First-principle calculations of structural,elastic and thermodynamic properties of Fe–B compounds

The structural, elastic and thermodynamic properties of FeB, Fe₂B, orthorhombic and tetrahedral Fe₃B, FeB₂ and FeB₄ iron borides are investigated by first-principle calculations. The elastic constants and polycrystalline elastic moduli of Fe–B compounds are usually large especially for FeB₂ and FeB₄, whose maximum elastic constant exceeds 700 GPa. All of the six compounds are mechanically stable. The Vickers hardness of FeB₂ is estimated to be 31.4 GPa. Fe₂B and FeB₂ are almost isotropic, while the other four compounds have certain degree of anisotropy. Thermodynamic properties of Fe–B compounds can be accurately predicted through quasi-harmonic approximation by taking the vibrational and electronic contributions into account. Orthorhombic Fe₃B is more stable than tetrahedral one and the phase transition pressure is estimated to be 8.3 GPa.     

Stability,structure and electronic properties of γ-Fe23C6 from first-principles theory

First-principles’ calculations of GGA and GGA + U type have been performed for γ-Fe₂₃C₆, a complex iron carbide with 116 atom in the unit cell. GGA results were found to be in better agreement with experimental data than GGA + U results. Various occupancies for Wyckoff positions and corresponding magnetic orderings have been explored. Our calculations reveal that the crystal structure is composed of a framework of strongly linked Fe atoms, and additional stabilizing Fe and C atoms positioned in cavities. The local electronic and magnetic properties vary strongly among the non-equivalent Fe sites in γ-Fe₂₃C₆. The lattice parameters of γ-Fe₂₃C₆ match those of austenite well. Surprisingly, pure γ-Fe₂₃C₆ is found to be more stable than commonly occurring θ-Fe₃C cementite. Moreover, the calculations show low vacancy energy (about 0.37 eV) for Fe at 4a sites in γ-Fe₂₃C₆. Conditions of formation and factors hampering the formation of γ-Fe₂₃C₆ in steel manufacturing processes are discussed.     

Structural, elastic and electronic properties for M2XC （M--Ti and Cr, X--Ga and A1） phases from ab initio calculations

The first-principles study of the structural, elastic and electronic properties of the M 2 XC phases depending on the type of M transition metal (M are Ti and Cr) and on X (X are Ga and Al) was reported. The calculations are performed using the pseudo-potential plane-wave approach in both the local density and generalized gradient approximations. The elastic constants are calculated using the static finite strain technique. Features such as structural and elastic parameters, Debye temperature, sound velocities and their pressure dependence have been investigated. In agreement with experimental and previous theoretical findings, it is found that the compressibility along a and c axis depends on the valence electron concentration (VEC). Correlations revealing the governing role of the X and M elements on the machinability indices of the material have been examined. The electronic properties have been discussed in terms of chemical bonding showing that bonding is due to Md-Cp and Md-Xp hybridizations. M-C bonds are stiffer than M-X ones and Al-Ti (Cr-C) bonds are stiffer than those corresponding to Ti-C (Al-Cr). It is shown that the stiffness of the M-X and M-C bonds increases with increasing the number of VEC.     

Thermodynamic properties of CexTh1−xO2 solid solution from first-principles calculations

A systematic study based on first-principles calculations along with a quasi-harmonic approximation has been conducted to calculate the thermodynamic properties of the Ce_xTh_1?xO₂ solid solution. The predicted density, thermal expansion coefficients, heat capacity and thermal conductivity for the Ce_xTh_1?xO₂ solid solution all agree well with the available experimental data. The thermal expansion coefficient for ThO₂ increases with CeO₂ substitution, and complete substitution shows the highest expansion coefficient. On the other hand, the mixed Ce_xTh_1?xO₂ (0 < x < 1) solid solution generally exhibits lower heat capacity and thermal conductivity than the ThO₂ and CeO₂ end members. Our calculations indicate a strong effect of Ce concentration on the thermodynamic properties of the Ce_xTh_1?xO₂ solid solution.     

First-principles phase stability and elastic properties of Al–La binary system intermetallic compounds

The phase stability and the elastic properties of Al–La binary system intermetallic compounds were thoroughly investigated using first-principles calculations. Firstly, the 0 K phase diagram for this system was calculated using the formation enthalpy convex hull construction, which indicates three metastable phases, namely Al₄La (I4/mmm), Al₄La (Imm2), and AlLa₃ (Pm-3m). Then, the stability of Al₁₁La₃ was examined at temperatures lower than 1000 K compared with the two Al₄La allotropes and the Al + Al₂La two-phase equilibrium. The results demonstrate that the needlelike phase in Mg–Al–La based alloys should be indexed as Al₁₁La₃, which is thermodynamically stable, with no decomposition under aging. Thirdly, AlLa₃ (Pm-3m) is more stable than AlLa₃ (P63/mmc) at temperatures higher than approximately 590 K, which well agrees with the experimental results. Finally, the elastic properties and vibrational properties for the stable Al–La intermetallic phases were calculated in this work.     

