Resistance of zinc crystals to shock deformation and fracture at elevated temperatures

Velocity profiles of the free surface of shock-loaded zinc crystals are measured in two different orientations. The test temperature is varied from room temperature to 410 °C. The results of the measurements show that the high-velocity deformation and fracture are athermal processes and that the fracture stresses are influenced by the preceding plastic deformation. Fiz. Tverd. Tela (St. Petersburg) 40, 1849–1854 (October 1998)     

Computer simulation of structure and mechanical properties of polymer liquid crystals

In this contribution, we report on a study of the self-organization and mechanical properties of polymer liquid crystals (PLCs). Both processes are computer simulated by the method of molecular dynamics. We investigated two real longitudinal PLCs (thermotropic polyesters) with macromolecules that consist of rigid and flexible parts arranged in a regular way. One rigid and one flexible part form a monomer containing 45 or 47 atoms. The total number of atoms in the macromolecules studied was 4700 (100 monomers) and 5400 (75 monomers). The self-organization was similar to that obtained earlier for a beads-on-a-string model, so compression calculations were done using this simpler model containing 1200 beads (100 monomers). Macroscopic characteristics such as the stress-strain relation, temperature change during deformation, as well as microscopic changes in structure, were investigated.     

Computer simulation of deformation of two-component compacts under liquid-phase sintering

A model is proposed for predicting the deformation of two-component compacts under diffusion accompanied by phase transformations. The model is based on the movable cellular automata method. Dilatancy effects of a Cu−Al powder mixture under liquid phase sintering are described by means of computer simulations. Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 69–74, March, 1999.     

Computer simulation study of a flexible-rigid-flexible model for liquid crystals

Small molecules that form liquid crystals typically consist of a rigid core with flexible tails on one end or on both ends. To date, most computer simulation studies have used completely rigid models such as hard spherocylinders: cylinders, characterized by their length/diameter ratio L/D, with hemispherical end caps. We have studied a model consisting of spherocylinders with L/D = 4, with a flexible tail attached to each end. The tails are ‘ideal’ in the sense that they have no volume. Using Monte Carlo simulations the phase behaviour of this model was studied and, for comparison, the behaviour of hard spherocylinders with L/D = 4 without tails was studied as well. The addition of the tails is found to stabilize the smectic-A phase at a lower pressure, and the nematic phase disappears. In the smectic-A and crystal phases, the smectic layers are further apart when tails are added. The structure of the layers and the smectic-A–crystal transition pressure change only a little. For both models close to melting the crystal consists of ordered layers, but there is almost no correlation between particle positions in neighbouring layers. In fact, the layer coupling is so weak that in a long simulation the layers are found to glide over each other. As the pressure is increased the crystal gradually becomes more ordered and the crystalline layers ultimately ‘lock’ into place.     

Computer simulation of Ti3Al intermetallic cleavage fracture

The decohesion energy and the energy of unstable stacking faults for all cracking planes and dislocation slip systems observed experimentally are calculated using the molecular dynamics method with N-particle atomic potentials. A dimensionless parameter characterizing the brittle behavior of the material is calculated for basis, prism, and pyramid faces in terms of the model elaborated by Kelly et al. and extended by Rice and Thompson. Cleavage in Ti₃Al is due to low decohesion energy values, which facilitates cracking, and high energies of unstable stacking faults, which prevents the formation of a plastic zone and stress relaxation at its vertex.     

Molecular dynamics simulation of deformation and fracture of graphene under uniaxial tension

The paper reports on molecular dynamics simulation of deformation and fracture of graphene under uniaxial tension. Dependences of Young’s modulus, critical force and fracture strain on the strain rate, temperature and angle between the tension direction and the graphene lattice are derived. The effect of defects on fracture of graphene is studied.     

Numerical simulation of deformation and fracture of a material with a polysilazane-based coating

The paper studies the localization of plastic deformation and fracture in a material with a porous coating. A dynamic boundary value problem in the plane strain formulation is solved. The numerical simulation is performed by the finite difference method. The composite structure corresponds to the experimentally observed one and is specified explicitly in the calculation. A generation procedure of the initial finite-difference grid is developed to describe the coating structure with adjustable porosity and geometry of the substrate-coating interface. Constitutive equations for the steel substrate include an elastic-plastic model of an isotropically hardening material. The ceramic coating is described by a brittle fracture model on the basis of the Huber criterion which accounts for crack nucleation in triaxial tension zones. It is shown that the specific character of deformation and fracture of the studied composite results from the presence of local tensile regions in the vicinity of pores and along the coating-substrate interface, in both tension and compression of the coated material. The interrelation between inhomogeneous plastic flow in the steel substrate and crack propagation in the coating is studied.     

