Materials Structure & Micromechanics of Fracture V

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Authors: Alena Spielmannová, Anna Machová, Petr Hora
Abstract: The paper presents results of molecular dynamic (MD) simulations in 3D bcc iron crystals with edge pre-existing cracks (001)[110] and (110) [110] (crack plane/crack front) loaded uni-axially in tension mode I at temperature of 300 K. The iron crystals in MD have the same orientation and similar geometry as in our recent fracture tests performed at room temperature on iron (3wt.%Si) single crystals [1].
Authors: Sergiy Kotrechko, O. Filatov, O. Ovsjannikov
Abstract: Atomic mechanisms of the beginning of plastic deformation and failure initiation in nanoparticles of b.c.c. transition metals are presented in this report. It is shown that strength level of nanoparticles of b.c.c. transition metals is pre-determined by the lattice instability within the local region of the crystal. At uniaxial tension even at low temperatures perfect crystal becomes unstable to shear („orthorhombic“ path), i.e. local shear instability is the main mechanism of stress relaxation in nanoparticles of b.c.c. metals. Specific features of local instability of nanoparticle under hydrostatic tension are considered. A model of the temperature dependence of strength is offered. It is shown that nanoparticle strength decreases as square root function of temperature with temperature growth. Just this is essential difference of the temperature dependence of nanoparticle strength from the same for “ordinary” single- and polycrystals.
Authors: Andriy Ostapovets, Václav Paidar
Abstract: The parameters of exponential many-body Finnis-Sinclair potentials corresponding to qualitatively different crystal lattice stability were selected and their behaviour was studied. Furthermore, a model with pairwise Lennard-Jones potential was also considered. The attention was paid to the stability of different crystal structures and the properties of simple interfaces such as stacking faults and twin boundaries were investigated.
Authors: Miroslav Černý, Jaroslav Pokluda
Abstract: Influence of biaxial stress applied perpendicularly to the [100] loading axis on the tensile stress-strain response is studied from first principles. Crystals of four cubic metals Mo, W, Ir and Au were selected as particular case studies. The results obtained show that, within a limited range of biaxial stresses, the tensile stress increases almost linearly with increasing transverse biaxial stress. The factor that expresses the slope of the linear function changes with applied tensile strain. A region of tensile stability of cubic crystals is also discussed.
Authors: Dominik Legut, Mojmír Šob
Abstract: The ideal tensile strength along the [111] direction in the Fe3Al and Ni3Al intermetallic compounds with the D03 structure has been calculated from the first principles using the fullpotential linearized augmented plane-wave method (FP LAPW) within the density functional theory (DFT). The strains corresponding to the maximum sustainable stresses in both materials were determined and compared. The behavior of atomic magnetic moments as a function of strain was analyzed. The tensile test simulations have been theoretically simulated employing both the local density approximation (LDA) and generalized gradient approximation (GGA) for the exchangecorrelation potential.
Authors: Dana Zöllner, Peter Streitenberger
Abstract: A Monte Carlo algorithm for single-phase normal grain growth has been implemented, which allows one to simulate and observe the temporal development of large grain microstructures in three dimensions. The relaxation process to the self-similar coarsening regime has been studied by following the temporal development of quantities like the average grain size, the standard deviation of the grain sizes and topological correlations.
Authors: Pavel Lukáč, Zuzanka Trojanová
Abstract: The mechanical properties and deformation behaviour of magnesium alloys are significantly influenced by the testing temperature, texture and grain size. The paper gives an overview on the influence of the grain size on the mechanical properties of Mg and Mg-2Al alloys at various temperatures. The yield stress and the fracture stress increase with decreasing grain size. It was shown that the elongation to fracture increases with inverse square root of grain size. The effect of grain size on the tensile strength and the elongation to fracture is influenced by the testing temperature. The observed grain size dependence of the elongation to fracture is explained by the activity of non-basal slip systems.
Authors: Christian F. Robertson, Christophe Déprés, Marc Fivel
Abstract: The combined effect of cyclic thermal shocks and static tensile loading is investigated, in a 304L stainless steel. During these experiments, the stress state in the cylindrical specimen walls is nearly equi-biaxial (σZZ ≈ σθθ). In dislocation dynamics (DD) simulations carried out with σZZ = σθθ, the predominant slip directions b are nearly aligned with the free surface normal vector n, regardless of their associated activation ratio (A.R.). This effect is related to the "surface connected volume" (SCV) of the predominant slip systems. Hence, surface grains with n = <110> possess "large SCV slip systems" and therefore, constitute preferential sites for micro-crack initiation in thermal fatigue. During the tests, a marked effect of the superimposed static tensile loading (or mean stress) is also noted. This effect is explained with the help of DD simulations performed with a positive mean stress: slip irreversibility in the individual persistent slip bands systematically augments with increasing mean stress.
Authors: Vladimir I. Betekhtin, Andrey G. Kadomtsev, Petr Král, Jiří Dvořák, Milan Svoboda, Ivan Saxl, Vàclav Sklenička
Abstract: This paper deals with an experimental investigation of the effect of various microdefects induced by equal-channel angular pressing (ECAP) on mechanical and creep properties of ultrafinegrained pure aluminium, an Al-0.2%Sc alloy and copper.

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