Atomistic simulation of the motion of dislocations in metals under phonon drag conditions

The mobility of dislocations in the over-barrier motion in different metals (Al, Cu, Fe, Mo) has been investigated using the molecular dynamics method. The phonon drag coefficients have been calculated as a function of the pressure and temperature. The results obtained are in good agreement with the experimental data and theoretical estimates. For face-centered cubic metals, the main mechanism of dislocation drag is the phonon scattering. For body-centered cubic metals, the contribution of the radiation friction becomes significant at room temperature. It has been found that there is a correlation between the temperature dependences of the phonon drag coefficient and the lattice constant. The dependences of the phonon drag coefficient on the pressure have been calculated. In contrast to the other metals, iron is characterized by a sharp increase in the phonon drag coefficient with an increase in the pressure at low temperatures due to the α-∈ phase transition.     

3D DDD modelling of dislocation–precipitate interaction in a nickel-based single crystal superalloy under cyclic deformation

Strain-controlled cyclic deformation of a nickel-based single crystal superalloy has been modelled using three-dimensional (3D) discrete dislocation dynamics (DDD) for both [0?0?1] and [1?1?1] orientations. The work focused on the interaction between dislocations and precipitates during cyclic plastic deformation at elevated temperature, which has not been well studied yet. A representative volume element with cubic γ′-precipitates was chosen to represent the material, with enforced periodical boundary conditions. In particular, cutting of superdislocations into precipitates was simulated by a back-force method. The global cyclic stress–strain responses were captured well by the DDD model when compared to experimental data, particularly the effects of crystallographic orientation. Dislocation evolution showed that considerably high density of dislocations was produced for [1?1?1] orientation when compared to [0?0?1] orientation. Cutting of dislocations into the precipitates had a significant effect on the plastic deformation, leading to material softening. Contour plots of in-plane shear strain proved the development of heterogeneous strain field, resulting in the formation of shear-band embryos.     

Criterion for the softening effect in crystalline materials under their doping

A dislocation model of the effect of softening (i.e., decrease in the microhardness or yield stress of the doped material as compared to the initial material) is developed. The motion of dislocations in metals with the body-centered cubic structure, semiconductors, and other materials in accordance with the kink mechanism is described taking into account the effect of impurity atoms creating the centers for benign generation of kink pairs and obstacles for propagation of kinks. The conditions under which the velocity of dislocations increases after the introduction of impurities are determined using the equation describing the kinetics of passage of dislocations through the Peierls barrier in the doped material.     

Influence of the Peierls relief on size effects in plastic deformation of micro- and nanocrystals of body-centered cubic metals

The influence of the Peierls relief and Peierls stresses on the size effects in micro- and nanocrystals of metals with a body-centered cubic (bcc) lattice has been theoretically discussed in the framework of the dislocation-kinetic approach. It has been found that, as compared to micro- and nanocrystals with a facecentered cubic (fcc) lattice, for the bcc crystals, the exponent n in the power law σ ～ D ^?n (where σ is the flow stress and D is the transverse size of the crystal) is the smaller, the higher is the critical temperature T _c above which the Peierls relief ceases to control the motion of dislocations in the bcc metals. The specific features of the influence of the Peierls relief on the coefficient of strain-rate sensitivity of the flow stresses in microcrystals of the bcc lattice have also been discussed. The theoretical results have been illustrated by the experimental data available in the literature.     

TEM investigation of the interaction between dislocation and nitride precipitates in deformed high manganese-chromium austenitic steels

The interaction between dislocations and nitride precipitates during high-temperature creep deformation of high manganese austenitic steels has been investigated by transmission electron microscopy. Most of the dislocations activated by applied stress were dissociated into Shockley partials. The fine TiN precipitates are pinning and/or incorporating the bow-type moving dislocations and they turned out to be more effective than the coarser TaN in disturbing the dislocation movement.     

Influence of the size and shape of free nanoparticles on the local changes in the lattice parameter and on the structural stability of body-centered cubic zirconium and iron

The influence of the shape and size of nanocrystals on the lattice relaxation of body-centered cubic metals (zirconium, iron) at a constant temperature has been investigated using the molecular dynamics method with the many-body interatomic interaction potential obtained in terms of the embedded-atom model. The calculations have been performed for isolated clusters with sizes ranging from 2.5 to 17 nm for zirconium and from 2 to 14 nm for iron. It has been demonstrated that, in free zirconium and iron particles, the relaxation of the lattice constant along the [100], [010], and [001] directions has an oscillatory character. Irrespective of the size of the zirconium and iron particles, the equilibrium distances between atoms at the center of cubic clusters are minimum compared to those observed in near-surface layers and the equilibrium value of the lattice parameter for the bulk sample. In spherical clusters, the region of a maximum contraction corresponds to a depth approximately equal to 0.2 particle diameter from the surface. An increase in the size of both cubic and spherical clusters leads to a decrease in the deviation of the local lattice parameter from the equilibrium value for the bulk sample. It has been established that the size and shape of the cluster substantially affect the temperature and mechanism of the structural transformation from the body-centered cubic phase into the hexagonal close-packed phase.     