Mechanisms of plastic deformation,hardening, and fracture in single crystals of nitrogen-containing austenitic stainless steels

We have carried out a systematic investigation of the mechanisms for solid-solution hardening by nitrogen atoms and dispersion hardening by nitride particles in single crystals of austenitic stainless steels with different stacking fault (SF) energies _SF=0.02–0.2 J/m². We show that alloying with nitrogen C_N=0–0.7 mass % and precipitation of dispersed particles leads to the appearance of an orientation dependence of the critical shearing stresses _cr, asymmetry phenomena, an orientation dependence of the slip and twinning deformation mechanisms, superelasticity, and transition from ductile fracture to brittle fracture. We develop dislocation models for solid-solution hardening by interstitial atoms, the orientation dependence and the asymmetry of _cr, based on taking into account the effect of the external stress field on the splitting of a/2110 dislocations into partial Schockley a/6211 dislocations and the change in the position of the interstitial atoms from octahedral interstitial sites to tetrahedral sites with a shift of the twinning a/6211 dislocations by a Burgers vector. We establish the role of strain localization, splitting of gliding dislocations, twinning, and a high stress level in creation of strain hardening, plastic flow instabilities, and the conditions for the "brittle-ductile" transition.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 5–32, March, 1996.     

Effect of stress state on deformation and fracture of nanocrystalline copper:Molecular dynamics simulation

Deformation in a microcomponent is often constrained by surrounding joined material making the component under mixed loading and multiple stress states. In this study, molecular dynamics(MD) simulation are conducted to probe the effect of stress states on the deformation and fracture of nanocrystalline Cu. Tensile strain is applied on a Cu single crystal,bicrystal and polycrystal respectively, under two different tension boundary conditions. Simulations are first conducted on the bicrystal and polycrystal models without lattice imperfection. The results reveal that, compared with the performance of simulation models under free boundary condition, the transverse stress caused by the constrained boundary condition leads to a much higher tensile stress and can severely limit the plastic deformation, which in return promotes cleavage fracture in the model. Simulations are then performed on Cu single crystal and polycrystal with an initial crack. Under constrained boundary condition, the crack tip propagates rapidly in the single crystal in a cleavage manner while the crack becomes blunting and extends along the grain boundaries in the polycrystal. Under free boundary condition, massive dislocation activities dominate the deformation mechanisms and the crack plays a little role in both single crystals and polycrystals.     

Plastic deformation of single nanometer-sized crystals

We report in situ electron microscopy observations of the plastic deformation of individual nanometer-sized Au, Pt, W, and Mo crystals. Specifically designed graphitic cages that contract under electron irradiation are used as nanoscopic deformation cells. The correlation with atomistic simulations shows that the observed slow plastic deformation is due to dislocation activity. Our results also provide evidence that the vacancy concentration in a nanoscale system can be smaller than in the bulk material, an effect which has not been studied experimentally before.     

Physics of the plasticity and fracture of high-strength crystals

Systematic studies of the mechanism of plastic deformation, strain hardening, and fracture of high-strength single crystals of heterophase alloys based on copper and austenitic stainless steels with nitrogen are reported. It is shown that the attainment of high resistance to the motion of dislocations results in the appearance of new mechanical behavior: strong orientation dependence of the critical shear stresses, a change in the deformation mechanism from slip to twinning, loss of mechanical flow stability at early stages in deformation, and a transition from viscous to brittle fracture.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 3–24, September, 1992.     

Mechanisms of deformation and fracture of the Al-Zn-Mg alloy

Laws of dislocation substructure evolution in the Al-Zn-Mg alloy subjected to compression and tension in different structural states are compared with laws of forming a deformation relief. It is established that long aging of the alloy changes the deformation localization mechanisms compared to its evolution in the alloy subjected to short-term aging. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 62–67, November, 2007.     

Computer simulation to the porosity

Abstract

Computer simulations to the porosity of starting materials for the synthesis of polycrystalline diamond (PCD) bodies are described. The logarithmic normal distributed spheres were deposited in three dimensions by means of random coordinates. Conditions for distances between the particles have to be satisfied. Thus one can realize a defined overlapping, contact or distance. Results were obtained for the porosity dependence as a function of standard deviation of the size distribution and particle overlapping. The comparison of calculated and measured porosity under normal pressure showed good agreement. The filling of pores yielded the estimation of pore size distribution and a clear reduction of the pore volume.