Magnetic studies of the dislocation structure of iron single crystals deformed at 195K and 77K

Measurements of the coercive field, the initial susceptibility and the reversible susceptibility in the approach to ferromagnetic saturation show that during low-temperature deformation of iron single crystals mainly screw dislocations are created. Long-range internal stresses are found to be significantly smaller than in crystals deformed at room temperature. Macroscopic slip occurs on several slip systems. In the parabolic region of the work-hardening curves at 195 K the relation is valid, where τ isthe shear stress andN is the dislocation density. In the region of saturation of the shear stress the dislocation density further increases. After room-temperature prestrain the relation appears to hold for 77K-deformation also. Exhaustion hardening of edge dislocation is found at the beginning of the low-temperature deformation.     

Peculiarities of dislocation structure at surface of deformed crystals

An experimental study of the distribution of edge and screw dislocation components in deformed LiF crystals at external surface parallel to Burgers vector of mobile dislocations was performed. At the surface we observed a layer with low density of edge and screw dislocations compared to the dislocation density in the bulk. The thickness of this layer was tens of microns. The experimental results are explained on the basis of analysis of dislocation structure evolution at surface layers during the plastic deformation.     

Domain structure of silicon iron single crystals

The model of a new domain structure arising after the magnetization of silicon iron single crystals in planes of the (110) type at an angle of 0°<Θ<-55° to the axis of easy magnetization is considered. Using this model the angular dependence of the domain-structure characteristics is established; it agrees closely with direct observations. On magnetizing a single crystal in the angular range 55° <Θ≤ 90° to the easy axis, layers with a uniform resultant magnetization parallel to the [001] direction are formed.     

Multiphysical simulation analysis of the dislocation structure in germanium single crystals

To grow high-quality germanium crystals is one of the most important problems of growth industry. The dislocation density is an important parameter of the quality of single crystals. The dislocation densities in germanium crystals 100 mm in diameter, which have various shapes of the side surface and are grown by the Czochralski technique, are experimentally measured. The crystal growth is numerically simulated using heat-transfer and hydrodynamics models and the Alexander–Haasen dislocation model in terms of the CGSim software package. A comparison of the experimental and calculated dislocation densities shows that the dislocation model can be applied to study lattice defects in germanium crystals and to improve their quality.     

Vortex structure in superconducting iron pnictide single crystals

The vortex structure in the iron pnictide single crystals Ba_1?x K _x Fe₂As₂ and Sr_1?x K _x Fe₂As₂ of the 122 type and SmFeAsO_1?x F _x of the 1111 type has been investigated using the decoration method. In all of the crystals under investigation, no regular vortex lattice has been revealed in the magnetic field range up to 200 Oe. The disordered vortex structure is discussed in view of the vortex pinning in single crystals.     

Internal friction of deformed zinc single crystals

Zinc single crystals with the side in a (0001) basal plane were elongated and the internal friction measurements were carried out as a function of temperature in the mode of a flexural vibration. Two relaxation peaks were observed to appear: one at around 500°K and the other at about 570°K. Each activation energy was obtained 0.70±0.08eV and 1.01±0.06eV, respectively, by the peak shift method. The experimental results were discussed in terms of dislocations in pyramidal slip system {$\text{11}$22} 〈11$\text{2}$3〉 and twinning dislocations in the planes {$\text{1}$012} 〈10$\text{1}$1〉, respectively.     

Determination of the dislocation structure in paratellurite single crystals using the piezo-optic effect

It is shown theoretically and experimentally that dislocations in paratellurite single crystals can be revealed using the piezo-optic effect. The mechanical-stress distribution near dislocations is calculated. Dislocations in the samples are observed using the photoelastic method and chemical etching. Data on the spatial distribution of dislocations in Czochralski-grown paratellurite crystals are reported.     

Magnetic properties of iron and nickel single crystals magnetized in the trigonal plane

Equations explaining the magnetization curves, the magnetostriction curves, and the galvanomagnetic effect for the trigonal plane of iron and nickel crystals in fields near the saturation magnetization are found from the Akulov theory.Translated from Izvestiya VUZ. Fizika, No. 3, pp. 86–91, March, 1970.     

Magnetic properties of stoichiometric iron sulfide single crystals near the alpha transition temperature

The magnetic susceptibility of nearly stoichiometric iron sulfide single crystals grown by normal freezing was measured in the vicinity of the crystallorgraphic transition temperature T_α ～ 155°C. The existence of two anomalies in the χ vs T curve clearly indicated that the spin-flop transition occurs at a much higher temperature (T_s ～ 190°C).     

