Ab initio study of C14 laves phases in Fe-based systems

Structural properties and energetics of Fe-based C14 Laves phases at various compositions (i.e. Fe2Fe, Fe2X, X2Fe, X2X, where X stands for Si, Cr, Mo, W, Ta) were investigated using the pseudopotential VASP (Vienna Ab initio Simulation Package) code employing the PAW-PBE (Projector Augmented Wave - Perdew Burke-Ernzerhof) pseudopotentials. Full relaxation was performed for all structures studied including the reference states of elemental constituents and the equilibrium structure parameters as well as bulk moduli were found. The structure parameters of experimentally found structures were very well reproduced by our calculations. It was also found that the lattice parameters and volumes of the unit cell decrease with increasing molar fraction of iron. Thermodynamic analysis shows that the Fe2X configurations of Laves phases are more stable than the X2Fe ones. Some of the X2Fe configurations are even unstable with respect to the weighted average of the Laves phases of elemental constituents. Our calculations predict the stability of Fe2Ta. On the other hand, Fe2Mo and Fe2W are slightly unstable (3.19 and 0.68 kJ.mol-1, respectively) and hypothetical structures Fe2Cr and Fe2Si are found unstable as well

Ab initio simulation of a tensile test in MoSi\u3csub\u3e2\u3c/sub\u3e and WSi\u3csub\u3e2\u3c/sub\u3e

The tensile test in transition metal disilicides with C11b structure is simulated by ab initio electronic structure calculations using full potential linearized augmented plane wave method (FLAPW). Full relaxation of both external and internal parameters is performed. The theoretical tensile strength of MoSi2 and WSi2 for [001] loading is determined and compared with those of other materials

Magneto-structural transformations via a solid-state nudged elastic band method: Application to iron under pressure

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter - in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volume collapse during the pressure-induced transformation in iron - from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface (yet all the atomic and cell degrees of freedom are continuous); the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples. Our results agree with experiments, but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom.Comment: 7 pages, 7 figure

Interatomic Forces and Atomic Structure of Grain Boundaries in Copper-Bismuth Alloys

The many-body empirical potentials that describe atomic interactions in the copper-bismuth system were constructed using both experimental data and physical quantities obtained by ab initio full-potential linear muffin-tin orbital calculations for a metastable Cu3Bi compound. These potentials were then used to calculate the structure of a grain boundary in copper containing bismuth, which was at the same time studied by high-resolution electron microscopy (HREM). Excellent agreement between the calculated and observed structures is shown by comparing a through-focal series of observed and calculated images. This agreement validates the constructed potentials, which can be used with a high confidence to investigate the structure and properties of other grain boundaries in this alloy system. Furthermore, this study shows, that HREM combined with computer modeling employing realistic empirical potentials can decipher with great accuracy the structure of boundaries containing multiple atomic species

Elasticity, Stability and Ideal Strength of β\beta -SiC in plane-wave-based ab initio calculations

On the basis of the pseudopotential plane-wave(PP-PW) method and the local-density-functional theory(LDFT), this paper studies energetics, stress-strain relation, stability and ideal strength of $\beta$-SiC under various loading modes, where uniform uniaxial extension and tension, biaxial proportional extension are considered along directions [001] and [111]. The lattice constant, elastic constants and moduli of equilibrium state are calculated, and the results agree well with the experimental data. As the four Si-C bonds along directions [111], [$\bar{1}$11], [11$\bar{1}$] and [1$\bar{1}$1] are not the same under the loading along [111], internal relaxation and the corresponding internal displacements must be considered. We find that, at the beginning of loading, the effect of internal displacement through shuffle and glide plane diminishes the difference among the four Si-C bonds length, but will increase the difference at the subsequent loading, which will result in a crack nucleated on \{111\} shuffle plane and a subsequently cleavage fracture. Thus the corresponding theoretical strength is 50.8 GPa, which agrees well with the recent experiment value, 53.4 GPa. However, with the loading along [001], internal relaxation is not important for tetragonal symmetry. Elastic constants during the uniaxial tension along [001] are calculated. Based on the stability analysis with stiffness coefficients, we find that the spinodal and Born instabilities are triggered almost at the same strain, which agrees with the previous molecular dynamics simulation. During biaxial proportional extension, stress and strength vary proportionally with the biaxial loading ratio at the same longitudinal strain.Comment: 9 pages, 10 figure