Vibrational properties of straight dislocations in bcc and fcc metals within the harmonic approximation

We calculate the vibrational spectra of straight screw and edge dislocations in several body-centered cubic (bcc) (Mo and Fe) and face-centered cubic (fcc) (Cu and Al) metals within the harmonic approximation. We take advantage of the translational symmetry of straight dislocations to efficiently calculate their phonon eigenstates in the harmonic limit. This allows us to calculate the low-temperature contribution of straight screw and edge dislocations to the heat capacity of each respective metal, and show that the dominant temperature dependence below 5 K is linear. Comparison with heat capacity measurements of heavily cold-worked Cu reveals very good agreement with our calculations. At higher temperatures, the contribution from the non-linear terms becomes significant. As a result, maxima in the straight dislocation heat capacities are observed in the temperature range from 9% to 16% of the Debye temperature. We investigate the appearance of localized and resonance peaks in the vibrational spectra induced by dislocations, and study in detail their spatial spread around the dislocation cores by projecting vibrational eigenstates onto individual atoms. We study the deviation of these atomic-level vibrational free energies from that of the perfect crystal as a function of distance to the dislocation cores, and establish that, similar to the dislocation energy, the vibrational free energy of an isolated dislocation behaves logarithmically in the long-range limit. Finally, we obtain vibrational spectra for propagating waves along the dislocation line and find that the dispersion for these waves is consistent with the notion of kink formation and motion for screw dislocations.     

TEM study of NbC heterogeneous precipitation in ferrite

The heterogeneous precipitation of NbC in ferrite has been quantitatively characterized by transmission electron microscopy in a Fe–C–Nb model alloy for different isothermal heat treatments. The elongation and size distribution of precipitates were derived using dark field imaging. For each precipitation state, the precipitation of NbC occurs on dislocations due to the as-quenched state. This precipitation mechanism leads to characteristic arrays of precipitates in which precipitates grow in a self-similar manner. A detailed study of these arrays has shown that most dislocations decorated by these arrays are edge dislocations with ?112? type line vectors. There is only one variant on a given dislocation. This selection can be interpreted by the interaction between dislocation and precipitate strain fields.     

Electrical properties of plastically deformed silicon due to its interaction with an iron impurity

The change in the recombination properties of plastically deformed silicon as a result of its interaction with an iron impurity introduced by diffusion at 1000°C has been studied using scanning electron microscopy and deep level transient spectroscopy. The recombination activity of dislocations and slip planes behind dislocations has been found to increase due to the accelerated formation of iron precipitates at these defects. Based on the measurements of the electron-beam-induced current, the concentration of the formed precipitates has been estimated, and their sizes have been calculated to be several hundred nanometers.     

Molecular dynamics investigation of the size effect upon the β → α transformation in Zr nanocrystals

The stability of the β phase in cubic zirconium nanoparticles has been calculated as a function of the size r (r varies in the range from 2.5 to 11.5 nm) by the molecular dynamics method with the many-body interatomic interaction potential obtained within the embedded-atom model. It has been demonstrated that the temperature T _k at which the cubic cluster of body-centered cubic zirconium becomes structurally unstable depends nonlinearly on the particle size. The curve T _k (r) exhibits a pronounced maximum in the range r ≈ 4.3−4.7 nm. It has been established that the mechanism of the structural transition from the body-centered cubic phase to the hexagonal close-packed phase depends substantially on the particle size. For particles with sizes in the range from 2.5 to 5.0 nm, there exists a temperature range in which the transition from the body-centered cubic phase to the hexagonal close-packed phase remains incomplete for a long time. In this case, two phases coexist and the initial particle undergoes a strong deformation along the habit plane.     

用类O原子波函数对H+9团簇的体心立方结构与能量的研究

在单中心球模型近似下,选用类O原子解析函数,用变分法计算了H+9团簇体心立方结构与能量。结果表明当中心氢原子核到顶角氢原子核之间的距离R=1.97a0时,体系能量有一极小值E=-4.376h0(a0=0.529177×10-10m,h0=27.2eV)。这表明H+9团簇的体心立方结构是稳定的结构,H+9团簇是存在的。     

Study of the atomic structure of bcc and fcc crystals upon instantaneous plastic deformation

The evolution of the atomic structure of face-centered cubic (fcc) and body-centered cubic (bcc) crystals under the conditions of pulsed external loads and large plastic strains is investigated on the basis of computer experiments. The crystals are strained in steps to 32%. After each deformation step (2%), the system is relaxed by molecular dynamics to a new equilibrium state at 300 K. The results of the computer experiments show that plastic deformation can take place under instantaneous external loads either as a result of the motion of partial dislocations, twinning, or the turning and displacement of atomic planes, depending on the stage of the process. The laws governing the variation of the potential energy of the system and the rotation angle of the atomic planes as functions of the degree of plastic strain of the crystal are found. Zh. Tekh. Fiz. 67, 100–102 (December 1997)     

体心立方Ta的广义面错能及在Ⅱ型裂纹尖端初始塑性研究中的应用

基于嵌入原子势考察体心立方(bcc)金属Ta的广义层错能和广义孪晶能并获得广义层错能和广义孪晶能曲线. 研究表明,bcc Ta的广义层错能曲线与面心立方金属的广义层错能曲线有明显差异,Ta的广义层错能曲线不存在明显的能量极小值,位错主要以全位错的形式发射. 不同原子厚度的广义孪晶能曲线表明4个原子层的孪晶能曲线开始出现亚稳定的能量极小值,5个原子层的孪晶能曲线出现稳定的能量极小值. 为进一步验证广义层错能和广义孪晶能曲线揭示的塑性变形机理,采用准连续介质力学多尺度方法研究Ⅱ型裂纹尖端的初始塑性变形过程. 关键词： 广义层错能 广义孪晶能 体心立方金属钽 Ⅱ型裂纹     

Structure,impurity composition,and photoluminescence of mechanically polished layers of single-crystal silicon

The introduction of optically active defects (such as atomic clusters, dislocations, precipitates) into a silicon single crystal using irradiation, plastic deformation, or heat treatment has been considered a possible approach to the design of silicon-based light-emitting structures in the near infrared region. Defects were introduced into silicon plates by traditional mechanical polishing. The changes in the defect structure and the impurity composition of damaged silicon layers during thermal annealing (TA) of a crystal were examined using transmission electronic microscopy and x-ray fluorescence. Optical properties of the defects were studied at 77 K using photoluminescence (PL) in the near infrared region. It has been shown that the defects generated by mechanical polishing transform into dislocations and dislocation loops and that SiO₂ precipitates also form as a result of annealing at temperatures of 850 to 1000°C. Depending on the annealing temperature, either oxide precipitates or dislocations decorated by copper atoms, which are gettered from the crystal bulk, make the predominant contribution to PL spectra.     

Maximal symmetry of the Skyrme crystal

The symmetry of the classical Skyrme crystal considered by Klebanov and by Wüst, Brown and Jackson can be enlarged, implying that the crystal is a body-centered cubic lattice of half-skyrmions instead of a primitive cubic lattice of skyrmions.     

Concentration dependence of the spin diffusion constant for a diluted Heisenberg magnet

The concentration dependence of the diffusion constant for a dilute isotropic Heisenberg magnet in the high temperature limit is examined in the cases of simple cubic, body-centered cubic and face-centered cubic lattices with nearest-neighbour exchange.     

Effective-Field Theory on High Spin Systems with Biaxial Crystal Field

Based on the effective-field theory with self-spin correlations and the differential operator technique,physical properties of the spin-2 system with biaxial crystal field on the simple cubic, body-centered cubic, as well as faced-centered lattice have been studied. The influences of the external longitudinal magnetic field on the magnetization,internal energy, specific heat, and susceptibility have been discussed in detail. The phenomenon that the magnetization in the ground state shows quantum effects produced by the biaxial transverse crystal field has been found.     

Effective-Field Theory on High Spin Systems with Biaxial Crystal Field

Based on the effective-field theory with self-spin correlations and the differential operator technique, physical properties of the spin-2 system with biaxial crystal field on the simple cubic, body-centered cubic, as well as faced-centered lattice have been studied. The influences of the external longitudinal magnetic field on the magnetization, internal energy, specific heat, and susceptibility have been discussed in detail. The phenomenon that the magnetization in the ground state shows quantum effects produced by the biaxial transverse crystal field has been found.     

Evolution of magnetism in iron from the atom to the bulk

The evolution of the magnetic moment in iron clusters containing 20-400 atoms is investigated using first-principles numerical calculations based on density-functional theory and real-space pseudopotentials. Three families of clusters are studied, characterized by the arrangement of atoms: icosahedral, body-centered cubic centered on an atom site, and body-centered cubic centered on the bridge between two neighboring atoms. We find an overall decrease of magnetic moment as the clusters grow in size towards the bulk limit. Clusters with faceted surfaces are predicted to have magnetic moment lower than other clusters with similar size. As a result, the magnetic moment is observed to decrease as function of size in a nonmonotonic manner, which explains measurements performed at low temperatures.