Dislocation emission from interphase boundaries in deformed nanocomposites

A theoretical model for emission of lattice dislocations from small-angle interphase boundaries characterized by both orientational and dilatational misfit in deformed nanocomposites is proposed. With allowance for the free surface of the material, the forces acting upon the dislocation structures of the interphase boundaries are calculated, through which the dependences of the critical shear stress for dislocation emission on different parameters of the boundary are found. It is shown that the influence of dilatational misfit and proximity of the interphase boundary to the free surface on dislocation emission is insignificant. It is established that the ability of interphase boundaries to emit dislocations is not uniform: emission of certain dislocations is facilitated as compared to ordinary small-angle grain boundaries, while emission of other dislocations may be inhibited.     

Dislocation nucleation from symmetric tilt grain boundaries in body-centered cubic vanadium

We perform molecular dynamics (MD) simulations with two interatomic potentials to study dislocation nucleation from six symmetric tilt grain boundaries (GB) using bicrystal models in body-centered cubic vanadium. The influences of the misorientation angle are explored in the context of activated slip systems, critical resolved shear stress (CRSS), and GB energy. It is found that for four GBs, the activated slip systems are not those with the highest Schmid factor, i.e., the Schmid law breaks down. For all misorientation angles, the bicrystal is associated with a lower CRSS than their single crystalline counterparts. Moreover, the GB energy decreases in compressive loading at the yield point with respect to the undeformed configuration, in contrast to tensile loading.     

Liquid nucleation at superheated grain boundaries

Grain boundaries with relatively low energies can be superheated above the melting temperature and eventually melt by heterogeneous nucleation of liquid droplets. We propose a thermodynamic model of this process based on the sharp-interface approximation with a disjoining potential. The distinct feature of the model is its ability to predict the shape and size of the critical nucleus by using a variational approach. The model reduces to the classical nucleation theory in the limit of large nuclei but is more general and remains valid for small nuclei. Contrary to the classical nucleation theory, the model predicts the existence of a critical temperature of superheating and offers a simple formula for its calculation. The model is tested against molecular dynamic simulations in which liquid nuclei at a superheated boundary were obtained by an adiabatic trapping procedure. The simulation results demonstrate a reassuring consistency with the model.     

Homogeneous nucleation of glide dislocation loops in nanoceramics

A theoretical model is proposed for the homogeneous nucleation of glide dislocation loops in nanocrystalline ceramics under deformation at low and high temperatures. The nucleation of a dislocation loop in a crystalline grain is considered an ideal nanoscopic shear whose magnitude (the Burgers vector of the dislocation) increases gradually as the loop is nucleating. The characteristics of the homogeneous nucleation of glide dislocation loops in nanocrystalline ceramics based on cubic silicon carbide are calculated. It is shown that, in general, the homogeneous nucleation of a dislocation loop in nanocrystalline ceramics at high temperatures proceeds in two stages, namely, the athermal nucleation of a loop of a “noncrystallographic” partial dislocation and its thermally activated transformation into an ordinary partial lattice dislocation loop.     

Molecular dynamics simulations of He bubble nucleation at grain boundaries

The nucleation behavior of He bubbles in single-crystal (sc) and nano-grain body-centered-cubic (bcc) Mo is simulated using molecular dynamics (MD) simulations, focusing on the effects of the grain boundary (GB) structure. In sc Mo, the nucleation behavior of He bubbles depends on irradiation conditions. He bubbles nucleate by either clustering of He atoms with pre-existing vacancies or self-interstitial-atom (SIA) punching without initial vacancies. In nano-grain Mo, strong precipitation of He at the GBs is observed, and the density, size and spatial distribution of He bubbles vary with the GB structure. The corresponding He bubble density is higher in nano-grain Mo than that in sc Mo and the average bubble size is smaller. In the GB plane, He bubbles distribute along the dislocation cores for GBs consisting of GB dislocations and randomly for those without distinguishable dislocation structures. The simulation results in nano-grain Mo are in agreement with previous experiments in metal nano-layers, and they are further explained by the effect of excess volume associated with the GBs.     

Plastic deformation transfer through the amorphous intercrystallite phase in nanoceramics

A three-dimensional model is proposed for plastic deformation transfer through the amorphous intercrystallite phase in mechanically loaded nanoceramics. In this model, glide dislocation loops are pressed against amorphous intercrystallite boundaries by the applied local shear stress and initiate in them local longitudinal plastic shears, which causes emission of new glide dislocation loops into neighboring grains. The energy characteristics of these processes and the critical applied stress required for barrierless nucleation of grainboundary and intragrain loops are calculated. As an example, a nanoceramic based on cubic silicon carbide is considered. It is shown that plastic deformation transfer through the amorphous intercrystallite phase in such nanoceramics is energetically favorable and can occur athermically over wide ranges of values of the applied stress and the structural characteristics of the material.     

Dislocation arrangement in deformed very dilute Cu-Cd alloy single crystals

The influence of the precipitation annealing on the CRSS and dislocation arrangement of the Cu-0·03 at. % Cd single crystals is observed experimentally. Dislocation distributions are determined by TEM of sections (111) and (101) in stage I and in the transition region. The CRSS increases due to precipitation annealing. There are not observed any second phase particles, but nevertheless there exist some differences in the dislocation arrangement of the solution annealed single crystals and the precipitation annealed ones.     

