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.     

The structural,elastic, electronic and thermodynamic properties of hexagonal η-Cu6−xNixSn5 (x = 0, 0.5, 1, 1.5 and 2) intermetallic compounds

Based on first-principles calculations, the effects of various Ni concentrations on the structural, elastic, electronic and thermodynamic properties of hexagonal η-Cu₆Sn₅ compound have been systematically investigated. The results demonstrate that higher Ni concentration in the η-Cu_6−xNi_xSn₅ (x = 0, 0.5, 1, 1.5 and 2) leads to thermodynamically stable compounds, and Ni atoms preferentially occupy Cu₂ + Cu_1c sites forming the η-Cu₄Ni₂Sn₅ compound. It is also found that the unit cell volume and lattice parameter of the ‘a’ axis decrease with increasing Ni concentration, which are consistent with the other experimental results. Furthermore, the polycrystalline elastic properties are obtained from single-crystal elastic constants. Our results indicate that the addition of Ni enhances the mechanical stability, brittleness, modulus and Debye temperatures of η-Cu₆Sn₅ compound. Analyzing the electronic structure and charge density distribution provides the explanation that Ni develops distinct bonding energy to Cu and Sn in the structure.     

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.     

Lattice stability of Ca, Sr and Yb disilicides

In this paper a study of the lattice stability of Ca, Sr and Yb disilicides is reported. The structural stability of six different prototype lattices was investigated by means of density functional theory calculations and pseudopotentials within the generalized-gradient approximation using the VASP code. The high-pressure equilibria for the CaSi₂ and SrSi₂ have been described deriving the transition pressures, the structural properties, the bulk moduli and the heats of formation for the various polymorphs. Results are discussed together with the experimental literature. For the ytterbium disilicide a specific study of the Si vacancies in the hR3 ground state structure was carried out. Six different supercells derived from the primitive hexagonal cell were considered and full atomic and lattice relaxations were performed in order to predict the energetically most favorable structure. The formation of vacancies in the Si-sublattice is driven by the lowering of the density of states at the Fermi energy. A new mP22 lattice is proposed to describe the structure of the defective YbSi_2 − x phase: the resulting stoichiometry is Yb₄Si₇.     

First principles calculations of the stability of the T2 and D88 phases in the V–Si–B system

The crystal and electronic structures of D8_l-V₅SiB₂ and D8₈-V₅Si₃B ternary compounds have been investigated by means of first principle calculations. The calculated structural parameters are in very good agreement with the experimental data. The calculated values of the enthalpies of formation at T = 0 K of the D8_l-V₅SiB₂ and D8₈-V₅Si₃B ternary compounds are −67.1 and −62.1 kJ/mol of atoms respectively. The total and partial electronic densities of states show a strong hybridization between the B p states and V d states. The defect enthalpies of formation as well as the mixing enthalpies have been computed. These data are essential for the modeling of the D8_l and D8₈ phases in the V–Si–B ternary system. A partial V–VSi₂–VB isothermal section at 298 K is proposed.     

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.     

NMR investigation of atomic bonding properties in Al–Li alloys

It has been suggested that intrinsic ductile versus brittle properties of alloys be connected with bonding characters in some sense while there is no much proof. In this investigation, ²⁷Al isotropic metallic shifts of Al–Li solid solutions were measured by ²⁷Al NMR spectroscopy. Previously observed anomalous elastic properties upon Li alloying were found to be closely related to ²⁷Al metallic shifts which were associated with s electron density of states at the Fermi level on Al sites. This result is relevant for better understanding of electronic origin of solid solution strengthening mechanisms in Al–Li alloys from the point of view of electronic structure.     

Transformation temperature changes due to second phase precipitation in NiTi-based shape memory alloys

This study investigated changes in the martensite start temperature (M_s) of NiTi-based shape memory alloys as a result of second phase precipitation. Precipitation of the second phase leads to a change in the matrix chemical composition and may alter the electron concentration (c_v) of the matrix. When the electron concentration of the matrix increases, the transformation temperature decreases, whereas the M_s temperature rises when the c_v of the matrix decreases. Additionally, if the precipitation does not result in a significant variation of the c_v of the matrix, the variation in M_s is small (<15 °C). In the present work it is shown that alteration of the M_s temperature as a result of precipitation is mainly due to the change in electron concentration of the matrix. The extrinsic effect of the precipitates on M_s is also discussed on the basis of their quantities in the as-quenched and aged microstructures and the strain energy induced in the matrix.