Interactions of glide dislocations in a channel of a persistent slip band

Two unlike dislocations gliding in parallel slip planes in a channel of a persistent slip band are considered. Initially they are kept apart in straight screw positions. As the dislocations are pushed by the applied stress between two walls in the opposite directions, they bow out and attract one another forming a dipole. With the increasing stress the dislocations become more and more curved, until they separate. The walls of the channel are represented by elastic fields of rigid edge dipoles. The dislocations are modelled as planar curves approximated by moving polygons. The objective of the simulations is to determine the stress in the channel needed for the dislocations to escape one another. The stress and strain controlled regimes considered provide upper and lower estimates of the escape stress. The results are compared with the studies by Mughrabi and Pschenitzka, and Brown and the recent dislocation dynamics estimates. Problems encountered in the dislocation dynamics evaluation of the escape stress are analyzed.     

A crystal plasticity model of a formation of a deformation band structure

The formation of deformation bands with the typically alternating sign of the misorientation across their boundaries is interpreted as spontaneous deformation instability caused by anisotropy of hardening. To analyse the nature of the fragmentation, a model of a rigid-plastic crystal domain deformed by symmetric double slip in a plane-strain compression is considered. The basic reason for the deformation band existence is that a local decrease in number of active slip systems in the bands is energetically less costly than a homogeneous deformation by multislip. However, such model of the bands predicts their extreme orientation and their width tends to zero. This trend is modified by hardening caused by a build up of the band boundaries and by a dislocation bowing (Orowan) stress. The model provides an explanation of observed orientation of the bands, their width and the significant change in the structural morphology seen as the band reorientation occurs at large strains. The predictions are in a favourable agreement with the available observations.     

Valence band formation in small silver clusters

UV photoemission spectra of valence electrons in small silver clusters have been compared with spectra from bulk silver samples using synchrotron radiation, 16 <hν< 27 eV. Spectra for single silver atoms supported on carbon are indicative of a completely filled 4d¹⁰ initial state configuration. With increased cluster size, both density of states and valence band modulations with respect to photon energy resemble the bulk metal more closely. Spectral modulation, characteristic of conservation of crystal momentum, appears to require a cluster consisting of ～150 atoms.     

Anomalous band formation in arrays of terahertz nanoresonators

We investigate band formation in one-dimensional periodic arrays of rectangular holes which have a nanoscale width but a length of 100 μm. These holes are tailored to work as resonators in the terahertz frequency regime. We study the evolution of the electromagnetic response with the period of the array, showing that this dependence is not monotonic due to both the oscillating behavior of the coupling between holes and its long-range character.     

Dynamics of slip band formation in fcc alloys

The problems induced by the inhomogeneity of deformation of alloys with high solute concentration are discussed using experimental results on Cu-2...10% Ni and Cu-30% Zn single crystals. Combining microscopic observations (etch-pit/stress-pulse method) with mesoscopic studies (microcinematography of slip line formation) and with macroscopic measurements (stress relaxation) in terms of the effective activation volumes, it is inferred that theories on the motion of a dislocation across the obstacle field in the slip plane can be compared with microscopic (etch-pit) results, while the macroscopic deformation kinetics and the critical resolved shear stress in the yield region of inhomogeneously deformed alloys might rather be controlled by the transfer process of slip to neighbouring slip planes.Fruitful discussions with Prof. Ch. Schwink and Dipl.-Phys. A. Hampel, as well as the continuous financial supports by the Deutsche Forschungsgemeinschaft, now in project A9 of SFB 319 are gratefully acknowledged.     

Simulation of the formation of porous-silicon structures

The formation of porous silicon is investigated by the Monte Carlo method in a model that takes account of the nonuniformity of the charge distribution over the silicon-electrolyte interface, hole diffusion, generation, and recombination processes, and size quantization. The structures obtained in a computer simulation for various doping levels of the crystalline substrate, temperatures, HF concentrations, and anode current densities are presented. Analysis of nanoporous structures shows that the porosity depends on the depth and reveals the presence of a fractal dimensionality on scales of less than 10 nm. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 685–690 (10 May 1997)     

Simulation of electromagnetic shower formation in a Germanium crystal

Abstract

When high-energy electrons penetrate crystalline matter, the successive processes of photon emission and pair production form an electromagnetic shower. If the incident electrons are directed along the crystal axis, the cross section for photon emission is drastically enhanced because electrons in ‘channeling’ states feel a strong electric field continuously. Experiments designed to detect this effect were performed at CERN. The results showed an anomalous peak in the energy loss spectrum of the emerging electrons. In this paper, we report results of a Monte-Carlo simulation of shower formation in a Germanium crystal. Our results agree with the experimental data more quantitatively than previous simulations. We simulated a shower formation by incident photons as well.     