Dislocation nucleation and precipitation of impurity in calcium-doped NaCl-KCl crystals

In calcium-doped mixed NaCl-KCl crystals, Ca²⁺ ion sites were selectively sensed by a suitable etchant. Different impurity substructures, viz. arrays, close-networks, unevenly distributed clusters were formed by the dopant ions in the lattice. Non-uniform distributions of the impurity ions in the lattice does not nucleate dislocations. Non-structural impurities like precipitates were found to take different forms, viz. elongated rods, rectangular forms, with different orientations. Dislocations showed strong preference for the nucleation of the precipitates. The nucleation of the precipitates at sites other than those of dislocations is discussed.     

Effect of dislocation distribution in twin boundaries on the nucleation of microcracks at the twin tip

The effect of dislocation distribution in the boundaries of an arrested twin on the nucleation of microcracks at its tip is investigated. The twin is simulated by a double step pileup (cluster) of twinning dislocations located in adjacent slip planes. The equilibrium equations for dislocations are solved numerically. Clusters with different total numbers of dislocations and with different ratios of the numbers of dislocations at the upper and lower twin boundaries are considered. The formation of microcracks as a result of coalescence of head dislocations according to the force and thermally activated mechanisms is analyzed. The equilibrium configurations of a single twin boundary and of the twin are calculated. It is found that the condition for microcrack formation at the twin tip considerably depends on the ratio of the numbers of dislocations in twin boundaries. In the limit, this condition coincides with the condition of crack formation at the tip of a single twin boundary with the same total number of dislocations. It is shown that thermally activated formation of a microrack corresponds to lower values of the critical stress.     

Dislocation nucleation mechanism in a one-dimensional model of a Frenkel-Kontorova crystal

Numerical experiments have demonstrated the possibility of the creation of a kink-antikink pair, modeled as a dislocation loop in the Frenkel-Kontorova model, in the interaction of two nonlinear localized breezer excitations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 70–74, February, 1996.     

Connecting diffusion and dynamical heterogeneities in actively deformed amorphous systems

In this Letter, we explore the relations between tracer diffusion and flow heterogeneities in amorphous materials. On the basis of scaling arguments and an extensive numerical study of an athermal elastoplastic model, we show that there is a direct link between the self-diffusion coefficient and the size of cooperative regions at low strain rates. Both depend strongly on rate and system size. The mean square displacement of passive tracers thus gives information about the microscopic rheology, such as the geometry of cooperative regions and their scaling with strain rate and system size.     

Dislocation structure and transport properties of low-angle tilt boundaries in high-temperature superconductors

A theoretical model that describes split grain-boundary dislocations in low-angle tilt boundaries of high-temperature superconductors is suggested. It is shown that the dissociation of dislocations in low-angle tilt boundaries is usually accompanied by a decrease in their elastic energy and causes an increase in the critical current density across the boundaries in high-temperature superconductors.     

Dislocation nucleation in shocked fcc solids: effects of temperature and preexisting voids

Quantitative behaviors of shock-induced dislocation nucleation are investigated by means of molecular dynamics simulations on fcc Lennard-Jones solids: a model argon. In perfect crystals, it is found that the Hugoniot elastic limit (HEL) is a linearly decreasing function of temperature: from near-zero to melting temperatures. In a defective crystal with a void, dislocations are found to nucleate on the void surface. Also, HEL drastically decreases to 15% of the perfect crystal when the void radius is 3.4 nanometers. The decrease of HEL becomes larger as the void radius increases, but HEL becomes insensitive to temperature.     

氦泡在bcc钨中晶界处成核长大的分子动力学模拟

钨(W)是潜在的聚变堆面向等离子体材料.聚变反应中产生的氦(He)不溶于金属W,并在其中易聚集形成He泡,使W基体发生脆化,从而导致W基体的性能发生退化.在前人工作的基础上,本文采用分子动力学研究了He泡在单晶bcc-W中以及bcc-W中∑3[211](110)和∑9[110](411)晶界处He泡形核长大初期的演化过程.结果发现,晶界处He泡的长大机制和单晶W中有所不同.单晶W中He泡通过挤出位错环促进长大.而He泡在∑3[211](110)晶界处的长大机制为:首先挤出并发射少量自间隙W原子,而后挤出1/2⟨111⟩位错线,随后,该位错线会沿晶界面上[111]方向迁移出去;在∑9[110](411)晶界处,He泡在我们的模拟时间尺度范围内没有观察到W自间隙子的发射和位错的挤出.     

Dislocation content of geometrically necessary boundaries aligned with slip planes in rolled aluminium

Previous studies have revealed that dislocation structures in metals with medium-to-high stacking fault energy, depend on the grain orientation and therefore on the slip systems. In the present work, the dislocations in eight slip-plane-aligned geometrically necessary boundaries (GNBs) in three grains of near 45° ND rotated cube orientation in lightly rolled pure aluminium are characterized in great detail using transmission electron microscopy. Dislocations with all six Burgers vectors of the ½?1?1?0? type expected for fcc crystals were observed but dislocations from the four slip systems expected active dominate. The dislocations predicted inactive are primarily attributed to dislocation reactions in the boundary. Two main types of dislocation networks in the boundaries were identified: (1) a hexagonal network of the three dislocations in the slip plane with which the boundary was aligned; two of these come from the active slip systems, the third is attributed to dislocation reactions (2) a network of three dislocations from both of the active slip planes; two of these react to form Lomer locks. The results indicate a systematic boundary formation process for the GNBs. Redundant dislocations are not observed in significant densities.