constant ranging from −12.8% to +4.9%.     

Studying charge ordering and parameters of exchange interaction in NaxCoO2

Results of first-principles calculations of the band structure of Na_xCoO₂ (x=0.33 and 0.61) are presented. In Na_0.33CoO₂, charge ordering of Co³⁺-Co⁴⁺ ions has been obtained. Ferromagnetic and antiferromagnetic orderings have been considered. Magnitudes of trigonal splitting of the t _2g band of Co and intersite hopping integrals have been determined in the basis of Wannier functions. Magnitudes of the parameter of exchange interaction have been calculated in terms of the Heisenberg model.     

Modeling of phase stability of the fcc phases in the Ni–Ir–Al system using the cluster/site approximation method coupling with first-principles calculations

A combined CSA (cluster/site approximation)/FP (first-principles) calculation approach was employed to investigate the phase stability and thermodynamic properties of the face-centered cubic (fcc) phases of the Ni–Ir–Al system. For the constituent binaries of the Ni–Ir–Al system, enthalpies of formation of the Ni_xIr_1−x, Ni_xAl_1−x and Ir_xAl_1−x fcc compounds were calculated by first-principles approach at x = 0.75, 0.5 and 0.25 at 0 K, respectively. The pair exchange energies of the Ni–Ir and Al–Ir systems in the CSA model were obtained from FP calculated enthalpies of formation, while those for the Ni–Al binary were adopted from previous work. Thermodynamic model parameters of the fcc phases for the Ni–Ir–Al ternary system were then obtained from the constituent binaries via extrapolation. The calculated isothermal section at 1573 K is in good agreement with the experimental data within the uncertainties of the calculations and experiments.     

Sintering of BaZrO3 and SrZrO3 perovskites: Role of substitutions by yttrium or ytterbium

The modification of perovskite sintering oxides A^IIB^IVO₃ was studied. The alkaline earth metal plays a role in the densification of perovskite. The effect of the substitution of Zr⁴⁺ by a lanthanide Yb³⁺ in barium zirconate (BZYb) increases its density and impedes BaO evaporation independently of the thermal cycle. A substitution of Zr⁴⁺ by Y³⁺ increases the density of barium zirconate (BZ) only after 4 h at 1650 °C. In all cases, the substitutions of Zr⁴⁺ by Y³⁺or Yb^{2+ or 3+} in the zirconate inhibit grain growth. The substitution in B site by a Ln³⁺ leads to the creation of vacancies (A^IIB^IV_(1−x)Ln^III_x O_3−x/2). To determine the vacancies’ role, three different controlled atmospheres were tested. Argon, dry air, and oxygen were used to study the impact on the linear shrinkage of barium zirconate when some Zr⁴⁺ are replaced by yttrium or ytterbium (BZY, BZYb) or not substituted (BZ). Strontium zirconate (SZ) is not as chemically stable as barium zirconate (BZ) during the thermal cycle when Yb is used as a substitution element. In effect, the presence of the secondary phase Zr₃Yb₄O₁₂ associated with the release of oxygen proves the bivalence of ytterbium (Yb²⁺ or Yb³⁺). Therefore, ytterbium can be substituted in either A or/and B sites. The study of reactive sintering between lanthanum strontium zirconate (SZYb) and Yb₂O₃ shows that a slight excess of Yb₂O₃ around SZYb grains stabilizes the chemical composition of SZYb. Perovskite symmetry and stability were determined by two factors, i.e., tolerance (t) and octahedral factors (r_B/r_O). It was demonstrated that the distortion of the perovskite unit cell increases the density of pellets.     

In-Situ (TiB+TiC) particulate reinforced titanium matrix composites: Effect of B4C size and content

This study investigated the microstructure and tensile behavior of (TiB+TiC) reinforced titanium matrix composites (TMCs) using an in-situ reaction between Ti and B₄C. Different B4C sizes (1,500 and 150 μm) and contents (0.94, 1.88 and 3.76 mass%) were added to pure Ti to produce 5, 10, and 20 vol% (TiB+TiC) reinforced TMCs. In-situ synthesized TiB and TiC reinforcements prepared with 150 μm B₄C were very fine, and were distributed more homogeneously than the 1,500 μm B₄C. As the TiB and TiC contents increased, the tensile strength increased and the ductility decreased compared to unreinforced pure Ti. The improvements in the tensile strength of TMCs were obtained by load transfer strengthening and an alpha-Ti matrix grain reduction of 9–26%. In addition, the TMCs produced using 150 μm B₄C showed a greater tensile elongation of approximately 61–117%, with a slightly improved strength compared to that with 1,500 μm B₄C. The tensile elongation of TMCs obtained with 150 μm B₄C was enhanced because the coarse reinforcements produced by 1,500 μm B₄C were more easily and frequently cracked at the fracture surface.     

