Narrow Results

The TiAl3/Al2O3 metal-ceramic composite was synthesized using high energy ball milling, powder compaction and thermal treatment. Micron sized powders of titanium oxide (TiO2) and aluminum were subjected to high energy ball milling under an argon protected atmosphere. Milling of this powder mixture although reduced crystallites sizes to a nano scale, did not result in a reaction between the reactants. Further compaction of the milled powder and annealing, paved the way to a reduction reaction and led to the formation of an ultrafine grained composite structure. The reaction appeared to proceed through two-steps. Titanium oxide was first reduced to TiO and later on, TiO was reduced to Ti. The resulting Ti was alloyed with extra Al to produce TiAl3 intermetallic in which alumina particles were dispersed. Also, mechanical activation was found to reduce the reaction temperature between Al and TiO2. The morphology and phase composition of the milling products were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis.

Amorphous/nanocrystalline 50Ni–50Ti powders were synthesized from elemental Ti and Ni powders by solid state synthesis utilizing low energy mechanical alloying with times up to 100 h. The produced powders were investigated by X-ray diffraction and differential scanning calorimetry to study phase transformations that occurred during heating in the calorimeter. It was found that at the first stage of the heating process, a disordered NiTi phase was formed at temperature of about 400°C. Further investigations indicated that this phase transformed into the Ni₃Ti and Ti₂Ni intermetallic compounds after heating at a temperature of about 800°C.

Nb-xSi(x=3,9,16)-22Ti-3Cr-3Al-2Hf (at.%) have been successfully prepared by directional solidification in an optical floating zone furnace. Microstructure analysis and phases identification of the alloys were examined by X-ray diffraction (XRD), Electro Probe Micro Analyzer (EPMA) and Energy Disperse Spectroscopy (EDS). Fracture toughness specimens without pre-crack were prepared, room temperature fracture toughness of alloys was tested by three-point bending method, and fracture mechanism was studied. The results showed that with increasing Si content, Nb₅Si₃ phase gradually increased and the phase transformed from γ-Nb₅Si₃ to the stable α-Nb₅Si₃ phase and β-Nb₅Si₃ phase. There appeared the Ti-rich Nb₅Si₃ phase when the Si content is 16 at%. In addition, more micro-cracks generated in the Ti-rich Nb₅Si₃ phase, which seriously affected room temperature fracture toughness of the alloys.

Structural, electronic, elastic and thermodynamic properties of Rh₃X(X = Zr, Nb, Ta) intermetallic compounds are investigated in the framework of density functional theory (DFT). The exchange-correlation (XC) potential is treated with the generalized gradient approximation (GGA) and local density approximation (LDA). The computed ground state properties agree well with the available theoretical and experimental values. The elastic constants are obtained by calculating the total energy versus volume conserving strains using Mehl model. The electronic and bonding properties are discussed from the calculations of band structures (BSs), densities of states and electron charge densities. The volume and bulk modulus at high pressure and temperature are investigated. Additionally, thermodynamic properties such as the heat capacity, thermal expansion and Debye temperature at high pressures and temperatures are also analyzed.

To study the mechanical, spin polarized electronic and magnetic behavior of TmX(X = Cu, Ag), full-potential linear augmented plane wave plus local orbital method has been used. The lattice parameter (a₀), bulk modulus (B₀) and its first-order pressure derivative have been calculated using optimization method. Mechanical properties have been studied in terms of elastic constants (C_ij), Young's modulus (Y), shear modulus (G) and Poisson's ratio (v) at ambient temperature and pressure which are found to be consistent with available experimental/theoretical values. Electronic properties have been investigated in terms of band structure and density of state histograms for spin up and spin down channel. Electronic and magnetic behavior of TmX shows that studied materials are metallic ferromagnets with high spin polarization in which Tm-f state electrons are contributed mainly.

The structural properties, formation heats, cohesive energies, elastic properties and electronic structures of common intermetallics in CrMnFeCoNiZr system high-entropy alloys are analyzed by using density functional theory. The calculation results show that Co₁Zr₁, Co₂Zr₁, Fe₂Zr₁ and Fe${}_{23}$Zr₆ phases may form in the casting process of the alloy, which indicates good agreement with the XRD results of the self-designed alloys. The calculated elastic constants reveal that Co₂Zr₁, Fe₂Zr₁ and Fe${}_{23}$Zr₆ are mechanically stable. In addition, the results show that Co₂Zr₁, Fe₂Zr₁ and Fe${}_{23}$Zr₆ with high shear modulus and elastic modulus would be excellent strengthening phases in high-entropy alloy and would improve the hardness of the alloys.