Microstructure and mechanical properties of Ti–Nb–Fe–Zr alloys with high strength and low elastic modulus

Zr was added to Ti–Nb–Fe alloys to develop low elastic modulus and high strength β-Ti alloys for biomedical applications. Ingots of Ti–12Nb–2Fe–(2, 4, 6, 8, 10)Zr (at.%) were prepared by arc melting and then subjected to homogenization, cold rolling, and solution treatments. The phases and microstructures of the alloys were analyzed by optical microscopy, X-ray diffraction, and transmission electron microscopy. The mechanical properties were measured by tensile tests. The results indicate that Zr and Fe cause a remarkable solid-solution strengthening effect on the alloys; thus, all the alloys show yield and ultimate tensile strengths higher than 510 MPa and 730 MPa, respectively. Zr plays a weak role in the deformation mechanism. Further, twinning occurs in all the deformed alloys and is beneficial to both strength and plasticity. Ti–12Nb–2Fe–(8, 10)Zr alloys with metastable β phases show low elastic modulus, high tensile strength, and good plasticity and are suitable candidate materials for biomedical implants.     

Existence patterns of Dy in β-NiAl from first-principles calculations

Intermetallic compound β-NiAl is a promising material in high temperature applications due to its high melting temperature,high strength,low density,and good oxidation resistance.However,its application remains limited because of its relatively poor cyclic oxidation resistance.Addition of reactive element(RE)Dy can improve the cyclic oxidation of NiAl alloys significantly.However,the mechanism of Dy addition is not clear.Even the existence pattern of Dy in NiAl is unspecified.Therefore,in the present study,the impurity formation energies of Dy in stoichiometric NiAl,Ni-rich,and Al-rich NiAl for the substitution cases were studied by first-principles density functional theory.The results show that Dy could hardly substitute for either Ni or Al atoms in NiAl.However,calculations for dissolution energies show that Dy could be easily dissolved in Al vacancies in all three types of NiAl,which provides a new existence pattern of Dy in NiAl beyond experimental detection.     

Surface and electrochemical characterization of Ni–Zr intermetallic compounds

In this paper the Ni–Zr system has been considered with the aim of investigating the role of composition and structure of an early–late transition metals system on the electrocatalytic activity, for the hydrogen evolution reaction. As a matter of fact, pure Ni and Zr show low activity, while their intermetallic compounds generate a higher catalytic efficiency. Five alloys, with increasing Ni content starting from Ni₃₃Zr₆₇ up to Ni₇₅Zr₂₅, have been prepared and characterized. The alloy of composition (Ni_0.55Mn_0.30V_0.10Co_0.05)_2.1Zr has also been considered, in order to investigate the catalytic efficiency related to a Laves structure. The thickness and composition of the surface oxides have been investigated and their effect on reducing the catalytic efficiency of the as-prepared alloys has been discussed. The activity of the samples submitted to a surface activation treatment with hydrofluoric acid, that removes the oxide layer and allows to evidence the properties of the compounds, has been observed to increase significantly. The trend of the electrocatalytic efficiency with the composition of the alloys is discussed considering a synergetic effect between Ni and Zr. The Laves phase appears slightly more active than the binary intermetallic compound.     

Experimental and atomistic study of the elastic properties of α′ Fe–C martensite

We calculate the elastic constants of Fe–C α′ single crystals and compare them to our own and previously published measurement data on polycrystals. Based on a recently developed interatomic interaction potential, discrepancies between our present experimental results and earlier measurements are discussed, and can be settled with the help of our simulation data. Atomistic data obtained with a different interatomic potential show less satisfactory agreement. Our results demonstrate a strong increase of the elastic anisotropy with carbon content, but only a mild dependence of the Debye temperature.     

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.     

Microstructure and properties of aging Cu–Cr–Zr alloy

The crystallography and morphology of precipitate particles in a Cu matrix were studied using an aged Cu–Cr–Zr alloy by transmission electron microscopy(TEM) and high-resolution transmission electron microscopy(HRTEM). The tensile strength and electrical conductivity of this alloy after various aging processes were tested. The results show that two kinds of crystallographic structure associated with chromium-rich phases, fcc and bcc structure, exist in the peak-aging of the alloy. The orientation relationship between bcc Cr precipitate and the matrix exhibits Nishiyama–Wasserman orientation relationship. Two kinds of Zr-rich phases(Cu4Zr and Cu5Zr)can be identified and the habit plane is parallel to {111}Cu plane during the aging. The increase in strength is ascribed to the precipitation of Cr- and Zr-rich phase.     

