Dynamic aging of dislocations in materials with a high crystalline relief: Competition between diffusion and impurity entrainment

A model of the dynamic interaction of dislocations with the impurity subsystem of crystals that have a high lattice potential relief (Peierls barriers) has been developed. It is shown that the microscopic structure of migration barriers for impurities near a dislocation core may cause qualitatively different behavior of the impurity atmosphere on a moving dislocation. It is justified that the impurity kinetics during atmosphere formation includes two stages. The first (initial) stage is fast and significantly nonequilibrium; it is followed by the second stage, characterized by a slower approach to equilibrium. The initial stage manifests itself at a sufficiently fast dislocation motion and may lead to an anomalous increase in the driving force (or the yield strength of the material) with an increase in the temperature in some range. Blocking of the dislocation motion by impurities may cause inverse brittle-ductile transition, which is observed in some materials with an increase (rather than the usual decrease) in temperature.     

碳化硅单晶位错研究进展

SiC作为代表性的第三代半导体材料,具有优异的物理化学性能。随着材料及应用的发展,SiC衬底在航天电源、电动汽车、智能电网、轨道交通、工业电机等领域的应用日益重要。相比第一代半导体材料如Si和第二代半导体材料如GaAs而言,SiC衬底质量还有很大的改善空间,是现阶段研发和产业的热点。其中SiC单晶缺陷,特别是一维位错缺陷的检测和降低,是近10年内重要的研究内容。本文重点对SiC中位错的形成原因、位错检测技术、位错密度降低方法及近年来SiC单晶中位错的优化水平进行总结归纳,并提出了SiC需要继续突破和发展的方向。     

Fluctuations in the step velocity and the generation of a dislocation spiral on the (101) face of monoclinic lysozyme crystals

The generation of a dislocation spiral and fluctuations in the step velocity on the (101) face of monoclinic lysozyme crystals have been studied by in situ atomic force microscopy (AFM). It is shown that the (101) face grows by the dislocation mechanism and that the steps move via the formation of one-dimensional nuclei. The velocity of a part of the step fluctuates, with the fluctuations increasing proportionally to the fourth-order root of time. In the process of spiral generation, a segment of the step attains a certain critical length and then moves with a constant velocity. Even under constant supersaturation, the fluctuations can give rise to changes in the segment length. The interstep distance in the step echelon also varies.     

Analytical description of the yield drop in the Alexander-Haasen model

The analytical solution of the equations of the Alexander-Haasen model, which describes the shape of the deformation-curve peak (the so-called yield drop), has been obtained for the case of a low initial dislocation density. It is shown that the self-development of the dislocation structure results in the specific kinetic transition with a dramatic decrease of the elastic-deformation rate and an increase of the plastic-flow rate. It is natural to interpret this phenomenon as an elastoplastic transition, and to consider the corresponding stress as the yield stress despite the fact that, being considered in the traditional way, its value does not coincide with either the upper or lower yield stress. The conditions of the existence of the yield drop are studied, and the quantitative criterion of the corresponding change in the deformation-curve shape is proposed.     

LCAO-HMO Study of the Bond Energy of Small Crystals

The binding energy of one-, two- and three-dimensional crystals of primitive cubic lattices consisting of one-electron atoms are studied using the Hückel LCAO-MO method. It is shown that the average bond energy is decreased with an increase in the number of atoms in the crystal, and a trend to a constant value, different for one-, two- and three-dimensional crystals is observed. On the basis of the growth process of a one-dimensional crystal it is shown that the second atom possesses the highest bond energy. The bond energy of each subsequently added atom is smaller. However, the atom which is even forms a stronger bond than the atom which is odd by sequence of adding. The even-odd oscillations in the bonding energies are damped parallel to an increase in size of one-dimensional crystal. A comparison is made of the ratio between the experimentally obtained values for the dissociation energy of diatomic molecule and the energy per bond in an infinitely large crystal, and the ratio of the same quantities calculated by the authors. Comparison shows that in general a correlation is observed between the two ratios for these extreme cases.     

Determination of the types and densities of dislocations in GaN epitaxial layers of different thicknesses by optical and atomic force microscopy

The change in the dislocation density on the surface of GaN epitaxial layers, which were grown by hydride vapor-phase epitaxy on sapphire substrates with c and r orientations, has been investigated by optical and atomic force microscopy (AFM). It is shown that the observed decrease in the density of threading dislocations with an increase in the layer thickness is related to the annihilation of mixed dislocations. The experimental and theoretical data on the change in the density of mixed dislocations with an increase in the epitaxial-layer thickness are in good correspondence.     