In this paper, structural, electronic and magnetic properties of the aluminum-rich intermetallic alloy Al${}_{13}$Fe₄ are investigated using full potential linearized augmented plane waves (FPLAPW) approach in the frame work of density functional theory (DFT). The calculated structural parameters are consistent with the experimentally reported data. In this alloy, Fe atoms possess different coordination numbers with Al atoms, i.e., Fe(1) and Fe(2) have the same coordination number 5, whereas Fe(3), Fe(4) and Fe(5) have 4, 7 and 9 coordination numbers, respectively. The compound is found metallic and ferromagnetic in nature. Post-DFT (BoltzTraP code) calculations confirm the ferromagnetic and anisotropic behavior. The Fe(5) atom plays a central role in the electronic and magnetic properties of the alloy due to the large coordination with Al atom as compared to the rest Fe atoms.

The mechanical, electronic and thermodynamic properties of Pd₃M (M=Sc, Y) compounds have been investigated using the Full Potential Linearized Augmented Plane Wave (FP-LAPW) formalism. The generalized gradient approximation (GGA) is used to treat the exchange–correlation terms. The calculated formation enthalpies and the cohesive energies reveal that the L1₂ structure is more stable than the D0${}_{24}$ one. The obtained lattice parameters and bulk modulus calculations conform well to the available experimental and theoretical results. The elastic and mechanical properties are analyzed and results show that both compounds are ductile in nature. The Debye temperature and melting temperature are also estimated and are in a good agreement with experimental findings. The total and partial densities of states are determined for L1₂ and D0${}_{24}$ structures. The density of states at the Fermi level, $N$($E_{\mathrm{\text{F}}}$), indicates electronic stability for both compounds. The presence of the pseudo-gap near the Fermi level is suggestive of formation of directional covalent bonding. The number of bonding electrons per atom $n_b$ and the electronic specific heat coefficient $\gamma$ are also determined. The quasi-harmonic Debye model has been used to explore the temperature and pressure effects on the thermodynamic properties for both compounds.

First-principles calculations within density functional theory (DFT) with the projector augmented wave (PAW) technique were used to investigate the stabilities of intermetallics in the Au–Rb system at 0 K. Four intermetallics: Au₇Rb₃, Au₃Rb₂, Au₅Rb and AuRb were investigated in their observed experimental structures. The Au₂Rb compound, reported in the Au–Rb phase diagrams without specifying explicitly its structure, was also investigated by inspecting several hypothetical structures. A suspect compound (AuRb₂) was also investigated. Results show that: (i) The Au₃Rb₂ and Au₇Rb₃ compounds, which were never reported in any Au–Rb phase diagram, are stable at 0 K. (ii) The Au₂Rb compound is not a ground state for all the tested structures. (iii) Stability of the Au₅Rb and AuRb compounds was confirmed. (iv) The new compound AuRb₂, not yet reported experimentally, is found mechanically stable at 0 K.

The structural, elastic and thermodynamic properties of the A15 structure V₃Ir, V₃Pt and V₃Au were studied using first-principles calculations based on the density functional theory (DFT) within generalized gradient approximation (GGA) and local density approximation (LDA) methods. The results have shown that both GGA and LDA methods can process the structural optimization in good agreement with the available experimental parameters in the compounds. Furthermore, the elastic properties and Debye temperatures estimated by LDA method are typically larger than the GGA methods. However, the GGA methods can make better prediction with the experimental values of Debye temperature in V₃Ir, V₃Pt and V₃Au, signifying the precision of the calculating work. Based on the E–V data derived from the GGA method, the variations of the Debye temperature, coefficient of thermal expansion and heat capacity under pressure ranging from 0 GPa to 50 GPa and at temperature ranging from 0 K to 1500 K were obtained and analyzed for all compounds using the quasi-harmonic Debye model.

Intermetallic formation at 425°C in the aluminum–copper system has been studied by scanning electron microscopy using welded diffusion couples. Several Al–Cu phases predicted by the equilibrium phase diagram of the elements and voids taking place in the diffusion zone have been detected in the couples. The predominant phases were found to be Al₂Cu₃ and the solid solution of Al in Cu, α. The growth of the intermetallic layer obeyed the parabolic law.

Ti-48%Al-2%Cr-2%Nb-(0, 0.5, 1) at.%W alloys were synthesized via the powder metallurgical route, and cyclically oxidized at 800, 900, or 1000°C in air for up to 100 h in order to find the effects of W on their oxidation characteristics. At 800°C, they oxidized relatively slowly, and the scales were thin and adherent. At 900°C, the scales began to spall locally. At 1000°C, they spalled repetitively during oxidation. Cr, Nb, and W improved the cyclic oxidation resistance of TiAl alloys. The oxides formed were TiO₂ and Al₂O₃, which contained the alloying elements of Cr, Nb, and W. Nitrides formed were TiN and Ti₂AlN.