Investigation of the structure and properties of rhombohedral Cu–Ge–Te alloys by ab initio calculations

Crystallization property of amorphous GeTe can be significantly improved by doping Cu. However, the effect of Cu on the structure and electrical properties of crystalline GeTe is not clear, which is of great importance for phase-change memory. In this work, we have studied the effect of Cu on the structure and properties of crystalline GeTe by means of ab initio calculations. The results show that it is energetically favorable to obtain rhombohedral structured Cu_nGe_32–m–nTe₃₂ films by co-sputtering defective Ge_32–mTe₃₂ and Cu as characterized by the negative formation energies. The doped Cu has slight effect on the structure property and chemical bonding of GeTe but has remarkable effect on the electrical properties. The results show that Cu_nGe_32–nTe₃₂ alloys might be a good candidate material for phase-change memory.     

Thermodynamic properties of the Al–Fe–Ni system acquired via a hybrid approach combining calorimetry,first-principles and CALPHAD

A reaction calorimeter coupled with first-principles calculations was employed to obtain enthalpies of formation for τ₁ (Al₉FeNi) and τ₂ (Al₁₀Fe₃Ni) compounds. The previous thermodynamic model for describing the disorder/order transition (fcc_A1/L1₂) in the Al–Fe–Ni system was modified to extrapolate this model to quaternary and higher-order systems. The first-principles energy calculations for the end-members of sub-lattice models in ternary compounds and L1₂ phase were performed to facilitate subsequent modeling. The existence of the experimentally observed miscibility gap for ternary B2-ordered phase is detected by the present calculation. Such a feature cannot be identified with available thermodynamic software due to the tiny difference between the Gibbs energies associated with different phase assemblages. A set of thermodynamic parameters for the Al–Fe–Ni system was obtained via thermodynamic modeling. Numerous experimental data including phase diagram, thermodynamic properties and site occupation of Fe in B2 phase are well accounted for by the present modeling.     

Effect of thermomechanical treatment on the microstructure,phase composition,and mechanical properties of Al–Cu–Mn–Mg–Zr alloy

The effect of the thermomechanical treatment on the microstructure, phase composition, and mechanical properties of heat-treatable AA2519 aluminum alloy (according to the classification of the Aluminum Association) has been considered. After solid-solution treatment, quenching, and artificial aging (T6 treatment) at 180°C for the peak strength, the yield stress, ultimate tensile strength, and elongation to failure are ~300 MPa, 435 MPa, and 21.7%, respectively. It has been shown that treatments that include intermediate plastic deformations with degrees of 7 and 15% (T87 and T815 treatments, respectively) have a significant effect on the phase composition and morphology of strengthening particles precipitated during peak aging T8X type, where X is pre-strain percent, treatments initiate the precipitation of significant amounts of particles of the θ′- and Ω-phases. After T6 treatment, predominantly homogeneously distributed particles of θ″-phase have been observed. Changes in the microstructure and phase composition of the AA2519 alloy, which are caused by intermediate deformation, lead to a significant increase in the yield stress and ultimate tensile strength (by ~40 and ~8%, respectively), whereas the plasticity decreases by 40–50%.     

Structure and electronic properties of the Watson–Crick base pairs: Role of hydrogen bonding

The hydrogen bonding patterns in the adenine–thymine (A–T) and guanine–cytosine (G–C) base pairs for B-DNA has been studied using the density functional theory. The H-bond for the crystal geometry is found to differ considerably from the geometry optimized structures for the free base pairs with larger deviation for the G–C pair compared to the A–T pair. Furthermore, the H-bonding patterns are found to be highly non-local and co-operative. For the N···HN bond in the G–C pair, the proton hops in between two symmetric double-well potentials localized on the donor and the acceptor, due to cancellation of the local polarizations of the top and down NH···O and O···HN bonds, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap (HOMO–LUMO gap) for the free bases decreases with increase in the strength of the H-bonds in between the base pairs with larger decrease for the G–C pair. The adiabatic electron affinity (AEA) for the free bases which are found to be all negative become positive on the formation of the H-bonds and more so for the G–C pair.     