Defects in LEC-grown Indium Phosphide Crystals Doped with Tin

Crystalline defects in Sn-doped LEC indium phosphide have been revealed by chemical etching and analyzed by TEM. Grown-in dislocations, various kinds of defect clusters and colonies of microdefects were found. The symmetrical defect clusters are shown to equate mostly with larger dislocation loops exhibiting shear components and/or other dislocation arrangements generated by a stress source which is positioned in the centre of the dislocation cluster. Those centres are often formed by a plate-like agglomeration composited of tiny inclusions and very small faulted dislocation loops. Such planarly arranged accumulations of microdefects lie on {111} planes. The direct vicinity of single threading grown-in dislocations is always enriched with tiny perfect dislocation loops and precipitates. Additionally, very large isolated interstitial-type perfect dislocation loops with b = a₀/2 〈110〉 have been found by TEM experiments. Mostly, the {110} habit plane of such loops is decorated with an high number of small dislocation loops and precipitates as a consequence of dislocation climb.     

Reconstruction of Dislocation Potential Relief by Means of Self-Blocking Effect

The effect of dislocation self-blocking in intermetallic compounds with an anomalous temperature dependence of yield stress has been theoretically and experimentally analyzed. In essence, this effect is a dislocation self-immersion in a deep potential relief valley (without external stress). The possibility of reconstructing the relief shape (from one-valley to two-valley) is shown and the relief parameters are reported. The allowed and forbidden regions for self-blocking are revealed. The method for determining the ratio of the valley depths by measuring the self-blocking limiting angles between the dislocation segments is proposed. Transmission electron microscopy images of the dislocation structure can be used to this end. As an example, this ratio is estimated for a superpartial dislocation sliding in the cube plane in Ni₃Ge.     

Mechanoluminescence in Centrosymmetric Crystals

The paper reports that the mechanoluminescence (ML) is not an inherent property of only the non-centrosymmetric crystals. The ML may appear in number of centrosymmetric crystals due to variety of processes. The ML of 82 centrosymmetric crystals are reported and different models are proposed for the ML excitation. The models proposed are: space charge electrification model, triboelectrification model, phase transformation model, gas adsorption model, chemical reaction model, thermal population model, molecular deformation model, cleavage electrification model, defective piezoelectric phase model, dislocation defect stripping model, dislocation unpinning model, dislocation annihilation model, charged dislocation model and incandecent light emission model. It is shown that on the basis of the proposed model, intense mechanoluminescent materials with desired nature and characteristics may be prepared.     

Dislocation arrangements in cyclically deformed polycrystalline molybdenum

The present paper deals with the cyclic stress-strain behaviour of polycrystalline molybdenum at room temperature and the dislocation structures built up within this material during the fatigue process. A cyclic stress-strain curve of molybdenum deformed in a strain-controlled and symmetrical push-pull test is shown. At strain amplitudes e_a < 3 × 10⁻³ arrangements of relatively homogeneously distributed dislocations are observed in the stage of the stabilization of mechanical properties. The characteristics of these dislocation arrangements are similar to those of dislocation structures of unidirectionally deformed molybdenum single crystals. At strain amplitudes e_a > 3 × 10⁻³ dislocation structures are developed with an inhomogeneous dislocation distribution (bundles structures). The dislocation density in the surface layers of fatigued specimens shows larger values than within the material. The cyclic deformation after a change from a small deformation amplitude to a larger one, or vice versa, is connected with characteristic changes of dislocation density.     

Fluctuations of impurity composition and their role in dislocation cross slip in crystals

The influence of impurities on the kinetics of split-dislocation cross slip caused by a change in the stacking-fault energy is studied theoretically. It is shown that the fluctuations in the impurity composition of a crystal make a considerable contribution to the kinetics of dislocation cross slip. The activation-energy spectrum and the average frequency of the processes of dislocation cross slip are calculated for a model of random impurity distribution in a crystal. The calculation shows that the fluctuations in impurity concentration, reducing the stacking-fault energy, play an important role in the low-temperature region.     

Self-blocking of dislocations: A new concept

The effect of self-blocking (transformation of dislocations at zero external stress from sliding configurations into blocked ones) was theoretically predicted and then found in Ni₃(Al, Nb) and TiAl alloys. The physical reasons for self-blocking are revealed. The thermally activated dislocation flip from a shallow potential relief valley into a deep one is shown to be the controlling process. It occurs through the formation of a double kink with its subsequent reorientation, leading to the formation of an asymmetric kink and the extension of dislocation along the preferred direction. The driving force of this process is calculated and its conditions are formulated. It is shown on the basis of a set of experimental data that the multivalley character of the potential relief plays a key role in the dislocation blocking and self-blocking. It is proven that both effects—the yield stress anomaly and self-blocking—have the same nature: a two-valley potential relief of dislocation.     