Mechanical properties and chemical bonding of the Os–B system: A first-principles study

The mechanical properties of Os–B compounds containing different boron contents have been investigated systemically by first-principles calculations. Two previously unreported crystal structures of Os₂B₅ and OsB₃, crystallizing in space groups R3m and P-6m2 respectively, are determined using the ab initio evolutionary structure prediction. The calculated elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and hardness for Os–B compounds are in good agreement with the available experimental values. Our results show that the hardness of osmium borides increases with increasing boron content. Os₂B₅ and OsB₃, with hardnesses of 34.4 and 36.9 GPa respectively, can almost be considered as potential superhard materials. Further analyses on density of states, crystal orbital Hamilton population, and electron localization function demonstrate that the electronic structure of Os–B compounds is directly responsible for their particular mechanical properties. High hardness in Os₂B₅ and OsB₃ is mainly attributed to the occurrence of strong B–B covalent bonds and the disappearance of some ductile Os–Os metallic bonds.     

Solid State Interaction in Ni-Mo-B System During Mechanical Alloying and Annealing

This work presents the results of a study of Ni_87?x Mo _x B₁₃ alloys (x?=?7, 10 and 14?at.%), which were obtained by mechanical alloying (MA) of elemental powder mixtures in a MAPF-2M high-energy planetary ball mill. The x-ray diffraction analysis and differential scanning calorimetry measurements were used. The single-phase fcc solid solutions of Mo and B in Ni were formed by MA of Ni-Mo-B mixtures of various compositions for 6-8?h. The coherent domain sizes of solid solutions calculated from the x-ray peak widths were 12-14?nm. The exothermic effects on the DSC curves, which corresponded to the phase transformations of supersaturated Ni(Mo,B) solid solutions, were observed during heating of the synthesized alloys. After heating to 700?°C, the alloys contained a fcc Ni(Mo) phase and a metastable hexagonal MoB₄ phase. Thermodynamically stable phase composition of Ni₈₀Mo₇B₁₃ and Ni₇₇Mo₁₀B₁₃ alloys, containing three phases: fcc Ni (Mo), Ni₂₁Mo₂B₆ with cubic lattice and Ni₃B with orthorhombic lattice, was reached after the isothermal annealing at 1000?°C. The ratio between the amounts of these phases in the alloys corresponds to their location in a three-phase area of the Ni-Mo-B equilibrium phase diagram.     

Correlation between the Hall Resistance and Magnetoresistance in the Mixed State of an Nd<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4 + δ</Subscript> Electronic Superconductor

The Hall resistance and the magnetoresistance in the mixed state of the Nd_{2 ? x}Ce _x CuO_{4 + δ} quasi-two-dimensional system near the antiferromagnetic-superconductor (AF-SC) phase transition have been measured at doping levels x = 0.14 and 0.15, and a correlation has been established. This correlation can be analyzed using the following power relationship: ρ _xy (B) ~ [ρ _xx (B)]^β. It was found that index β varied from 0.94 ± 0.03 in the region of AF and SC coexistence (x = 0.14) to 0.6 ± 0.1 in the SC region with the maximum critical temperature (x = 0.15) at low temperatures and weak magnetic fields. This reduction suggests that the symmetry of carrier pairing changes at the boundary of the transition from the phase of antiferromagnetic ordering and spin density waves to the superconducting phase in the presence of antiferromagnetic spin fluctuations.     

Theory of atomic defects and diffusion in ordered compounds,and application to B2-FeAl

By means of a generalized grandcanonical approach the relations between the effective formation energies, entropies and volumes of atomic defects in ordered compounds and the parameters characterizing the microscopic properties of the single defects are derived, whereby the microscopic parameters are calculated by the ab initio electron theory. The migration energies for several possible self-diffusion paths are determined by the transition-state theory in combination with the ab initio electron theory. Results are obtained for B2-Fe_xAl_1−x at or close to stoichiometry, and they are compared with experimental data.     