Effect of magnetic field on the dislocation structure and mechanical properties of alkali halide crystals deformed by ultrasound

The dislocation structure and mechanical properties of LiF and NaCl crystals under joint action of magnetic field and ultrasound in the range of longitudinal strain amplitudes corresponding to dislocation multiplication have been investigated. Ultrasonic deformation was implemented in a piezoelectric oscillator at a frequency of 80 kHz. It is established that the presence of a magnetic field facilitates cross slip; initiates displacement of block boundaries; and leads to the formation of labyrinth structures (characteristic of high-temperature loading), which strengthen the crystal.     

Ultrasonic investigation of sandwich single crystals designed for studies of dislocation resonance damping

A novel sandwich single crystal (SSC) technique has been designed for improved MHz-pulse-echo investigations of dislocation damping. In the present SSC the high purity Cu crystal to be investigated is epitaxially overgrown by a buffer crystal doped with 200 ppm Mn. The buffer avoids interference between ultrasonic transducer and sample by shielding the investigated dislocation structure against stresses caused e.g., by different thermal expansion of the quartz transducer. The frequency dependence of dislocation resonance damping measured on the SSC has been analyzed and compared with measurements on normal single crystalline samples of high purity Cu and Cu doped with 200 ppm Mn.     

Temperature dependence of the coercive force of deformed silicon iron crystals

The temperature dependence of the coercive force of deformed silicon iron (Fe-3.15 Si) crystals was studied. It was shown that the current theories relating coercive force to dislocation density do not explain changes observed in H_c. The latter were attributed to changes in the domain structure whose necessity was proved thermodynamically.     

Evolution of a dislocation structure in the surface layers of single crystals during high-temperature annealing

The dislocation structure of KCl crystals annealed in a forevacuum has been shown to display a nonmonotonic variation of the density of dislocations in the direction from the surface. Our experiments and estimates showed that the observed redistribution of dislocations near the surface is due to the diffusion of impurities from the atmosphere and the generation of dislocations in the diffusion zone.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 48–52, September, 1987.     

Extended x-ray absorption fine structure studies of oriented single crystals

Linearly polarized synchrotron radiation is used to study the orientation dependence of the Extended X-ray Absorption Fine Structure (EXAFS) spectrum of single crystal zinc. The spectrum some 5–1200 eV above the K photoabsorption threshold is found to be markedly anisotropic, as would be expected from the established formalism generalized to crystals of lower than cubic symmetry. Zinc thus provides a prototype for molecular crystals of low symmetry where one wishes to study particular bond directions.     

Dislocations in Fe-3% Si alloy single crystals deformed at a higher rate

The method of etching dislocations is used to study the distribution of dislocations and twins in Fe-3% Si alloy single crystals prepared from the melt after plastic deformation with higher speed. The crystals are deformed by twinning in the 〈111〉 directions along the {112} planes and by slip in the 〈111〉 directions along the {110} planes. The results prove that the dislocations causing plastic deformation move in the {110} planes during both fast and slow deformation. The difference in the slip surfaces during fast and slow deformation is explained by the different number of cross slips per unit dislocation path.     

Post-yielding dislocation retraction of nano-lamellar TiAl single crystals

The perfect single crystal has ultra-high strength but is often accompanied by catastrophic failures after yielding. This study reveals that nano-lamellar TiAl single crystals alleviate the catastrophic failure due to a post-yielding dislocation retraction through atomistic simulations and theoretical analyses. This dislocation retraction leads to a retained post-yielding strength of1.03 to 2.33 GPa(about 50% of the yielding strength). It is shown that this dislocation retraction is caused by local stress relaxation and interface-mediated image force. The local stress relaxation is due to successive dislocation nucleation in different slip systems, and the interface-mediated image force is caused by the heterogeneous interface. Based on dislocation theory, this study demonstrates that the size effect also plays a vital role in dislocation retraction. Theoretical modeling shows that the dislocation retraction occurs when the lamellar thickness is less than approximately 12 nm. Additionally, the post-yielding dislocation retraction is more pronounced at higher temperatures, making it more effective in alleviating catastrophic failures.These findings demonstrate a viable option for avoiding catastrophic failure of single crystals through nanoscale-lamellar design.