Analysis of the plasticizing effect of hydrogen dissolved in a crystal on the evolution of plastic deformation at the tip of a crack

The effect of interstitial hydrogen atoms on the evolution of plastic deformation in a crystal at the tip of a tensile crack is estimated taking into account gas exchange at the crack banks. It is found that, for an initial concentration of not less than 10^?4, the plasticizing effect of dissolved hydrogen causing a dislocation expulsion is significant and can be responsible (at least, partially) for plasticization. As regards the evolution of the distribution of hydrogen atoms, a monotonic drain of dissolved hydrogen atoms into the hollow of the crack is observed for concentrations below 5×10^?4, while at higher concentrations the impurity concentration at the banks of the crack varies periodically: complete drain is replaced by the accumulation of hydrogen corresponding to a “blocking” of the drain by the gas pressure. Numerical calculations are made for an α-Fe crystal.     

Evolution of the concentration of point defects at the tip of a crack

The evolution of the distribution of interstitial impurity atoms in the plastic zone around the tip of a tension crack is analyzed. The transport of point defects is determined by: 1) the hydrostatic component of the elastic stress at the crack tip, created by the superposition of the elastic fields of the crack and dislocations; 2) the elastic field of moving dislocations (“sweeping out” of interstitial impurity atoms); 3) the dislocation-driven transport of point defects present in the dislocation cores. The contributions of each mechanism of transport of point defects to the crack tip are calculated over the entire time from the start of loading of a sample containing a crack until an equilibrium distribution of plastic deformation is established after the cessation of loading. Numerical calculations are carried out for interstitial hydrogen atoms dissolved in an α-Fe crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1580–1585 (September 1997)     

Phonon spectrum and related thermodynamic properties of microcrack in bcc-Fe

The phonon spectrum and the related thermodynamic properties of microcracks in bcc-Fe are studied with the recursion method by using the Finnis--Sinclair (F--S) N-body potential. The initial configuration of the microcracks is established from an anisotropic linear elastic solution and relaxed to an equilibrium by molecular dynamics method. It is shown that the local vibrational density of states of the atoms near a crack tip is considerably different from the bulk phonon spectrum, which is closely associated with the local stress field around the crack tip; meanwhile, the local vibrational energies of atoms near the crack tip are higher than those of atoms in a perfect crystal. These results imply that the crack tip zone is in a complex stress state and closely related to the structure evolution of cracks. It is also found that the phonon excitation is a kind of local effect induced by microcracks. In addition, the microcrack system has a higher vibrational entropy, which reflects the character of phonon spectrum related to the stress field induced by cracks.     

Multiparadigm modeling of dynamical crack propagation in silicon using a reactive force field

We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for several thousand atoms near the crack tip, while more than 100,000 atoms are described with a nonreactive force field. ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including changes in crack dynamics for different crack orientations.     

Effect of plastic deformation at the crack tip in a crystal on the direction of the tip growth under a mixed load

Evolution of plastic deformation at the tip of a wedge-shaped crack in a crystal under planar strain (modes I and II) was calculated for different cleavage planes, easy-slip systems, angles at the wedge tip, and ratios of the external extension and shear loads. Time distributions are obtained for the plastic deformation, the effective shear stress, the stress intensity factor, and the crack growth direction under monotonic load of the crystal up to a specified limit and further relaxation to establishment of equilibrium distributions under a constant external load. Numerical calculations were performed for an α-Fe crystal.     