Study of crystal perfection in the KCl-KBr and KCl-RbCl systems

The dislocation densities in the crystals of the KCl-KBr and KCl-RbCl systems with varying KCl concentration have been determined from the integrated reflection intensities. The results obtained lead to the conclusion about the formation of disordered solid solutions. It is shown that the dislocation density in the systems is higher than the dislocation density in the starting components and attains the maximum value for the compositions close to equimolar. The change of the dislocation and microstress densities depending on the KCl concentration are well correlated, which indicates the appearance of high stresses during the formation of a solid solution.     

Influence of impurity and indentation time on dislocation rosettes

It has been shown that the impurity type and indentation time affect appreciable on dislocation rosettes of NaCl single crystals. It was established that the growth of indentation diagonal was not practically followed by the increase of the dislocation zone size. This phenomenon connected with the cross slip processes, which are perhaps more intensive for prolonged indentation time and high temperatures. The assumption is made that the two mechanisms of responsible for indentation formation the movement of dislocations and their multiplication by the cross slip.     

Regularities of plastic deformation in indentation on the (111) plane of NaCl single crystals

Dislocation structures arising in indentation on the (111) face of NaCl single crystals are studied by etch-pit technique. The deformation temperature varied in the interval from 77 to 520 °K. The effect of radiation defects introduced by γ-irradiation on the shape of dislocation rosette was investigated. Three types of dislocation rosettes depending on the experimental conditions were observed. A possible explanation of this phenomenon is proposed. It is shown that the polarity of plastic deformation observed under indentation is an universal phenomenon which is revealed in the temperature range from 77 to 470 °K. The phenomenon of dislocation rosette inversion was first observed.     

A model for the growth of anomalous polytype structures in vapour grown SiC

A number of polytype structures observed in vapour grown SiC crystals have a unit-cell which is an integral multiple of the unit-cell of the basic 6H, 15R or 4H structure. The growth of such anomalous structures cannot be understood in terms of spiral growth round a single screw dislocation in a basic matrix. However many of these polytype crystals display a single growth spiral on their (0001) face indicating that they have resulted from spiral growth round a single screw dislocation. It is shown that this anomaly can be resolved if the basic matrix is assumed to contain stacking faults near the surface at the time of the origin of the screw dislocation ledge. This possibility, overlooked in the earlier deduction of polytype structures, must be taken into consideration since vapour grown SiC crystals frequently contain a high concentration of random stacking faults, producing continuous streaks on their X-ray diffraction photographs. The most probable fault configurations that can occur in 6H, 15R and 4H structures of SiC have been deduced from a calculation of their stacking fault energy. These fault configurations are then considered to lie at different distances from the surface at the time of the origin of a screw dislocation ledge. Such a faulted ledge gives rise to polytype structures during subsequent spiral growth even if the screw dislocation has an integral Burgers vector. The most probable series of polytype structures that can result from such a faulted matrix model are deduced. It is shown that nearly all the polytype structures of SiC hitherto regarded as anomalous (such as 36H, 54H, 66H, 45R, 90R etc.) are among the expected structures and there is no need to postulate a complicated configuration of cooperating dislocations to account for their growth.     

On the elements of the dislocation structure of deformed corundum

It is found by the method of transmission electron microscopy that the main elements of dislocation structure of deformed corundum are spatial triple dislocation nodes, recombination zones and dislocation network fragments. The structure of dislocation nodes is analyzed. It is shown that the nodes in corundum are the “rigid” constructions.     

Correlation between microhardness and dislocation mobility in crystals with a different type of bond

The complex investigations of microhardness and dislocation mobility in temperature interval 77–873 K have been performed on ionic crystals, semimetals and covalent crystals. It is shown that the correlation between the microhardness and dislocation mobility exists as a whole only. The temperature change of microhardness is not often connected with the change of dislocation mobility. An “anomalous” behaviour of semimetals has been established – they have a small microhardness and a low dislocation mobility at a time.     

Dislocation formation and motion in germanium single crystals

The dislocation structure of germanium single crystals used in IR optics is investigated. The effect of high-temperature annealing on the dislocation distribution in Ge single crystals is shown.     

X-ray investigations of the defect structure of aluminium after plastic deformation

With plastic deformation of polycrystalline metals deformation textures are build up. Our experimental results show that the dislocation density increases, even with small plastic deformation. Simultaneously a decrease in the value of coherence length was revealed. The formation of a distinctly marked sub-structure, which is accompanied by an increase of the dislocation density, is essentially completed when plastic strain reaches 30 per cent. Further increase of the degree of deformation is followed by a change of orientation of the subgrains. When plastic strain is 60 per cent the deformation texture is almost completely accomplished.