The Systems Tantalum (Niobium)-Cobalt-Boron

Constitution of the ternary systems Nb-Co-B and Ta-Co-B was studied, employing optical and electron microscopy, x-ray powder, single crystal diffraction, electron probe microanalysis, DTA and Pirani melting point measurements. Ternary phase equilibria were determined within an isothermal section at 1100 °C. For the Co-rich part (≥50 at.% Co) of the system, a liquidus surface projection and a corresponding Schulz-Scheil reaction scheme were constructed in combination with data for primary crystallization from as-cast samples determined by SEM and EPMA measurements. The crystal structures of novel ternary compounds have been elucidated by x-ray powder and single crystal diffraction and were supported by TEM. {Nb,Ta}CoB with NbCoB-type exhibits a high temperature modification (ZrAlNi-type, a = 0.5953 nm, c = 0.3248 nm; a = 0.5926 nm, c = 0.3247 nm for Nb and Ta respectively), which was only present in as-cast alloys, but found to be stabilized by the addition of Fe to annealing temperatures of 1400 °C. Ta₃Co₄B₇ (a = 0.3189 nm, b = 1.8333 nm, c = 0.8881 nm) was proven to be isotypic with Nb₃Co₄B₇. The novel orthorhombic compounds {Nb,Ta}Co₂B₃ (TaCo₂B₃-type with space group Pnma; a = 0.53628 nm, b = 0.32408 nm, c = 1.24121 nm for TaCo₂B₃; a = 0.53713 nm, b = 0.32442 nm, c = 1.2415 nm for NbCo₂B₃) adopt unique structure types with branched boron zig-zag chains. {Nb,Ta}Co₂B were found to crystallize in a unique monoclinic structure type (space group P2 ₁/c; a = 0.9190 nm, b = 0.64263 nm, c = 0.63144 nm; β = 109.954°, for Nb) very close to an orthorhombic setting (Cmce, a = 0.63162 nm, b = 1.72810 nm, c = 0.64270 nm, for Nb). Substitution of Co by Ni stabilizes a smaller orthorhombic lattice with Re₃B-type structure (Cmcm) although no homologue compound in the Ni-system exists. The crystallographic relations among the structure types of Re₃B and pseudo-orthorhombic as well as monoclinic {Nb,Ta}Co₂B were defined in terms of a Bärnighausen scheme. DFT calculations revealed very close stabilities for the three competing structure types for {Nb,Ta}Co₂B. Detailed transmission electron microscopy (TEM) for Nb(Co,Fe)B, {Nb,Ta}Co₂B,  {Nb,Ta}(Co,Ni)₂B, and Ta₃Co₄B₇ confirmed lattice geometries and crystal symmetry. Vickers hardness was measured for {Nb,Ta}Co₂B, {Nb,Ta}(Co,Ni)₂B, {Nb,Ta}_2?x Co_21+x B₆ and {Nb,Ta}Co₂B₃ exhibiting the highest value of hardness of HV = 22.4 ± 1.1 GPa for TaCo₂B₃. Magnetic, specific heat and electrical resistivity measurements on the compounds TaCo₂B and Ta₂Co₂₁B₆ reveal paramagnetic and ferromagnetic metallic ground states, respectively.     

Enhancement of plastic deformation in FeCoNbB bulk metallic glass with superhigh strength

The effect of B on the glass-forming ability of FeCoNbB alloys was investigated. Bulk metallic glasses with critical diameters up to 3 mm and superhigh yield strength of 4860 MPa were synthesized in (Fe_0.5Co_0.5)_71−xNb₆B_23+x (x = 0, 2, 3, 4 and 5) alloy system. Besides, Cu was added to the Fe_33.5Co_33.5Nb₆B₂₇ alloy with the aim at enhancing mechanical properties, and it was found that proper amount of Cu addition could effectively improve the compressive plasticity from 1.4% to 3.7% without obvious strength decreasing. The enhancement of plasticity is ascribed to the formation of clusters in Cu-contained FeCoNbB bulk metallic glasses.     

