Jaroslav Pokluda

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.

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.

The theoretical tensile strength of fcc crystals predicted from shear strength calculations.

This work presents a simple way of estimating uniaxial tensile strength on the basis of theoretical shear strength calculations, taking into account its dependence on a superimposed normal stress. The presented procedure enables us to avoid complicated and time-consuming analyses of elastic stability of crystals under tensile loading. The atomistic simulations of coupled shear and tensile deformations in cubic crystals are performed using first principles computational code based on pseudo-potentials and the plane wave basis set. Six fcc crystals are subjected to shear deformations in convenient slip systems and a special relaxation procedure controls the stress tensor. The obtained dependence of the ideal shear strength on the normal tensile stress seems to be almost linearly decreasing for all investigated crystals. Taking these results into account, the uniaxial tensile strength values in three crystallographic directions were evaluated by assuming a collapse of the weakest shear system. Calculated strengths for [Formula: see text] and [Formula: see text] loading were found to be mostly lower than previously calculated stresses related to tensile instability but rather close to those obtained by means of the shear instability analysis. On the other hand, the strengths for [Formula: see text] loading almost match the stresses related to tensile instability.

Ab initio calcuation of ideal strength for cubic crystals under three-axial tension

Ab initio calculation of ideal strength of cubic crystals under three-axial tension was performed using the LMTO-ASA method. Computed values are in a good agreement with those obtained previously by means of semi-empirical polynomial approach. The values of equilibrium lattice parameter obtained in the framework of ab initio method are well comparable with the experimental data whereas a less satisfactory agreement was achieved in the case of bulk moduli.

Theoretical Strength of Metals and Intermetallics from First Principles

The state of the art of ab-initio calculations of the theoretical strength (TS) of materials is summarized and a database of selected theoretical and experimental results presented. Differences between theoretical and experimental TS values are discussed by assessing the stability conditions.

Dynamic stability of fcc crystals under isotropic loading from first principles

Lattice dynamics and stability of four fcc crystals (Al, Ir, Pt and Au) under isotropic (hydrostatic) tensile loading are studied from first principles using the linear response method and the harmonic approximation. The results reveal that, contrary to former expectations, strengths of all the studied crystals are limited by instabilities related to soft phonons with finite or vanishing wavevectors. The critical strains associated with such instabilities are remarkably lower than those related to the volumetric instability. On the other hand, the corresponding reduction of the tensile strength is by 20% at the most. An analysis of elastic stability conditions is also performed and the results obtained by means of both approaches are compared.

Damage and Performance Assessment of Protective Coatings on Turbine Blades

Many structural components operate in very severe environments characterized by a high temperature, increasing temperature gradients, thermo-mechanical stresses and a presence of oxidizing and corrosive atmosphere. In addition, an impact of hard particles can cause failure by erosion mechanisms. Turbine blades of aircraft engines stand as a perfect example of components working in such a severe environment. They are the most loaded parts of the engine owing to the high working temperature and mechanical stresses induced by forces and moments; only centrifugal forces acting on each blade can reach several tens of kN. Temperature patterns of incoming gases are inhomogeneous and, due to transient regimes, their temperature can increase in about 500 °C during a few seconds (Eskner, 2004). These components must withstand such a complex loading for a required performance and lifetime. They are often made from layered material systems in which interfaces between layers play a key role for a prediction of durability. Each layer has different thermal and mechanical properties. Elastic energy of the layered material is concentrated into small volumes the fracture resistance of which is lower than that of the bulk material (Bose, 2007). Therefore, quantitative approaches based on fracture mechanics can be used to assess the damage due to crack initiation and growth. Structural materials of rotor blades and their coatings determine a maximal permissible temperature of gases incoming to the high pressure turbine. The metallic coatings on blades serve as physical barriers between the underlying substrate and the outer environment. From the point of view of corrosive and oxidizing effects, they can be divided into two categories: diffusion and overlay coatings. Diffusion aluminide coatings (DAC) are based on the intermetallic compound ┚-NiAl that forms under the influence of the substrate (usually Ni superalloys). On the other hand, the composition of the overlay coatings MCrAlX remains independent of the substrate alloy. In the MCrAlX alloy system, where M means Ni, Co, Fe (or a combination of these) and X means Y, Si, Ta, Hf, etc., the properties can be controlled and balanced for a specific application. Thermal barrier coatings (TBCs) are composed of ceramics and represent another group of so-called multilayered coatings. These surface barriers insulate the underlying substrate from the heat flux of gases, thus contributing to an improvement of the engine performance. They consist of three constituents: (i) The thermally insulating outer layer (the TBC itself),

Stability and strength of covalent crystals under uniaxial and triaxial loading from first principles

The response of three covalent crystals with a diamond lattice (C, Si and Ge) to uniaxial and a special triaxial (generally nonhydrostatic) loading is calculated from first principles. The lattice deformations are described in terms of variations of bond lengths and angles. The triaxial stress state is simulated as a superposition of axial tension or compression and transverse (both tensile and compressive) biaxial stresses. The biaxial stresses are considered to be adjustable parameters and the theoretical strengths in tension and compression along <100>, <110>, <111> crystallographic directions are calculated as their functions. The obtained results revealed that the compressive strengths are, consistently to fcc metals, almost linear functions of the transverse stresses. Tensile transverse stresses lower the compressive strength and vice versa. The tensile strengths, however, are not monotonic functions of the transverse biaxial stresses since they mostly exhibit maxima for certain values of the transverse stresses (e.g., tensile for <100> and <110> loading of Si and Ge or compressive for <100> loading of C).

Elasticity and Stability of Fe-P Ordered Systems from First Principles

Elastic properties of Fe-P ordered system (the bulk modulus and the theoretical strength under isotropic tension) are computed by means of ab initio computational program code VASP. Different configurations and relative amounts of constituent atoms are considered in the crystal cells of known stable phases as well as of some hypothetical structures. The influence of a relative content of P in the alloy on computed properties is studied. Magnetic ordering is taken into account by means of collinear spin-polarization. The results of calculations reveal that, somewhat surprisingly, no dramatic changes of elastic moduli are to be expected up to the 67% atomic concentration of P.

Grain boundary segregation of elements of groups 14 and 15 and its consequences for intergranular cohesion of ferritic iron

The experimentally determined data—enthalpy and entropy—of the grain boundary segregation of substitutional solutes of 14th and 15th groups of the periodic table, i.e., silicon, phosphorus, tin, and antimony, in α-iron are compared. The consequences of the grain boundary segregation of these elements for the intergranular strengthening or embrittlement are also shown. It is documented that all these solutes except silicon segregate at the grain boundaries interstitially at enhanced temperatures although substitutional segregation is preferred at zero K. Despite some variations, the values of the strengthening/embrittling Gibbs energy of all solutes are nearly linearly dependent on the differences in the sublimation energies of the host and solute.