Threshold character of the magnetoplastic effect in twinning of bismuth crystals

It is found that, when a bismuth crystal is subjected simultaneously to concentrated loading and a dc magnetic field, the magnetoplastic effect manifests itself as an abrupt decrease in the average length of wedge twins on the (111) cleavage plane (and, hence, as a decrease in the total twinned volume of the crystal) as a threshold value of the magnetic field is reached. However, the magnetic field stimulates glide deformation, which is indicated by an increase in the size of dislocation rosettes. The microhardness of a crystal varies smoothly and tends to increase with increasing magnetic field. The last phenomenon is accounted for by hardening of the crystal under the indenter in the presence of a magnetic field, because several slip systems become operative.     

Magnetoplastic effect in NaNO2 ferroelectric crystals

This paper reports on a laser interferometric study of the effect of a dc magnetic field (MF) on the rate of plastic deformation (creep) $\dot \varepsilon$ of NaNO₂ ferroelectric crystals under compression. It is established that the application of a dc MF to a loaded specimen results in an increase in the creep rate and that removal of the MF brings about a decrease in $\dot \varepsilon$ . Subjecting an unloaded specimen to a dc MF beforehand also affects its strain rate under the subsequent loading. The observed magnetoplastic effect is most clearly pronounced within a certain $\dot \varepsilon$ interval, and the magnitude of this effect for the NaNO₂ ferroelectric is several times larger than that for LiF crystals.     

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)     

Magnetoplastic effect in LiF crystals and longitudinal spin relaxation

New features of the dependence of the average travel distance l of dislocations on the magnetic field B have been found in an investigation of the magnetostimulated dislocation mobility in LiF crystals: A transition has been found from ordinary proportionality l∝B ² to saturation l≈ const in high fields B. It is shown that the experimental points can be described satisfactorily by the theoretical dependence l∝ [(B ₀/B)²+1]⁻¹ (B ₀≈0.8 T), typical for the mechanism of longitudinal spin relaxation in a system of radical pairs, which are supposedly formed when dislocation nuclei interact with paramagnetic impurity centers. According to the theory, the level of the field B ₀ is determined by the characteristic frequency of the oscillations of the internal fields in the lattice, which for B ₀∼1 T is of the order of 10¹¹s⁻¹, which corresponds to the typical frequency of characteristic oscillations of dislocation segments between pinning centers, which, naturally, does not depend on temperature. This in turn explains the fact that the measured values of B ₀ are the same at 293 K and 77 K. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 749–753 (10 December 1999)     

Deformation of alkali-metal halide crystals by a concentrated load

The parameters characterizing the effect of a concentrated load on crystals subjected to various preliminary treatments and their possible relationship to the mechanical characteristics manifested during ordinary (one-dimensional) deformation are investigated. The significant upward deviation of the microhardness from the values of the elastic limit and yield strength of crystals is shown to be caused not so much by the larger values of the strain under an indenter as by the larger value of the strain hardening coefficient in the case of deformation by a concentrated load. It is also shown that the types of hardening caused by uniaxial deformation and by reactor irradiation influence the microhardness of LiF crystals differently. Fiz. Tverd. Tela (St. Petersburg) 41, 1999–2003 (November 1999)     

Investigation of the electromechanical effect in twinning of bismuth crystals in the temperature range 77–530°K

A study was made of the influence of high-density current pulses on twinning of bismuth single crystals by a concentrated load at temperatures of 77–530° K. Twinning was stimulated by current pulses throughout the investigated temperature range and at high temperatures these pulses widened the interval of twinning temperatures. The electroplastic effect in twinning appeared most clearly at low temperatures. The temperature dependences of the quantitative characteristics of twinning had three regions in which there were considerable differences in the qualitative pattern of the distribution of twins originating from indentations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 29–33, July, 1980.     

Magnetoplastic effect in InSb

We have observed dislocation motion in InSb semiconductor crystals under the action of a static magnetic field in the absence of a mechanical load. The dependence of the average dislocation travel distance and of the relative number of diverging and converging half-loops on the magnetic induction and the “magnetic treatment” time is obtained. The activation energy of the motion of diverging dislocations in a magnetic field in the temperature range 120–250°C is estimated. Possible reasons for the observed phenomenon are discussed. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 298–302 (25 August 1999)     

On twinning in smectic crystals

It is shown that mechanical twinning in smectic crystals is possible. The structure of the boundary of twins for a small disorientation of crystallites is determined. The periodic twin structure, which should appear at the tension of the smectic layer, is proposed. Original Russian Text ? V.I. Marchenko, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, p. 841.     

On twinning in smectic crystals

It is shown that mechanical twinning in smectic crystals is possible. The structure of the boundary of twins for a small disorientation of crystallites is determined. The periodic twin structure, which should appear at the tension of the smectic layer, is proposed.     

Magnetoplastic effect and spin-lattice relaxation in a dislocation-paramagnetic-center system

A magnetic induction threshold B _c above which the magnetoplastic effect — depinning of dislocations from paramagnetic pinning centers — can be observed in samples placed in a magnetic field is predicted and observed in Al, NaCl, and LiF crystals. The existence of a threshold is associated with the fact that for B<B _c the spin-lattice relaxation time τ_sl in a dislocation-paramagnetic-center system is less than the time required for spin evolution in a magnetic field resulting in the removal of the spin forbiddenness of an electronic transition that “switches off” the dislocation-pinning-center interaction. It is shown that the threshold field B _c is sensitive to temperature and x-ray irradiation of the samples. A new method for measuring the spin-lattice relaxation time in paramagnetic centers on dislocations is proposed on the basis of the data obtained. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 628–633 (25 April 1996)     

Effect of a constant magnetic field and a pulse electric current on the mean linear density of twinning dislocations in bismuth crystals

Bismuth crystals are studied under the joint action of a pulse electric current and a constant magnetic field. It is shown that the combined effect of a constant magnetic field and pulse current leads to a substantial decrease in the mean linear density of twinning dislocations piled up at the boundaries of wedge twins. The decrease in the mean linear density of twinning dislocations is accompanied by a decrease in the microhardness of the samples.     

Magnetoplastic Effect under Stress Relaxation in NaCl Crystals

JETP Letters - The effect of a weak magnetic field on stress relaxation in NaCl crystals after the termination of active deformation has been experimentally studied for the first time. The results...     

A new classification of twinning in crystals

A comprehensive and informative classification of twinning in crystals is proposed. It is based on the nature of the twin mapping operation. If the mapping operation is a symmetry element of a certain prototype space group (in Aizu's sense), the twin is called an “Aim twin”. Otherwise it is called a “Bollmann twin”. Aim twins are essentially transformation twins. They may be further divided into ferroic twins and translation twins. Ferroic twins, in turn, can be of two types: ferroelastic or F-twins (e.g. the 90° twins of BaTiO₃), and nonferroelastic-ferroic or N-twins (e.g. the Dauphiné twins of quartz). The antiphase domains in Cu₃Au are a typical example of translation twins (T-twins). The three types of Aim twins (F, N and T) have distinctive macroscopic physical properties. Bollmann twins are divided into two main categories: C-twins and M-twins, where C stands for coincidence lattice and M for miscellaneous. C-twins are further categorized into two types, depending on the “total” or “partial” nature of the coincidence sublattice. M-twins can be of three types, depending on the dimensionality of the dichromatic pattern being 0, 1 or 2. Illustrative examples are discussed. A compact and informative twin symbol is introduced.     

Effect of a dislocation forest on the bauschinger effect during the twinning of zinc single crystals

A study was made of the effect of a dislocation in the {{11¯22}} 11¯23 system (pyramidal slip) on the characteristics of the Bauschinger effect during the twinning of single crystals of zinc and of the alloy Zn + 0.6% Cu. The dislocation forest retards the backward motion of the twinning dislocations, leading to the effect.Translated from Izvestiya VUZ. Fizika, No. 5, pp. 101–104, May, 1971.In conclusion the authors thank candidates of physicomathematical sciences F. F. Lavrent'yev for useful discussion.     

Mechanical twinning of yttrium oxide single crystals

The deformation structure of an yttrium oxide single crystal under point load was studied by microhardness and electron microscopy. It was concluded that mechanical twinning is the main mechanism of plastic deformation of these single crystals. A twin was simulated by rotating the structure around the three-fold axis. It was shown that the average change of the distance between anions was about 5% and that between cations was 15%. The comparison with the structure of the regions of the fluorite single crystals and optical ceramics deformed in the analogous manner was performed.     

Anisotropy and twinning in cubic zinc sulfide crystals

The structure of zinc sulfide single crystals grown from the vapor has been studied by a unique combination of X-ray crystallography and thermal conductivity measurements. Weissenberg patterns reveal the face centered cubic structure with rotational twinning in the [111] direction, which could be ascribed to two overlapping lattices. These could be explained by normal stacking faults or by inverted twins. A strong anisotropy in thermal conductivity was measured and interpreted as favoring the inverted twin model.     

Energy estimates of twinning processes in cubic single crystals

The role of twinning processes giving rise to the fragmentation and rotation of a structure caused by single-crystal deformation, as well as the joining of grains during the manufacturing of optical ceramics, is investigated. Models describing the twins in the single crystals under study, which are formed by rotating a cubic lattice about the threefold symmetry axis through an angle of 60°, are constructed. The twin formation energies are estimated.