Materials Structure & Micromechanics of Fracture VIII

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Authors: Vaclav Vitek, Yi Shen Lin, Matous Mrovec
Abstract: In this paper we present bond-order potentials (BOPs) based on the tight-binding method. The potentials have been developed for bcc non-magnetic metals of group V.B (V, Nb, Ta) and group VI.B (Cr, Mo, W) as well as for the ferromagnetic bcc iron. The testing of the transferability of BOPs involves energies of alternate structures, formation energies of vacancies and self-interstitials, transformation paths between different structures and phonon dispersion relations. An example of the application of these potentials is modeling of the structure and glide of 1⁄2<111> screw dislocations under the effect of applied shear and tensile/compressive stresses.
Authors: Yoji Shibutani, Daisuke Matsunaka
Abstract: Dynamics and statics of defects interaction among crack, dislocations and twin boundary (TB) observed in magnesium were investigated using molecular dynamics and elasticity with the complex stress functions to clarify the effect of long-range elastic stress field. An atomic model containing a crack parallel to (10-11) TB was gradually elongated under KI-mode tension by molecular dynamics simulations. Changing the distance between the crack and the TB, four kinds of crack propagation manners were observed, one of which showed the path transition from the crack to the TB itself by shielding effect of piled-up dislocations around the crack tip. The stress intensity factor of the nanosized crack in bulk is 0.28 MPam1/2, which is smaller than that of crack on the TB. The shielding effect due to the piled-up dislocations drastically decreases stress concentration around the crack tip and the stress intensity factor diminishes down to the 0.22, and thus the crack nucleated from the void nucleation and coalescence on the TB was propagated instead. The elastic stress distributions obtained by the superposition of some complex stress functions suggest that the stress field around the crack tip is disturbed by the localized stress due to the TB in the case of crack closest to TB and also by the back stress due to the piled-up dislocations in the case of crack far from TB.
Authors: Hideki Mori
Abstract: The Peierls stress and barrier of a screw dislocation in body-centered cubic iron at finite temperature is investigated by using the free energy gradient method. The Peierls barrier is shown to decrease from 12 to 5 meV per unit length of the Burgers vector with increasing temperature from 0 to 400 K. The entropy term of the Peierls barrier is estimated to be 0.2kB. The Peierls stress also decreases from 900 to 400 MPa with increasing temperature from 0 to 300 K. The change in the Peierls stress due to the entropic effect is larger than that of the Peierls barrier because of thermal softening.
Authors: Dmitrij Sergeevich Kryzhevich, Aleksandr Vyacheslavovich Korchuganov, Konstantin Petrovich Zolnikov, Sergey Grigorievich Psakhie
Abstract: A molecular dynamics simulation of the behavior of nanocrystalline materials in the fields of external influences was carried out. Crystallites of the fcc copper and bcc iron under different schemes of mechanical loading were investigated. Revealed specific localized non-equilibrium states served as the mechanism of formation and evolution of partial dislocations in fcc materials and twin growth in bcc materials. These non-equilibrium states were realized on the basis of local transformation of the martensitic type when the nearest surrounding of atoms – the centers of local rearrangements – changed according to the A-B(C) scheme, where A, B and C are types of crystal lattice. The bcc-fcc-bcc local rearrangements during twin growth were typical for bcc iron. The fcc-bcc-hcp and hcp-bcc-fcc local rearrangements during the partial dislocation movement were typical for fcc copper.
Authors: Aylin Ahadi, Per Hansson, Solveig Melin
Abstract: Nanoindentation is a useful experimental method to characterize the micromechanical properties of materials. In this study molecular dynamics and peridynamics are used to simulate nanoindentation, with a spherical indenter targeting a thin single crystal Cu film, resting on an infinitely stiff substrate. The objective is to compare the results obtained from molecular dynamic simulations to those obtained using a peridynamic approach as regards the force-displacement curves and the deformation patterns after that the material parameters in the peridynamic model have been fitted to the force displacement curve from the molecular dynamic simulation.
Authors: Andriy Ostapovets, Olena Vatazhuk
Abstract: The Schmid law says that yielding takes place when resolved shear stress on slip plane reaches the critical value. It is valid for wide variety of materials. However, it is well known that breaking of Schmid law takes place in bcc materials due to non-planar splitting of dislocation cores. The non-Schmid behavior is also possible for plastic deformation of fcc and hcp materials. Particularly, it is sometimes reported for deformation twinning. Present paper demonstrates the non-Schmid phenomena in hcp magnesium by means of computer simulations. We consider influence of non-glide stress components on motion of screw <a> dislocation as well as migration of twin boundaries.
Authors: Lei Zhang, Sheng Li Jiang, Jun Chen
Abstract: The issue of HMX phase transition under hydrostatic compression is not clear and experiments show conflicting results. Effective solution via first-principles simulation is challenged by difficulty of accurate prediction of Van der Waals interaction, which exists ubiquitously and is crucial for determining the structure of molecules and condensed matter. We have contributed to this by constructing a set of pseudopotentials and pseudoatomic orbital basis, specialized for molecular systems with C/H/N/O elements. The reliability of the method is verified from the interaction energies of 45 complexes (comparing to the results of coupled cluster with singles and doubles (Triple) (CCSD)(T)) and the crystalline structures of 7 typical explosives (comparing to experiments). Using this method, we complete the phase diagram of HMX under static compression up to 50 GPa. We make it clear that no β→δ/ε→δ phase transition occurs at 27 GPa, which has long been a hot debate in experiments. A possible γ→β phase transition is found at around 2.10 GPa in the environment of vapour. We have also predicted the equation of states for α-, δ-, and γ-HMX, which are experimentally absent.
Authors: Martin Zelený, Ivo Dlouhý
Abstract: The influence of Co and Cu doping on Ni-Mn-Ga alloy is investigated using the different ab initio methods for description of chemical disorder. The exact muffin-tin orbital method in combination with the coherent-potential approximation provides almost identical profiles of total energies along the tetragonal deformation path compared to the supercell approach used within the projector-augmented wave method. On the other hand, the simple virtual crystal approximation exhibits different results and thus it is not able to describe doping in Ni-Mn-Ga alloy properly.
Authors: Daisuke Matsunaka, Yoji Shibutani
Abstract: To study the physical origin of the periodic arrangement of the quadrople solute-enriched layers in Mg-based LPSO structures, we carry out first-principles calculations of the formation energy of the L12 cluster and investigate effects of phonon on the inter-planer ordering of the solute-enriched layers using the 1-dimensional chain model with mass change. The formation energy of the L12 cluster increases as the period of the LPSO structure decreases. Thus, it is found that the electron-mediated interaction is short-range repulsive. On the other hand, in the case of considerably heavy mass change, the ordering of the mass changes is stabilized by phonons and the energy gain increases with the concentration of the mass changes, i.e., the short LPSO period is favorable. A promising mechanism of the inter-planer ordering of the LPSO structures is the phonon-mediated interaction of the quadrople layers where heavy solute atoms are enriched as the L12 clusters at SFs.

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