Collective effects in an ensemble of dislocations and vacancies with the formation of the localized deformation zone

Abstract

Paperi have reported about the formation of the superplastic zone as the result of defect production in high strength alloys with the fine precipitates of a non-metalic phase under the intensive stress of about (10^-2 ÷ 10^-1)μ, where μ is a shear modulus. It turned out, that the stopping of slipping dislocations near the precipitates leaded to vacancy influx, which promoted dislocations climbing, on the one hand, and increased its concentration on the other. The higher vacancy concentration, the higher dislocation density increases     

The effect of deformation and forest dislocations density on the activation volume in zinc single crystals

The obstacle density calculated from the experimental values of the activation volume is compared with the density of forest dislocations. The density of obstacles is about two or three order higher than that of the forest dislocations. It is assumed that nonconservative jog motion is the rate-controlling mechanisms in stage A, while in stage B the activation volume is more directly determined by interaction with the forest dislocations.     

Mechanism of the formation of deformation steps of nanometric sizes at the surface of plastically deformed crystals

The mechanism of the formation of nanometric-size deformation steps at the surface of plastically deformed crystals is discussed theoretically. Such steps are detected by means of a scanning tunneling microscope or by high-resolution speed filming. The analysis shows that the exponential step distribution by height is due to the double cross slip (DCS) of screw dislocations and that the growth kinetics of separate steps is deter-mined by the kinetics of the Frank-Read dislocation sources appearing as a result of the DCS.     

Formation of dislocations with local mechanical damage to a silicon surface at room temperature

A study is made of the formation of dislocations in silicon with local damage to the surface at room temperature and subsequent annealing within the range 1073–1473 K. The damages to the surface are modeled with the use of micro-hardness indentations. Measurements of mean linear dislocation density in a ray of the indentation rosette show that the number of dislocations in the rosette is independent of both the temperature and duration of isothermal annealing. It was found that annealing at 573–773 K leads to partial relaxation of elastic stresses from the indentation due to the formation of sections of silicon with a hexagonal structure near the indentation. Further annealing at high temperatures leads to the disappearance of these sections and the formation of a normal dislocation rosette, with the number of dislocations in the rays corresponding to the case of one-stage annealing. The results are empirical confirmation of the hypothesis of incomplete shear. In accordance with the latter, dislocations are formed during deformation at room temperature, not during subsequent annealing.Zaporozh'e University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 78–82, May, 1992.     

Plastic deformation of the precipitation hardened Cu-0·7 at.% Ti single crystals

Single crystals Cu-0·7 at.% Ti were deformed at room temperature in tension. The effect of the annealing time on the shape of the stress-strain curves has been investigated. The change of the stress-strain curve consisting of three regions to that one with one region only is observed if the annealing time is increased. There is a maximum in the dependence of CRSS on the annealing time fort=140 hours.The author wishes to thank to Dr. P.Kratochvil who stimulated this research and helped him. It is also the author's pleasure to thank to Ing. J.Bican of the Research Institute of Metals, who kindly prepared of Cu-Ti alloys. Also the determination of Ti content in all samples by Dr. J.Vrbský and Dr. Z.Ksandr of the Chemical College is acknowledged.     

Dynamic interaction of edge dislocations with point defects and prismatic dislocation loops at high-strain-rate deformation of crystals

The motion of an ensemble of edge dislocations at high-strain-rate deformation of a crystal with a high concentration of prismatic dislocation loops and point defects has been analyzed. It has been shown that, under certain conditions, the drag of an edge dislocation by prismatic dislocation loops has the character of dry friction, and the magnitude of the drag force of the dislocation is determined by the relationship between the concentration of prismatic dislocation loops and the density of mobile dislocations. An increase in the density of mobile dislocations leads to an enhancement of their collective interaction, thus facilitating the overcoming of prismatic dislocation loops by edge dislocations. The total drag force of an edge dislocation is a nonmonotonic function of the concentration of point defects, which, under certain conditions, has a minimum.     