Transmission electron microscopy study of the interaction between a glide dislocation and a dislocation node

Using in-situ tensile straining in conjunction with stereo imaging in a transmission electron microscope, real-time observations have been made in thin copper foils of the interaction of glide dislocations with a dislocation node. A mechanism is observed by which a dislocation approaching the node effectively bypasses the node by exchanging segments with one of the dislocations constituting the node.     

The formation of the position-dependent band gaps in a semiconductor placed in the strong quantized inhomogeneous magnetic field

The influence of an inhomogeneous quantized magnetic field on the energy spectrum of a semiconductor is investigated. It is shown that the curvature and splitting of the energy levels of the conduction and valence bands lead to the appearance of the position-dependent band gaps.     

S波段磁绝缘线振荡器的数值模拟

 设计了一种阶梯阴极型S波段磁绝缘线振荡器,通过对其色散关系的研究,选择了合理的结构参数。通过对开放腔模型的分析,得到了磁绝缘线振荡器的谐振频率和有载品质因数。粒子模拟表明,在外加电压523 kV、束流49.7 kA时,微波输出功率4.35 GW,频率2.10 GHz,功率转换效率16.7%。     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

周期性层状结构材料中光子带隙的形成

结合经典电磁场理论和Bloch定理,讨论了光波在周期性层状结构材料中传播的色散关系,并通过传输矩阵方法得到光波的透射谱,指出在周期性层状结构中波阻抗是形成光子带隙的本质因素.     

The contribution of Fermi resonance to the formation of the Raman valence band of water

We study the role of Fermi resonance in the formation of the valence band of Raman scattering of light by liquid water molecules. We experimentally study the Raman valence and bending bands of water under changes in temperature from 20 to 99°C and in concentrations of D₂O and KI, KBr, KCl and KF salts in the range of their full solubility. We propose and implement a method for calculating the interaction constant of Fermi resonance in water. Use of genetic algorithms together with variational methods has made it possible to increase the accuracy in calculating the interaction constant of Fermi resonance and for the first time to quantitatively estimate the contribution of resonance and the intensity of the Raman valence band of water.     

Theory of the diffusive glide of dislocations with narrow kinks

A theory of glide velocity of a dislocation with narrow kinks which encounter spatially periodic, relatively high energy barriers is developed. The thermally activated generation of double kinks is considered as the mechanism of the dislocation movement. It is assumed that the saddle point is determined by the elastic interaction between the two kinks and that diffusion of the kinks is rate controlling. According to this theory velocities of screw dislocations in -iron are calculated in dependence on temperature and the applied stress with 2E _k =0·68 eV andE=0·07 eV (E _k is the energy of an isolated kink,E is the second-order Peierls energy). Relations to three other theories, which may be considsred for calculation of velocities of screw dislocations in b.c.c. metals are discussed and demonstrated by numerical calculations for iron. It appears that there are no serious objections suggested by experiments which might be raised against the screw dislocation velocities in iron calculated according to the presented theory.     

Topological kinematic constraints: dislocations and the glide principle

Topological defects play an important role in the physics of elastic media and liquid crystals. Defect kinematics in elastic media is restrained by rigid constraints of purely topological origin. An example is the glide motion of dislocations, a topic which has been extensively studied through the years by metallurgists. To date, most theoretical investigations of this phenomenon were heuristic or numerical. Here, we outline a mathematical derivation of this universal effect and report on new generalizations. Our formalism makes it possible to address the full non-linear theory of relevance at short distance where violations of the standard glide constraint become possible. Our new derivation enables us to systematically predict and estimate corrections to the standard, linear order, glide motion. Our analysis is very broad and pertains to both classical and quantum media. To fully capture the generality of this effect, we arrive at a mathematical definition of the glide constraint which has a universal status. When fused with the mass continuity equations, this then dictates glide motion within linear elasticity and leads to new non-linear corrections in a general elastic medium. It further enables us to study the kinematics of dislocations in arbitrary spatial dimensions (or space-time dimensions in the quantum arena). As an example, we analyze the restricted climb associated with edge dislocations in 3±1D. Quite generally, the climb constraint is equivalent to the condition that dislocations do not communicate with compressional stresses at long distances.