Magnetic,electric and thermoelectric properties of ternary intermetallics from the Ce–Co–Ge system

Magnetic susceptibility, magnetization, electrical resistivity and thermoelectric power of polycrystalline Ce₂CoGe₃, Ce₃CoGe₂, CeCo_0.86Ge₂, Ce₅Co₄Ge₁₃, CeCoGe₃, and Ce₂Co₃Ge₅ were studied in wide temperature and magnetic field ranges. The results of magnetic properties and electrical resistivity measurements carried out for Ce₃CoGe₂, CeCo_0.86Ge₂, CeCoGe₃ and Ce₂Co₃Ge₅ corroborate the data reported in literature. Ce₂CoGe₃ and Ce₅Co₄Ge₁₃ were found to be ferromagnetically ordered below 10 and 7 K, respectively. Moreover, the resistivity of Ce₂CoGe₃ shows Kondo-like behavior. The thermoelectric power exhibits in all six phases a large broad maximum, located between 50 and 150 K, most likely due to the presence of 4f electrons.     

Diffusion kinetics,mechanical properties,and crystallographic characterization of intermetallic compounds in the Mg–Zn binary system

Pure Mg was diffusion bonded to pure Zn at 315 °C for 168 h to produce equilibrium intermetallic compounds of the Mg–Zn system. All equilibrium phases at 315 °C, Mg₂₁Zn₂₅, Mg₄Zn₇, MgZn₂, Mg₂Zn₁₁, were observed to develop. Concentration profiles by electron probe microanalysis, electron diffraction patterns by transmission electron microscopy, and load–displacement curves by nano-indentation were examined to characterize the phase constituents, crystal structure, diffusion kinetics, and mechanical properties. Mg₂₁Zn₂₅ with trigonal, Mg₄Zn₇ with monoclinic, and Mg₂Zn₁₁ with cubic structures were found and their lattice parameters were reported herein. Mg₄Zn₇ and Mg₂Zn₁₁ were observed to have a range of solubility of approximately 2.4 at% and 1.6 at%, respectively. Interdiffusion in MgZn₂ occurred most rapidly, was an order of magnitude slower in Mg₄Zn₇ and Mg₂Zn₁₁, and was the slowest in Mg₂₁Zn₂₅. Composition-dependence of interdiffusion within each intermetallic phase was negligible. The intermetallic phases exhibited insignificant creep, but evidence of discontinuous yielding was observed. The average hardness and reduced moduli were similar for Mg₂₁Zn₂₅, Mg₄Zn₇, and MgZn₂ phases, ∼5 GPa and ∼90 GPa, respectively. However, the Mg₂Zn₁₁ phase had lower hardness of 3.76 GPa and higher modulus of 108.9 GPa. The mechanical properties in the characterized intermetallic phases, exclusive of Mg₂₁Zn₂₅, were strongly concentration-dependent.     

Influence of Al on the microstructure and mechanical properties of Cr–Zr–(Al–)N coatings with low and high Zr content

Low Zr (S1) and high Zr (S2) quaternary Cr–Zr–(Al–)N coatings with increasing Al content were deposited by d.c. reactive magnetron sputtering. The structure, fracture cross-section morphology and mechanical properties of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation, scratch testing and Vickers micro-indentation testing. All the coatings present an fcc NaCl-type B1 structure; in the low Zr content coatings, the diffraction peaks shift towards higher angles as the Al content increases. The grain size is approximately constant in a range from 6 to 8 nm, except for high Zr content films where a significant decrease in crystalline order is observed (grain size ~ 2.5 nm). In both series, the microstructure changed from equiaxed to columnar with increasing Al content. The highest hardness and strongest adhesion values were achieved in coatings with lower Zr and Al content. Conversely, the coatings with high Zr and the highest Al content exhibited an abrupt decrease in hardness, adhesion strength and toughness.     

ZrNi–H2: microstructural analysis of the thermodynamically controlled hydride phase growth,and electronic properties

The ZrNi–H₂ phase diagram indicates the presence of two stable hydrides: a triclinic monohydride ZrNiH and an orthorhombic trihydride ZrNiH₃ [1]. In this paper, we present the results of some thermodynamic, microstructural and electronic properties of the ZrNi–H₂ system. A strict control of the thermodynamic variables (P, T) during the hydride phase growth allowed us to prepare two samples of overall composition ZrNiH_0.70 and ZrNiH_2.5. In a first step, the microstructure evolutions induced by hydrogen absorption are investigated during the activation process. The intermetallic and hydrided compounds are studied by energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction (XRD) methods. The nature of defects generated in ZrNi and its hydrides compounds are observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Ab-initio electronic structure calculations have been performed. They clearly show the importance of chemical effects besides geometrical factors in explaining the preferential site occupancy of hydrogen atoms in the ZrNiH phase.