Pavel Šandera

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.

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.

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.

Linear-Elastic and Elastoplastic Mode II and III Crack Tip Stress-Strain Fields in Cylindrical Specimens with Circumferential Crack

A numerical analysis by means of the ANSYS code was performed in order to identify the ratio of both stress intensity factors and crack tip opening displacements for a cylindrical specimen with circumferential V-notch loaded by remote pure shear stress. This kind of loading produces pure mode II and III loading in four points on the circumferential crack front while the mix mode II+III exists in all other crack front points. In the linear-elastic range, the ratio of maximum values of mode III and mode II stress intensity factors was found to be . On the other hand, the ratio of crack tip opening displacements in the elastoplastic range approaches . These results can be used for the construction of fatigue crack growth rate curves in austenitic and ferritic steels measured in the near-threshold and near-fracture regions by means of a special testing device.

Onset of Microplasticity in Copper Crystal during Nanoindentation

The nanoindentation test in the dislocation free crystal of copper is simulated by finite element calculations coupled with ab initio calculation of ideal shear strength. The onset of microplasticity, associated with the pop-in effect identified in experimental nanoindentation tests (creation of first dislocations), is assumed to be related to the moment of achieving the value of the ideal shear strength for the copper crystal. This value also depends on the normal stress in the critical shear system in an approximately linear way, as follows from recently published first principle calculations. The calculated values of the critical shear stress (related to the ideal shear strength) lie exactly at the lower limit of the range of experimentally observed pop-ins in the copper crystal.

Specimens for Simultaneous Mode II, III and II+III Fatigue Crack Propagation: Elasto-Plastic Solution of Crack Tip Stress-Strain Field

Determination of fatigue crack growth characteristics under shear-mode loading is a rather complicated problem. To increase an efficiency and precision of such testing, special specimens enabling simultaneous propagation of shear cracks under II, III and II+III loading modes started to be used rather recently. K-calibration of these specimens was performed and, after unique pre-crack and heat-treatment procedures, effective thresholds in several metallic materials could be measured. However, a description of crack growth rate in terms of appropriate fracture mechanics quantities demands a precise assessment of plastic zone size under various shear-mode loading levels. This contribution is focused on the numerical elasto-plastic analysis of stress-strain field at the crack tip in specimens made of a pure polycrystalline (ARMCO) iron. The results reveal that the small scale yielding conditions are fulfilled in the near-threshold region. Starting from ΔK values approximately two times higher than the threshold, however, the ΔKJ or ΔJ approach should already be utilized. Probably the most interesting result of the analysis lies in a simple procedure that enables us to separate individual loading components ΔKJ,II and ΔKJ,III, applied in the mixed-mode II+III part of the specimen, by comparing elasto-plastic and elastic solutions.

Effect of Grain Boundary Segregation on Mechanical Properties of P-Doped Fe-Si Base Alloys

Abstract The grain boundary segregation in two Fe-Si base alloys with the different bulk phosphorus contents was investigated. The phosphorus grain boundary concentrations determined with the use of Auger electron spectroscopy were related to the microstructure, fracture mode, and mechanical properties of the alloys. Transgranular cleavage was found to be the dominant fracture mode of the samples tested by impact at room temperature. The correlations between the phosphorus grain boundary concentration, the portion of intergranular fracture, and the dynamic fracture toughness were presented, which fulfil the theoretical expectation. Two possibilities were proposed to explain a discrepancy in the temperature dependence of the phosphorus grain boundary concentration for the alloy with the higher bulk phosphorus content. Introduction Impurity segregation and precipitation of secondary phases at the grain boundaries are the fundamental diffusion-controlled processes contributing to the intergranular embrittlement of commercial steels [1]. With intention to quantify the particular effects of the above processes, simple alloys have been investigated. In this respect the P-doped Fe-Si alloys, mostly free of precipitates, are suitable for the segregation studies [2,3]. As was described by McLean [4], the equilibrium grain boundary concentration of the impurity depends directly on its bulk concentration and the annealing temperature. The segregation kinetics can also be influenced by other factors as are grain size, type of the microstructure etc. [5]. It is generally known that small changes in the grain boundary chemistry can provide significant changes in mechanical properties of the alloy steels. This process was excellently characterised on the phenomenological level [6]. However, the precise quantification of the relationship between the grain boundary composition and the energy of intergranular fracture is still missing [7].

A Fractographic Study of Bending/Torsion Fatigue Failure in Metallic Materials with Protective Surface Layers

Results are given of a fractographic study of biaxial in-phase bending/torsion fatigue fractures in specimens made of nitrided steel and nickel-based superalloy with protective coatings (diffusion coatings and plasma-sprayed thermal barrier coatings). Fracture surfaces were examined by optical and scanning electron microscopes while stereophotogrammetry and optical profilometry were employed to obtain 3D surface data of selected fracture surface regions. The studied materials exhibited a wide range of fracture mechanisms depending on the microstructure and applied mechanical loading.