Surface structure of large germanium single crystals

The dependence between structural-defect generation and the growth kinetics of germanium single crystals pulled from melts is investigated via selective chemical etching and the optical, atomic-force and scanning electron microscopy techniques. It is ascertained that the surface microrelief of germanium crystals grown from melts by means of directional crystallization contains protrusions and cavities with spatial periodicities of 5 and 50 μm. The values of the kinetic coefficients are estimated. It is demonstrated that the main part of the crystals is formed according to the normal mechanism with the kinetic coefficients β_k = 2 × 10^?5 m s^?1 K^?1.     

Structural defects in germanium single crystals

The origin and distribution of structural defects (dislocations, small angle grain boundaries, slip lines, polycrystalline inclusions) is studied in large single crystals of germanium applied in manufacturing optical elements.     

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.     

Nuclear spin-lattice relaxation in germanium single crystals

The temperature dependence of the spin-lattice relaxation time corresponding to the inelastic scattering of phonons by the⁷³Ge quadrupole moment in Ge single crystals is calculated in the framework of the adiabatic bond charge model. The results obtained agree with the experimental data.     

Isotope effect in the thermal conductivity of germanium single crystals

The thermal conductivity of chemically, structurally, and isotopically highly pure germanium single crystals is investigated experimentally in the temperature range from 2 to 300 K. It is found that the thermal conductivity of germanium enriched to 99.99% ⁷⁰Ge is 8 times higher at the maximum than the thermal conductivity of germanium with the natural isotopic composition. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 463–467 (25 March 1996)     

Photoluminescence in germanium with a quasi-equilibrium dislocation structure

The dislocation-related photoluminescence of n-Ge single crystals with a quasi-equilibrium structure of 60° dislocations is investigated at a temperature of 4.2 K. It is shown that the dislocation-related photoluminescence spectra are described by a set involving from 8 to 13 Gaussian lines with a width of less than 15 meV. With due regard for the data available in the literature, the Gaussian lines with maxima at energies in the range 0.47 < E _m ≤ 0.55 eV are assigned to the emission of 90° Shockley partial dislocations involved in quasiequilibrium segments of 60° dislocations with different values of the stacking fault width Δ (Δ = Δ₀, Δ < Δ₀, and Δ > Δ₀). It is revealed that the d8 line at the energy E _m = 0.513 eV, which corresponds to the emission of straight segments with the equilibrium stacking fault width Δ₀, dominates in the photoluminescence spectra only at dislocation densities N _D < 10⁶ cm^?2. As the dislocation density N _D increases, the intensity of the d8 line decreases with the d7 line (E _m ≈ 0.507 eV) initially and the d7 and d6 lines (E _m ≈ 0.501 eV) then becoming dominant in the photoluminescence spectrum. The d7 and d6 lines are attributed to the emission of segments with stacking fault widths Δ < Δ₀. Possible factors responsible for the formation of stacking faults with particular widths Δ ≠ Δ₀ for quasi-equilibrium dislocations are discussed.     

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.     

Magnetic studies of the dislocation structure of iron single crystals deformed at 295 K

The dislocation density in iron single crystals deformed at 295 K has been studied by measuring the coercive field, the initial susceptibility, the Rayleigh constant, and the reversible susceptibility in the approach to ferromagnetic saturation as functions of the resolved shear stress. The influence of different dislocation types on the saturation susceptibility has been calculated. In this way it is possible to distinguish dislocation structures composed of screw or edge dislocations and to reveal long-range internal stresses, which govern the work-hardening in the deformation stage II/III. The dislocation density increases in stage I linearly and in stage II/III quadraticaly with the resolved shear stress. In stage O mainly isolated screw dislocations are created.     

