第一性原理研究Al-Y合金在压力作用下的晶胞结构、力学性质、热力学性质和电子结构

The influence of structural, elastic properties, thermodynamics and electronic properties Al-Y alloy were investigated by using first-principles. The equilibrium lattice constant, elastic constants, and elastic modulus as calculated here agree with results of previous studies. Calculated results of bulk modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio v and Debye temperature all increase as pressure increase, but the opposite is true for heat capacity cp. In addition, the Debye temperature for the phases reduces gradually as follows: Al2Y > Al3Y> AlY. Additionally, the G/B ratio indicates that AlY and Al3Y are ductile materials, while Al2Y is a brittle material, and that the ductility of AlY and Al3Y can be improved with increased pressure, while the brittleness of Al2Y does not improve with increased pressure. Finally, the paper presents and discusses calculations of density of states and charge populations as they are affected by pressure.     

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,elastic and electronic properties of the Rh–Zr compounds from first-principles calculations

A systematic investigation on structural, elastic and electronic properties of Rh–Zr intermetallic compounds is conducted using first-principles electronic structure total energy calculations. The equilibrium lattice parameters, enthalpies of formation (E_for), cohesive energies (E_coh) and elastic constants are presented. Of the eleven considered candidate structures, Rh₄Zr₃ is most stable with the lowest E_for. The two orthogonal-type, relative to the CsCl-type, are the competing ground-state structures of RhZr. The result is in agreement with the experimental reports in the literature. The analysis of E_for and mechanical stability excludes the presence of Rh₂Zr and RhZr₄ at low temperature mentioned by .Curtarolo et al. [Calphad 29, 163 (2005)]. It is found that the bulk modulus B increases monotonously with Rh concentration, whereas all other quantities (shear modulus G, Young's modulus E, Poisson's ratio σ and ductility measured by B/G) show nonmonotonic variation. RhZr₂ exhibits the smallest shear/Young's modulus, the largest Poisson's ratio and ductility. Our results also indicate that all the Rh–Zr compounds considered are ductile. Furthermore, the detailed electronic structure analysis is implemented to understand the essence of stability.     

Effect of boron doping on the mechanical properties of ordered Ni–Mo alloys

HASTELLOY B-2 alloy was found to exhibit environmental embrittlement when tested in air and hydrogen at ambient temperature after atomic ordering introduced by a heat treatment at 700 °C for 24 h. Molybdenum in the HASTELLOY B-2 alloy was probably reactive enough to dissociate water vapor in air to generate atomic hydrogen, resulting in hydrogen embrittlement. The percentage of transgranular fracture increased as the test environment changed from hydrogen gas to moist air or water vapor, and vacuum or oxygen. With the addition of 100 wt ppm B, the environmental embrittlement was completely eliminated, with the tensile properties independent of both test environment and strain rate. The fracture mode remained the same, i.e. ductile dimpling, after B-doping when tested in different environments. The immunity of the B-doped B2 alloy to environmental sensitivity remained even after long-term heat treatment. Auger analysis does not detect any boron segregation at the grain boundaries. The mechanism of boron doping in eliminating the environmental embrittlement in the Ni–Mo alloy is apparently different from that in many L1₂-type alloys such as Ni₃Al and Ni₃Si.     

Effects of Ni vacancy,Ni antisite,Cr and Pt on the third-order elastic constants and mechanical properties of NiAl

Effects of Ni vacancy, Ni antisite in Al sublattice, Cr in Al sublattice, Pt in Ni sublattice on the second-order elastic constants (SOECs) and third-order elastic constants (TOECs) of the B2 NiAl have been investigated using the first-principles methods. Lattice constant and the SOECs of NiAl are in good agreement with the previous results. The brittle/ductile transition map based on Pugh ratio G/B and Cauchy pressure P_c shows that Ni antisite, Cr, Pt and pressure can improve the ductility of NiAl, respectively. Ni vacancy and lower pressure can enhance the Vickers hardness H_v of NiAl. The density of states (DOS) and the charge density difference are also used to analysis the effects of vacancy, Ni antisite, Cr and Pt on the mechanical properties of NiAl, and the results are in consistent with the transition map.     

Structural stability of some CsCl structure HfTM (TM = Co, Rh, Ru, Fe) compounds

In order to investigate Hf-TM (TM = Fe, Co, Rh, Ru) phase diagrams in the region of 50:50% atomic ratio, we performed ab initio Full-Potential Linearized Augmented Plane Waves calculations of the most stable Hf and TM elemental phases and HfTM compounds of the CsCl and CuAu structure types. The obtained electronic structures, cohesive energies and enthalpies of formation are discussed and compared to some of the existing models and available experimental data. The non-existing compound HfFe is found to be at least metastable, and the reason for its absence from the phase diagram is discussed.     

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.     

Ab initio based study of finite-temperature structural,elastic and thermodynamic properties of FeTi

We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials.