Dislocation dynamics in solid solutions of covalent crystals

The dislocation mechanism of solid solution strengthening of covalent semiconductor crystals has been studied. The change in the regularities of dislocation dynamics in solid solutions from those in the components of the solution is connected with the manifestation of the nonlinear drift of dislocation kinks. The theory developed suggests an explanation of specificities of the dislocation mobility in a Ge_1–cSi_c solid solution.     

Dislocation motion in NaCl crystals under combined loading by electron-beam-produced mechanical and electromagnetic pulses

A study is reported of the effect of a pulsed electromagnetic field on edge dislocation mobility in NaCl crystals under mechanical loading by an electron beam. It is shown that taking this effect into account removes the discrepancy in determination of the dislocation dynamic damping coefficient by two methods, namely, with an electron beam and by mechanical impact loading. Fiz. Tverd. Tela (St. Petersburg) 41, 2004–2006 (November 1999)     

Dislocation spectroscopy of crystals

A new method of studying the energy characteristics of dislocations is proposed, which is based on the investigation of the interaction of moving dislocations with purposefully introduced electronic and hole centers. A study has been made of KCl, NaCl, KBr, LiF, and KI alkali halide crystals containing electronic F and hole V _K and Me ⁺⁺ (Cu⁺⁺, Ag⁺⁺, Tl⁺⁺, In⁺⁺) centers. Investigation of the temperature dependence of the dislocation interaction with the F centers permitted determination of the position of the dislocation-induced electronic band (DEB) in the band diagram of the crystal. In KCl, the DEB is separated by ≈2.2 eV from the conduction-band minimum. It is shown that dislocations transport holes from the centers lying below the dislocation-induced hole band (DHB) (X ⁺, In⁺⁺, Tl⁺⁺, V _K) to those above the DHB (the Cu⁺ and Ag⁺ centers). Such a process is temperature independent. The DHB position in the crystal band diagram has been determined; in KCl it is separated by ≈1.6 eV from the valence-band top. The effective radii of the dislocation interaction with the electronic F and hole X ⁺, V _K, and Tl⁺⁺ centers have been found. Fiz. Tverd. Tela (St. Petersburg) 41, 2139–2146 (December 1999)     

Dislocation dynamics in nanocrystalline nickel

It is believed that the dynamics of dislocation processes during the deformation of nanocrystalline materials can only be visualized by computational simulations. Here we demonstrate that observations of dislocation processes during the deformation of nanocrystalline Ni with grain sizes as small as 10 nm can be achieved by using a combination of in situ tensile straining and high-resolution transmission electron microscopy. Trapped unit lattice dislocations are observed in strained grains as small as 5 nm, but subsequent relaxation leads to dislocation recombination.     

Z-like-centers in vanadium doped NaCl crystals

The absorption, thermostimulated luminescence (TSL) and electron emission (TSE) of prepared and X-irradiated NaCl crystals doped with vanadium were studied. From the TSE-, TSL- and thermo-optical bleaching (TOB-) curves it is suggested that the new 647 nm absorption band and the new TSE- and TSL-peaks at about 480–490 °K are due to Z-centers, i.e. to the aggregates ofF-centers with dipole complexes (divalent vanadium ions and cation vacancies).     

Dislocation dynamics in cyclic plastic deformation

The paper presents a theoretical investigation of the slip avalanches (so-called strain bursts) which occur in single-glide-orientated face-centered cubic or hexagonal close-packed metals during stress-amplitude-controlled cyclic plastic deformation. The study is based on a model of the dynamics of dislocations that has been developed in a companion paper (Part I). It is shown that this model allows for a quantitative treatment of the strain-burst phenomenon. In particular, the scaling relations between different strain-burst-characteristic parameters which have been found by experiment are connected to the evolution of the dislocation microstructure and thus find a natural explanation.     

Dislocation interaction with radiation defects in alkali-halide crystals

A mechanism of room-temperature irradiation-induced strengthening of alkali-halide crystals (AHCs) is suggested. It is shown that the deformation-stimulated luminescence is a result of the destruction of hole color centers, which strengthen AHCs, by dislocations.     

Dislocation diffusion mechanism in Pd/Ag crystals

In epitaxial Pd/Ag crystals, dislocations in the palladium film have a large influence on the effective diffusion coefficient. In silver the diffusion coefficient hardly depends on the dislocation density. With a dislocation density of 10¹⁰m⁻² the diffusion coefficient in silver is larger than in the palladium, and at 773 K it is 10⁻¹⁸ m²/s. At a dislocation density of 3·10¹² m⁻², the diffusion coefficient in the palladium becomes larger than in silver, and at 773 K it is 3·10⁻¹⁸ m²/s. It is most likely that diffusion in silver takes place via the lattice, while in palladium it occurs at mobile dislocation sites. State Technical University, Samarsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 116–118, August, 1996.     

On relaxation phenomena in filamentary NaCl crystals

The internal friction and reversible creep of filamentary sodium chloride crystals are investigated. An internal friction peak, associated with the reorientation of the impurity-vacancy complexes in an elastic field is detected. The activation energy of the process is determined. The correlation between the internal friction level and the size of the filamentary crystal, as well as its electric charge, are established. The relaxation time of the reversible creep of filamentary crystals in a 10 kV/cm electrical field is estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 101–104, April, 1973.     