晶体相场法研究应力状态及晶体取向对微裂纹尖端扩展行为的影响

采用晶体相场模拟研究了单向拉伸作用下初始应力状态、晶体取向角度对单晶材料内部微裂纹尖端扩展行为的影响, 以(111)晶面上的预制中心裂纹为研究对象探讨了微裂纹尖端扩展行为的纳观机理, 结果表明: 微裂纹的扩展行为主要发生在<011>(111)滑移系上, 扩展行为与扩展方向与材料所处的初始应力状态及晶体取向紧密相关. 预拉伸应力状态将首先诱发微裂纹尖端生成滑移位错, 进而导致晶面解理而实现微裂纹尖端沿[011]晶向扩展, 扩展到一定程度后由于位错塞积, 应力集中, 使裂纹扩展方向沿另一滑移方向[101], 并形成锯齿形边缘; 预剪切应力状态下, 微裂纹尖端首先在[101]晶向解理扩展, 并诱发位错产生, 形成空洞聚集型长大的二次裂纹, 形成了明显的剪切带; 预偏变形状态下微裂纹尖端则直接以晶面解理形式[101]在上进行扩展, 直至断裂失效; 微裂纹尖端扩展行为随晶体取向不同而不同, 较小的取向角度会在裂纹尖端形成滑移位错, 诱发空位而形成二次裂纹, 而较大的取向角下的裂纹尖端则以直接解理扩展为主, 扩展方向与拉伸方向几近垂直.     

α-Fe裂纹的分子动力学研究

通过分子动力学方法，模拟了α-Fe裂纹的单轴拉伸实验中的形变过程.研究了不同晶体取向裂纹的形变特点和断裂机理，观察到各种形变现象，如位错形核和发射，位错运动，堆垛层错或孪晶的形成，纳米空洞的形成与连接等.计算结果表明，裂纹扩展是塑性过程和弹性过程相结合的过程，其中塑性过程表现为由裂尖发射的位错导致的原子切变行为，而弹性过程的发生则是由无位错区中的原子断键所导致.同时还研究了α-Fe裂纹的形变特点和断裂机理与温度场和应力场的依赖关系.     

Simultaneous near field imaging of electric and magnetic field in photonic crystal nanocavities

The insertion of a metal-coated tip on the surface of a photonic crystal microcavity is used for simultaneous near field imaging of electric and magnetic fields in photonic crystal nanocavities, via the radiative emission of embedded semiconductor quantum dots (QD). The photoluminescence intensity map directly gives the electric field distribution, to which the electric dipole of the QD is coupled. The magnetic field generates, via Faraday's law, a circular current in the apex of the metallized probe that can be schematized as a ring. The resulting magnetic perturbation of the photonic modes induces a blue shift, which can be used to map the magnetic field, within a single near-field scan.     

Crack extension force and rate of mechanical work of fracture in linear dielectrics and piezoelectrics

The physical significance of the crack extension force produced by mechanical loads and electric fields in linear dielectric and piezoelectric materials is examined using simple thermodynamic arguments. General expressions are derived for the crack extension force in dielectrics and piezoelectrics, under mechanical and electrical loads, in terms of the measurable parameters elastic compliance and electric capacitance . It is shown that the crack extension force produced by an electric field on an impermeable crack is always negative and it is argued that under combined electromechanical loads the total crack extension force in a piezoelectric cannot be separated into a mechanical component and an electrical component. Expressions for the crack extension force in terms of mechanical and electrical intensity factors are also given. Their derivation from available solutions for the electromechanical field at the crack tip (in a transversely isotropic material of crystal class 6 mm) is presented in detail to emphasize the physical significance of the coefficients that appear in the final expressions. In the light of these results, existing experimental observations that appear to be inconsistent with theoretical expectations are re-examined. The suggestion that the crack extension force is not a valid parameter to characterize the fracture behaviour of ferroelectrics is justified on physical grounds. Its importance is discussed and the rate of mechanical work of fracture is proposed as a more suitable parameter for those cases where the electric field does not produce dielectric rupture, nor degradation of the material at the crack tip.     

Finite strain stress fields near the tip of an interface crack between a soft incompressible elastic material and a rigid substrate

We present a numerical study of finite strain stress fields near the tip of an interface crack between a rigid substrate and an incompressible hyperelastic solid using the finite element method (FEM). The finite element (FE) simulations make use of a remeshing scheme to overcome mesh distortion. Analyses are carried out by assuming that the crack tip is either pinned, i.e., the elastic material is perfectly bonded (no slip) to the rigid substrate, or the crack lies on a frictionless interface. We focus on a material which hardens exponentially. To explore the effect of geometric constraint on the near tip stress fields, simulations are carried out under plane stress and plane strain conditions. For both the frictionless interface and the pinned crack under plane stress deformation, we found that the true stress field directly ahead of the crack tip is dominated by the normal opening stress and the crack face opens up smoothly. This is also true for an interface crack along a frictionless boundary in plane strain deformation. However, for a pinned interface crack under plane strain deformation, the true opening normal stress is found to be lower than the shear stress and the transverse normal stress. Also, the crack opening profile for a pinned crack under plane strain deformation is completely different from those seen in plane stress and in plane strain (frictionless interface). The crack face flips over and the tip angle is almost tangential to the interface. Our results suggest that interface friction can play a very important role in interfacial fracture of soft materials on hard substrates.     

Variation of the atomic absorption of alkali metals in a flame on displacement of the ionization equilibrium by an external electric field

A constant or alternating electric field applied to graphite electrodes affects the absorption of light by alkali metal atoms in a flame and also the emission intensity. The weakening of the atomic absorption of potassium, rubidium, and cesium near the cathode has been studied both over the width (between the electrodes) and over the height (along the electrode and higher). The effect is connected with the fall in the partial pressure of the metal atoms resulting from a shift of the ionization equilibrium to the right.     

氢在Nd晶体中行为的分子动力学模拟

由三维Mobius反演变换所得的金属Nd原子和H原子间的相互作用势和组合规则的方法得到的Nd H原子间的相互作用势 ,利用正则系统分子动力学算法研究了在一定加载应力强度因子K =0 .6MPam下 ,氢在Nd晶体中的行为。模拟结果表明 ,氢在Nd晶体裂尖富集成许多氢原子团或氢气团。这可用来部分地解释NdFeB稀土永磁体吸氢后的氢爆行为。     

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack are dealt with in this paper. Utilizing the conformal mapping technique, the closed-form solutions are derived for complex potentials and stress fields due to a screw dislocation located near the interfacial blunt crack. The stress intensity factor on the crack tips and the critical stress intensity factor for dislocation emission are also calculated. The influence of the orientation of the dislocation and the morphology of the blunt crack as well as the material elastic dissimilarity on the shielding effect and the emission criterion is discussed in detail. The results show that positive screw dislocations can reduce the stress intensity factor of the interfacial blunt crack tip (shielding effect). The shielding effect increases with the increase of the shear modulus of the lower half-plane, but it decreases with the increase of the dislocation azimuth angle. The critical loads at infinity for dislocation emission increases with the increase of emission angle and curvature radius of blunt crack tip, and the most probable angle for screw dislocation emission is zero. The present solutions contain previous results as special cases.     

Local fracture of thin metallic films during electromagnetic loading

The pulsed electrodynamic fracture of thin (10–50 nm) aluminum films deposited onto polymer substrates is experimentally studied. The fracture of the films manifests itself in the form of discontinuity (crack) channel growth across an applied electric field. The roles of a magnetic pressure, thermomechanical stresses, and Joule heating at the tip of a growing crack are analyzed at high current densities (∼10¹⁰−10¹² A/m²). In contrast to the results well known for the electrodynamic fracture of bulk metallic samples, the first two factors in the films are insignificant. The effect of the electric explosion of the crack tip is found to play a key role in the beginning of fracture.     

Effect of the charge dragging in a graphene-based superlattice under a constant electric field

The effect of a constant electric field on the charge dragging by an electromagnetic wave is studied for graphene-based superlattice. An expression is derived for the electric current density in a graphene-based superlattice under a constant electric field in an approximation of a constant time of relaxation. It is demonstrated that the current as a function of wave intensity is of a nonmonotonous character.     