Development of FeNiNbSiBP bulk metallic glassy alloys with excellent magnetic properties and high glass forming ability evaluated by different criterions

Fe₃₈Ni₃₈Nb_2.5B_21.5−x−yP_xSi_y (x, y = 1–8) bulk metallic glassy alloys with high glass forming ability and excellent magnetic properties were developed. Bulk samples with maximum diameters of 3 mm are fabricated by copper mold casting method. The glassy alloys have large ΔT_x of 40–70 K. The alloys exhibit excellent magnetic properties like extremely low H_c of 0.5–0.8 A/m, high μ_e of 1.6–2.85 × 10⁴ and comparatively high B_s of 0.6–0.8 T which changes regularly with the content variations of P, B and Si. By ascertaining applicability of the empirical GFA criterions, T_rg, α, β and γ can be used in evaluating the GFA of FeNiBSiPNb system alloys.     

Reaction synthesis of TiB2–TiC composites with enhanced toughness

In situ toughened TiB₂–TiC_x composites were fabricated using reaction synthesis of B₄C and Ti powders at high temperatures. The resulting materials possessed very high relative densities and well developed TiB₂ plate-like grains, leading to a rather high fracture toughness, up to 12.2 MPa⋅m^1/2. The microstructure was examined by means of XRD, SEM, TEM and EDAX. The reaction products mainly consisted of TiB₂ and TiC_x. No other phases, e.g. Ti₃B₄, TiB, Ti₂B₅ and free Ti, were observed regardless of whether the starting composition was Ti:B₄C=3:1 or 4.8:1, and whether the sintering temperature was 1700 or 1800°C. The microstructural morphology is characterised by TiB₂ plate-like grains distributed uniformly in the TiC_x matrix. Some inclusions and defects were found in TiB₂ grains. The very high reaction temperature was believed to be responsible for the formation of plate-like grains, which, in turn, is responsible for the much improved mechanical properties. The main toughening mechanisms were likely to be crack deflection, platelet pull-out and the micro-fracture of TiB₂ grains.     

Crystal Structure of W1−x B3 and Phase Equilibria in the Boron-Rich Part of the Systems Mo-Rh-B and W-{Ru,Os,Rh,Ir,Ni,Pd,Pt}-B

The crystal structure of W_1?x B₃ has been reinvestigated by x-ray single crystal diffraction and revealed isotypism with the Mo_1?x B₃ structure type (space group P6₃/mmc; a = 0.52012(1), c = 0.63315(3) nm; R _F = 0.040). As a characteristic feature of the structure, planar hexagonal metal layers (1/3 of atoms removed from ordered positions) sandwich planar boron honeycomb layers. One of the two W-sites shows a random defect of about 73%. Strong metal boron and boron-boron bonds are responsible for high mechanical stability. Although W_1?x B₃ at about 80 at.% B is the metal boride richest in boron, it contains no directly linked three-dimensional boron framework. The solubility of Rh, Ir, Ni, Pd and Pt in W_1?x B₃ as well as of Rh in Mo_1?x B₃ has been investigated in as cast state and after annealing. Furthermore, phase equilibria in the boron rich part of the corresponding isothermal sections W-TM-B (TM = Rh, Ir at 1100 °C, TM = Ni, Pd at 900 °C and TM = Pt at 800 °C) and Mo-Rh-B (at 1100 °C) have been established. A ternary compound only forms in the system W-Ir-B: τ₁-W_1?x Ir _x B₂ with ReB₂ structure type (space group P6₃/mmc; a = 0.2900, c = 0.7475 nm). The type of formation and crystal structure of diborides W_1?x TM _x B₂ (TM = Ru, Os, Ir) isotypic with ReB₂ were studied by x-ray powder diffraction and electron probe microanalysis in as cast state and after annealing at 1500 °C. Accordingly, W_0.5Os_0.5B₂ (a = 0.29127(1), c = 0.7562(1) nm) forms directly from the melt, whereas W_0.4Ru_0.6B₂ (a = 0.29027(1), c = 0.74673(2) nm) and W_0.6Ir_0.4B₂ (a = 0.29263(1), c = 0.75404(8) nm) are incongruently melting. Annealing at 1500 °C leads in case of the iridium compound to an almost single-phase product but the same procedure does not increase the amount of the ruthenium diboride.