Low-temperature fatigue behaviour and development of dislocation structure in aluminium single crystals with single-slip orientation

Al single crystals oriented for single slip were cyclically deformed under constant plastic strain amplitudes between 1?×?10^?3 and 5?×?10^?2 at 77?K. Al single crystals showed hardening to saturation at all applied shear stress amplitudes. The resultant cyclic stress–strain curve (CSSC) showed a stress plateau in a range of plastic strain amplitude from 2?×?10^?3 to 2?×?10^?2. Surface observation revealed that multiple slip systems were active even at the strain amplitude in the plateau region. At plastic strain amplitudes corresponding to the plateau of the CSSC, persistent slip bands (PSBs) were formed parallel to the primary slip plane. In the PSBs, well-developed dislocation walls parallel to the {100} planes were observed. The microstructure in the PSBs was explained by the fact of multiple activation of the primary and critical slip systems. The above results indicate that the high stacking fault energy of Al is an important factor affecting the fatigue behaviour even at 77?K.     

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.     

Estimating the strength of single-ended dislocation sources in micron-sized single crystals

Three-dimensional (3D) discrete dislocation dynamics simulations were used to calculate the effects of anisotropy of dislocation line tension (increasing Poisson's ratio, ν) on the strength of single-ended dislocation sources in micron-sized volumes with free surfaces and to compare them with the strength of double-ended sources of equal length. Their plastic response was directly modelled within a 1?µm³ volume composed of a single crystal fcc metal. In general, double-ended sources are stronger than single-ended sources of an equal length and exhibit no significant effects from truncating the long-range elastic fields at this scale. The double-ended source strength increases with ν, exhibiting an increase of about 50% at ν?=?0.38 (value for Ni) as compared to the value at ν?=?0. Independent of dislocation line direction, for ν greater than 0.20, the strengths of single-ended sources depend upon the sense of the stress applied. The value for α in the expression for strength, τ?=?α(L)µb/L is shown to vary from 0.4 to 0.84 depending on the character of the dislocation and the direction of operation of the source at ν?=?0.38 and L?=?933b. By varying the lengths of the sources from 933 to 233b, it was shown that the scaling of the strength of single-ended and double-ended sources with their length both follow a ln(L/b)/(L/b) dependence. Surface image stresses are shown to have little effect on the critical stress of single-ended sources at a length of ～250b or greater. This suggests that for 3D discrete dislocation dynamics simulations of the plastic deformation of micron-sized crystals in the size range 0.5–20?µm, image stresses making the surface traction-free can be neglected. The relationship between these findings and a recent statistical model for the hardening of small volumes is discussed.     

Mechanism of formation of the interlamella dislocation network in polyethylene single crystals

It was found experimentally that bilayered polyethylene single crystals with an orientational misfit angle larger than the critical angle, θ*, show moire pattern and only those with an angle smaller than θ* show the interlamella dislocation network. The intermediate pattern, which could neither be classified into the typical moire pattern nor the typical interlamella dislocation network, was found in the vicinity of θ*. Criteria for discrimination between the moire pattern and the interlamella dislocation network are discussed. The crystal with a misfit angle a little larger than θ* also comes to show the feature of the dislocation network during long storage of the crystal in the mother solution at the crystallization temperature. θ* is, therefore, a function of the time of storage. A mechanism is presented such that crystal lattices near the interfacial boundary are distorted to form the dislocation network by intermolecular force between the overlying crystals when the misfit angle is smaller than θ*.     

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.     

Starting stress for the onset of dislocation motion in ruby single crystals

For the purpose of preparing ruby single crystals with a high degree of structural perfection, the evolution of their structural state is investigated using four-point loading at T=1490°C and chemical etching pits. The starting stress τ_st for the onset of the dislocation motion is measured. The regularities of the arrangement of dislocations in single-crystal samples of ruby are established. The starting stress for the onset of the dislocation motion is found to be τ_st=2 MPa. The results obtained make it possible to determine the optimum parameters of thermomechanical treatment of the single crystals and, in the future, to solve the problem of their longterm serviceability.