On the redistribution of dislocations in silicon single crystals in the vicinity of stress concentrators

The mechanism of relaxation of stresses through the multiplication and motion of dislocations in the vicinity of a stress concentrator (scribe) in silicon single crystals is investigated. The energy-related and dynamic characteristics of the observed processes are determined for different angles of orientation of stress concentrators with respect to the [110] direction.     

Investigation of the conditions for feeding filamentary silicon crystals,growing in a reactor with hot walls

An estimate of the factors which influence the rate of growth of filamentary silicon crystals in a standard chloride system using a quartz reactor with hot walls is given. It is shown that a diffusion form of crystallization is observed under the conditions investigated.Voronezh State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 22–26, October, 1995.     

Specific features in the generation and motion of dislocations in silicon single crystals doped with nitrogen

The specific features in the generation and motion of dislocations are investigated in Si: N single crystals grown by the Czochralski method. The motion of dislocation loops is analyzed by the four-point bending technique in the temperature range 500–800°C. The dislocation loops are preliminarily introduced into the samples with the use of a Knoopp indenter at room temperature. It is found that doping with nitrogen leads to a considerable increase in the critical stress of the onset of dislocation motion from surface sources (indentations) and in the stress of the generation of dislocations from internal sources. The velocity of dislocation motion in Si: N crystals is less than that in undoped crystals (under comparable loads). The hardening effect of nitrogen is explained by the fact that nitrogen promotes the decomposition of a solid solution of oxygen in silicon during postcrystallization cooling.     

On elastic deformation of the indentation surface in alkali-haloid crystals

It is demonstrated experimentally that in the microindentation process, elastic deformation takes place of the crystal surface region under load containing the forming imprint. A technique is proposed for determining this elastic deformation in alkali-haloid crystals.     

Resistance of zinc crystals to shock deformation and fracture at elevated temperatures

Velocity profiles of the free surface of shock-loaded zinc crystals are measured in two different orientations. The test temperature is varied from room temperature to 410 °C. The results of the measurements show that the high-velocity deformation and fracture are athermal processes and that the fracture stresses are influenced by the preceding plastic deformation. Fiz. Tverd. Tela (St. Petersburg) 40, 1849–1854 (October 1998)     

Steps and facets at the surface of soft crystals

We consider the shape of crystals which are soft in the sense that their elastic modulus μ is small compared to their surface tension γ, more precisely μa < γ where a is the lattice spacing. We show that their surface steps penetrate inside the crystal as edge dislocations. As a consequence, these steps are broad with a small energy which we calculate. We also calculate the elastic interaction between steps a distance d apart, which is a 1/d ² repulsion. We finally calculate the roughening temperatures of successive facets in order to compare with the remarkable shapes of lyotropic crystals recently observed by Pieranski et al. [#!Pieranski!#,#!EPJ!#]. Good agreement is found. Received 25 June 2001     

Processes involved in the formation of silver clusters on silicon surface

We analyze scanning electron microscopy measurements for structures formed in the deposition of solid silver clusters onto a silicon(100) substrate and consider theoretical models of cluster evolution onto a surface as a result of diffusion and formation of aggregates of merged clusters. Scanning electron microscopy (SEM) data are presented in addition to energy dispersive X-ray spectrometry (EDX) measurements of the these films. Solid silver clusters are produced by a DC magnetron sputtering source with a quadrupole filter for selection of cluster sizes (4.1 and 5.6 nm or 1900 and 5000 atoms per cluster in this experiment); the energy of cluster deposition is 0.7 eV/atom. Rapid thermal annealing of the grown films allows analysis of their behavior at high temperatures. The results exhibit formation of cluster aggregates via the diffusion of deposited solid clusters along the surface; an aggregate consists of up to hundreds of individual clusters. This process is essentially described by the diffusion-limited aggregation (DLA) model, and thus a grown porous film consists of cluster aggregates joined by bridges. Subsequent annealing of this film leads to its melting at temperatures lower than to the melting point of bulk silver. Analysis of evaporation of this film at higher temperatures gives a binding energy in bulk silver of ɛ₀= (2.74 ± 0.03) eV/atom. The text was submitted by the authors in English.