Dislocation dynamics of crystallographic slip

The dynamics of dislocation loops involved in the formation of a crystallographic shear zone is considered. As a dislocation moves in the shear zone, the kinetic energy is shown to attain high values. When hindered near barriers bounding the shear zone, the dislocation exhibits damped oscillations in the vicinity of some equilibrium position. Tomsk State University of Architecture and Building. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 71–76, January, 2000.     

Defect centers in antimony doped NaCl crystals

The existence of both Sb²⁺ and Sb³⁺ in NaCl has been established by a series of correlated experiments like optical absorption, EPR, Electrical conductivity and dielectric loss. Sb²⁺ gives an optical band at 212 nm and in an arbitraryorientation gives an EPR signal whose direction dependence studies indicate three magnetically inequivalent sites. A vacancy occupying a second neighbour site with respect to the impurity ion satisfactorily, explains the observed spectra. The association energy of the impurity ion has been found to be 0·56 eV from ionic conductivity studies. Dielectric loss measurements give two prominent loss peaks which have been assigned to Sb²⁺ and Sb³⁺ on the basis of their temperature variation and other correlated experiments. Radiation damage reduces the Sb³⁺ content while Sb²⁺ is not affected.     

Thermoluminescence X-irradiated NaCl:Sr crystals

Strontium-doped thermally pretreated NaCl crystals, X-irradiated at room temperature, were heated to 500°K for glow - curve measurements. The recorded glow curves show two definite peaks at 340 and 460°K. Study of the growth of these peaks with X-irradiation time indicates that the 460°K peak may be related to impurity - vacancy dipole in association with a negative ion vacancy situated in the dislocation region of the crystal.     

Dielectric properties of nickel-doped NaCl crystals

The influence of nickel impurities upon dielectric losses in natrium chloride crystals is studied. Experiments have shown that in non-heated samples, stored over prolonged periods of time, the majority of nickel is present in the form of aggregates and higher complexes. Their decomposition and the building up of dipoles depends in the first instance on sample heating and cooling methods. The aggregation of dipoles in time at room temperature is studied.It is shown that with nickel concentrations reaching approximately up to 2. 10^–3 mol% in heated and quickly cooled samples, all nickel builds itself into the NaCl lattice substitutionally in the form of dipoles Ni²⁺ + cation vacancy, while at higher concentrations some nickel precipitates or builds itself in in another non-substitutional way.The author wishes to express his thanks to Dr. Funk for making the polarographic analysis of the samples and to Dr. Bohun for valuable discussions of the results obtained.     

Dislocation kink-pair energetics and pencil glide in body-centered-cubic crystals

When body-centered-cubic crystals undergo plastic deformation, the slip planes are often noncrystallographic. By performing atomistic simulation on the activation pathway of dislocation jumps in bcc iron, we show that the main reason for bcc crystals to exhibit this phenomenon is that one type of kink pair has significantly lower energy than all the other types on the same slip plane. Dislocation motion therefore cannot continue on the same slip plane, and the dislocation has to cross slip onto an intersecting slip plane after each atomic jump. Thus in the long run, the average slip plane would be zigzag and noncrystallographic.     

Magnetoplastic effect in irradiated NaCl and LiF crystals

The effect of low x-ray irradiation doses (≈10² rad) on the magnetoplastic effect — the detachment of dislocations from paramagnetic centers under the action of an external magnetic field B — in alkali-halide crystals has been investigated. The measurements were performed on LiF crystals and three types of NaCl crystals, differing in impurity content. The dependence of the mean free path l of the dislocations on the rotational frequency ν of a sample in a magnetic field was especially sensitive to low irradiation doses. In unirradiated crystals this dependence is a single-step dependence and is characterized by a critical frequency ν _c ∝B ² above which the magnetoplastic effect is not observed. The frequency ν _c depends only on the type of paramagnetic centers, and not on their density. Even the lowest irradiation dose employed (<100 rad) leads to a sharp restructuring of the dependence l(ν), converting it into a two-step dependence (for edge dislocations) with an additional critical frequency ν _c2, that is insensitive to the irradiation dose, and that corresponds to the appearance of magnetically sensitive stoppers of a new type under irradiation. The initial critical frequency ν _c1, as a rule, also varies with the dose, reflecting the change in state of the impurity complexes (Ca in NaCl and Mg in LiF). Specifically, it is shown for NaCl(Ca) crystals that as the irradiation dose increases, the frequency ν _c1 increases, gradually approaching the value ν _c2, so that by the time the dose is ≈300 rad, the dependence l(ν) once again becomes a single-step dependence, dropping sharply only for ν⩾ν _c2. It is shown that the addition of a small number of Ni atoms to a NaCl crystal makes the Ca complexes radiation resistant, and the critical frequency ν _c1 corresponding to them initially equals ν _c2 for crystals with no Ni. The recombination kinetics of radiation defects in the case in which the samples are irradiated under a tungsten lamp was investigated. A possible physical model of the observed dependences is discussed. Zh. éksp. Teor. Fiz. 111, 615–626 (February 1997)