Theory of a surface phase transformation induced by impurity atoms in binary alloys containing interstitial impurity atoms

Mutual influence of the surface segregation of atoms on lattice sites and in interstitials is studied in the self-consistent field approximation. The effect of occurrence of a local surface phase transformation is predicted, which is caused by a local loss of stability of the statistical distribution of atoms of components on the surface due to the presence of interstitial impurity atoms. A theory is constructed for an induced phase transformation for [001] plane of a single crystal of a binary alloy with an fcc lattice whose octahedral interstitials are occupied by interstitial atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 50–55, September, 1984.     

Mechanism for Conversion of the Type of Conductivity in p-Hg1–xCdxTe Crystals upon Bombardment by Low-Energy Ions

The main peculiarities of the p–n conversion of the type of conductivity in narrow-band p-Hg_1–x Cd _x Te solid solutions containing vacancies of mercury upon bombardment by low-energy ions are explained based on the traditional notions about the chemical diffusion of mercury. These peculiarities are related, on the one hand, to the features of the defect formation in Hg_0.8Cd_0.2Te (containing a small amount of high-mobility interstitial mercury atoms with a great amount of low-mobility vacancies) and, on the other hand, to the high concentrations of intrinsic electrons and holes efficiently screening the electric field of the defect layer. The high conversion rate realized upon ion bombardment, as compared to the conversion rate taking place upon annealing in mercury vapors, is due to the fact that nonequilibrium interstitial mercury atoms are produced in abundance near the surface of the crystal subject to bombardment. This effect depends substantially on the electric field appearing near the outer boundary of the converted layer; therefore, as the Hg content, and, hence, the width of the forbidden band, is increased, one should expect a noticeable decrease in p-n conversion rate.     

Non-local theory solution of two collinear mode-I permeable cracks in a magnetoelectroelastic composite material plane

The non-local theory solution of two collinear mode-I permeable cracks in a magnetoelectroelastic composite material plane was investigated using the generalized Almansi's theorem and the Schmidt method. The problem was formulated through Fourier transform into two pairs of dual integral equations, in which the unknown variables are the jumps in displacements across the crack surfaces. To solve the dual integral equations, the displacement jumps across the crack surfaces were directly expanded as a series of Jacobi polynomials. Numerical examples were provided to show the effects of crack length, the distance between two collinear cracks and the lattice parameter on the stress field, the electric displacement field and the magnetic flux field near the crack tips. Unlike the classical elasticity solutions, it is found that no stress, electric displacement or magnetic flux singularities are present at the crack tips in a magnetoelectroelastic composite material plane. The non-local elastic solutions yield a finite hoop stress at the crack tip, thus allowing us to use the maximum stress as a fracture criterion.     

Nanoscale experimental study of the morphology of a microcrack in silicon by transmission electron microscopy

A microcrack in a silicon single crystal was experimentally investigated using high-resolution transmission electron microscopy (HRTEM). In particular, the numerical Moiré (NM) method was used to visualize the deformations and defects. The lattice structure of the microcrack was carefully observed at the nanoscale. HRTEM images of the microcrack demonstrated that the lattice structure of most of the microcrack regions is regular with good periodicity. In addition, the microcrack cleavage expands alternately along different crystal planes, where the principal cleavage plane is the (1 1 1) crystal plane. The NM maps showed no sharp plastic deformation around the microcrack, but discrete edge dislocations can be found only near the crack tip.     

Luminescence spectrum in far wings of the resonance line of cesium atoms approaching a sapphire surface

The shape of the luminescence spectrum of Cs atoms flying near a sapphire surface is described phenomenologically for the detuning from the D2 line ranging within 5–50 cm^?1. The numerical fitting of one theoretical parameter and a small variation of another parameter ensures a satisfactory agreement with the experimental spectrum. The experimentally determined abnormally high intensity of the antistatic wing is explained by the effect of phase relaxation of the atomic transition, caused by fluctuations of the electric field of oscillating ions of the sapphire crystal lattice.