Mojmír Šob

Theoretical tensile stress in tungsten single crystals by full-potential first-principles calculations

Abstract Fully self-consistent ab initio electronic structure calculation of theoretical tensile strength is performed for the first time using full-potential LAPW method. As a specific example, tensile strength of single-crystalline tungsten loaded uniaxially along the (001) and (111) directions is analyzed. Although tungsten is elastically nearly isotropic ( C 44 ≈ C′), theoretical tensile strength exhibits a marked anisotropy ( σ 001 th = 0.289 Mbar, σ 111 th = 0.401 Mbar). This anisotropy is explained in terms of structural energy differences between bcc, fcc and simple cubic structures which occur on the calculated deformation paths. Theoretical results compare favorably with experimental value of 0.247 ± 0.036 Mbar obtained for tungsten whiskers grown along the (110) direction.

Local stability of higher-energy phases in metallic materials and its relation to the structure of extended defects

Abstract Relative structural stability of elemental cubic transition metals and some transition metal aluminides with respect to tetragonal and trigonal deformation is investigated. Total energy calculations show that all higher-energy cubic structures studied are locally unstable with respect to at least one of those deformation modes. In intermetallics, there may or may not be symmetry-dictated energy extrema corresponding to cubic lattices depending on the atomic ordering. However, other energy extrema along the deformation path, besides those required by symmetry, occur. Configurations corresponding to energy minima on the transformation paths may represent metastable structures that can play an important role in interfaces and other extended defects.

Ab initio calculation of tensile strength in iron

A tensile test in ferromagnetic iron for loading in [001] and [111] directions is simulated by ab initio electronic structure calculations using all-electron full-potential linearized augmented-plane-wave method within the generalized gradient approximation. The theoretical tensile strengths and Youngs moduli of ferromagnetic iron are determined and compared with those of other materials. The magnetic and elastic behaviours of iron under uniaxial tensile loading are discussed in detail and compared with the results for isotropic tension (i.e. for negative hydrostatic pressure). Marked anisotropy of theoretical tensile strength in [001] and [111] direction is explained in terms of higher-symmetry structures present or absent along the deformation paths.

Ab initio calculations of ideal tensile strength and mechanical stability in copper

A simulation of a tensile test of copper crystal along the [001] direction is performed using the Vienna ab initio simulation package (VASP). Stability conditions for a uniaxially loaded system are presented and analysed and the ideal (theoretical) tensile strength for the loading along the [001] direction is determined to be 9.4 GPa in tension and 3.5 GPa in compression. A comparison with experimental values is performed.

A study of the applicability of many-body central force potentials in NiAl and TiAl

The applicability and characteristics of the central force many-body potentials of Finnis-Sinclair-type for NiAl and TiAl are investigated by studying the variation of the energy of these compounds with structural transformations that correspond to three distinct paths: tetragonal (B2L10), trigonal (B2L11) and hexagonal (B2B19). The energy was computed using both the central force potentials and the full potential linearized augmented plane waves (FLAPW) ab initio method. Comparison of these two calculations provides a means for the analysis of the efficacy of the potentials. The central force many-body potentials reproduce the results of ab initio calculations very satisfactorily for NiAl. This propounds that they are sufficient in atomistic modelling of lattice defects in this compound. For TiAl the central force potentials mimic the results of ab initio calculations qualitatively but are unable to differentiate adequately between structures with practically the same separations of the first and second neighbours. However, the present study provides a justification for the use of these potentials when investigating extended defects in which separations of the first and second nearest neighbours differ significantly from those in the L10 structure.

Calculation of theoretical strength of solids by linear muffin-tin orbitals (LMTO) method

Ab initio calculation of theoretical strength of Si, Na, W, Cu and Ir cubic crystals under three-axial tension is performed using the linear muffin-tin orbitals-atomic-sphere approximation (LMTO-ASA) method. This method is particularly effective in case of isotropic deformation modes and can be applied by means of advanced personal computers. Computed values are compared with those obtained previously by other methods. The results obtained verify some semi-empirical pair and many-body potential approximations.

Defect and atomic ordering studies in highly deformed and disordered Fe72Al28 alloy

Abstract The results of positron annihilation and Mossbauer spectroscopies in Fe 72 Al 28 alloy with different structural morphology obtained by severe deformation and by quenching from a high temperature are presented. The changes in atomic ordering and in defect arrangements after recovery and ordering annealing in both highly deformed and highly disordered samples are also pursued and discussed. It is shown that the dominant component of 184 ps in the positron lifetime spectrum of a highly disordered sample corresponds to the annihilation in vacancies which prefer to occupy that sublattice in D0 3 superstructure containing both aluminium and iron atoms. Important changes in vacancy concentration take place only when an annealing above D0 3 →B2 (823 K) is performed. Temperature treatment below this phase transformation temperature is characterized above all by changes in the density of dislocations.

Ab initio calculations of lattice stability of sigma-phase and phase diagram in the Cr-Fe system

Ab initio calculations of lattice stability of sigma-phase andnphase diagram in the Cr-Fe system were performed.

Phase diagram calculation in CoCr system using Ab initio determined lattice instability of sigma phase

Abstract The calculations of phase equilibria in the Coue5f8Cr system were performed using the CALPHAD method on the basis of a new two-sublattices model of sigma phase. This model enables us to utilise the results of ab initio calculations of total energy differences between the sigma phase structure and the Standard Element Reference (SER) structures of pure metal at the relaxed lattice parameters ( Δ 0 E i σ −SER ). Total energies were calculated by Full-Potential Linear Augmented Plane Waves (FLAPW) method in the General Gradient Approximation (GGA). The entropy contribution to the Gibbs energy of the pure elements in the sigma phase structure, and the excess Gibbs energy of mixing of the sigma phase were adjusted to the experimental phase equilibrium data.

Positron affinity for precipitates in reactor pressure vessel steels

The sensitivity of positron annihilation spectroscopy to irradiation-induced precipitates in reactor pressure vessel steels is discussed in the light of recent positron affinity and lifetime calculations. Carbide and nitride precipitates are found to trap positrons only if they contain metal vacancies. Copper precipitates are also attractive to positrons but they are probably detected through annihilation at the precipitate-matrix interface. These findings are related to available experimental data.