Serrated plastic deformation in LiF single crystals at high temperatures

The serrated plastic flow in LiF single crystals has been studied in the mode of active deformation at high temperatures (T = 573–1093 K). The parameters of the jumps in the deforming stresses (normalized amplitude and relaxation time of stress oscillations) were determined at the stage of strain softening under conditions of uniaxial compression and tension. It was shown that the jump parameters are essentially dependent on the type of the stressed state and the deformation temperature. The activation energy of serrated deformation in shear bands was established to be close to the migration energy for cation vacancies.     

Stability of dislocations in MgO single crystals at high temperatures

The dislocation structures of MgO single crystals annealed at 1900 °C for 3 hrs and at 2000°C for 1 hr were observed electron microscopically, and it was found that the following reactions took place at high temperatures: a/2 〈11 0〉 + a/2 [110], = a [100], a/2 [110] + a/2 〈11 01〉 = a/2 [011]. The resultant dislocations were sessile. Their interaction with impurities would make the subgrain boundaries stable.     

Regularities of plastic deformation in indentation on the (111) plane of NaCl single crystals

Dislocation structures arising in indentation on the (111) face of NaCl single crystals are studied by etch-pit technique. The deformation temperature varied in the interval from 77 to 520 °K. The effect of radiation defects introduced by γ-irradiation on the shape of dislocation rosette was investigated. Three types of dislocation rosettes depending on the experimental conditions were observed. A possible explanation of this phenomenon is proposed. It is shown that the polarity of plastic deformation observed under indentation is an universal phenomenon which is revealed in the temperature range from 77 to 470 °K. The phenomenon of dislocation rosette inversion was first observed.     

Dynamic deformation of silicon-rich V3Si single crystals at elevated temperatures

The plastic deformation behaviour of the intermetallic phase V₃Si (Cr₃Si type) was investigated under dynamic conditions. The experiments revealed that V₃Si deforms plastically at a strain rate of about 4.7 × 10⁻⁵s⁻¹ above 1200°. Flow stress depends strongly on deformation temperature and strain rate. An alloy hardening by deviation from stoichiometric composition within the range of homogeneity is observed.     

Correlation between different parameters of plastic deformation in doped NaCl crystals

The yield stress (τ_y), the microhardness (H) and the dislocation mobility in the stress field of a concentrated load (γ) were measured in NaCl: Ca and NaCl: Pb single crystals within the temperature interval 77–673 K. It was shown that there was a good correlation between the concentration changes of τ_y, H and γ for NaCl:Ca crystals. Such clear results were absent for NaCl:Pb single crystals. An anomaly of τ_y at 77 K was revealed for these crystals; it was not followed by the H anomaly. In conclusion it was stated that the impurity hardening was connected with the increasing of dislocation mobility but such a parameter did not determine the temperature hardening.     

The growth and mechanical properties of paratellurite single crystals at high temperatures

α-TeO₂ single crystals have been grown by the Czochralski method on specially developed systems with automatic growth control. The mechanical properties of crystals under compression in the [100], [110], and [221] directions at a constant strain rate of about 10⁻⁴ s⁻¹ in the temperature range T = 733−993 K have been investigated. The distribution of plastic shear strain in deformed samples is has been experimentally studied. Manifestations of localized strain in covalent paratellurite crystals at T > 900 K are revealed for the first time. The active slip systems along which localized shear band are oriented are determined. The temperature dependence of the critical stress of transition to localized flow is investigated.     

Localization of plastic deformation in Cu and Ni single crystals

The patterns of plastic flow in fcc Cu and Ni single crystals oriented in a way to provide easy slip are studied. The main space-time characteristics of deformation localization at the stages of easy slip and linear strengthening in these single crystals are established. The relation between the orientation of the sites of deformation localization and the crystallography of the slip systems in the samples is studied. The velocity of the motion of the deformation-localization sites during sample tension is determined.     

Inhomogeneities of the plastic deformation of MgZn2 single crystals

Single crystals of the Laves phase MgZn₂ of different chemical composition within the range of homogeneity were deformed by dynamic compression at temperatures from 300 to 500°C. The surfaces of the specimens were observed by light microscopy. Special types of inhomogeneities of deformation (Lüders bands, macro-slip steps) are described and first conclusions are drawn concerning the behaviour of dislocations in this substance.     

Growth of single crystals of B28 at high pressures and high temperatures

A method of the high-pressure high-temperature synthesis of single crystals of orthorhombic high-pressure boron B₂₈ from metal solutions is presented. The method is based on the high-pressure multi-anvil technique. The feasibility of single-crystal growth was demonstrated in a number of experiments conducted at various pressure–temperature conditions with various precursors including β-boron of 99.99% purity and various metals (Cu, Au, and Pt) used as fluxes and capsule materials. It was found that after dissolution in metals at high pressures and high temperatures, boron crystallizes in the form of single crystals at low temperature. The process is accompanied by chemical reactions resulting in the formation of borides. The maximum length of the B₂₈ crystals achieved is ∼100 μm.     

The influence of OH−-ions on the mechanical properties in LiF and NaCl single crystals

The influence of the growing atmosphere on the mechanical properties was investigated on LiF and NaCl single crystals. The growing atmospheres were air or vacuum and argon, respectively. In a wide temperature range the air grown crystals have a higher flow stress and a smaller activation volume. Besides, the dislocations are less mobile. The differences arise probably from the OH⁻-ions, which were found in a considerable concentration in the air grown material. The OH⁻-ions may create lattice distortions by themselves or by the formation of different complexes with the divalent cation impurities. In this way the density of thermal and athermal obstacles for the dislocation motion increases.     

Variation of low temperature internal friction at microplastic deformation of high purity molybdenum single crystals

Amplitude and temperature spectra of ultrasound absorption in weakly deformed high purity molybdenum single crystals of different orientations were measured. The results were discussed in terms of parameter changes related to quasiparticle- or dislocation oscillations, respectively, dislocation-point defect interactions as well as defect generation at microplastic deformation.     

Study of the instability boundaries of plastic flow in Cu + 14 at.% Al single crystals at low temperatures

The authors experimentally revealed the existence of the temperature-strain rate region of jumplike deformation in Cu + 14 at.% Al single crystals between 4.2 and 25 K. For each particular temperature in this temperature range, there are a lower and an upper critical strain rates, between which deformation is jumplike. The character of transients occurring during strain rate variation was investigated by increasing the strain rate to the lower critical value. The experimental data are explained in terms of the thermomechanical instability concept and inertial effects of the sample-machine system.     

Preparation and properties of ultratransparent NaCl single crystals

The preparation of up to 100 mm diameter single crystalline ingots of ultratransparent NaCl by injecting the reactive atmosphere is reported. The resulting material exhibits high optical quality and bulk absorption coefficient at 10.6 μm of 0.91 × 10⁻³ cm⁻¹, close to the intrinsic value, which makes it suitable for transmissive components in high-power CO₂ lasers.     

Dislocation mobility in LiF at low temperatures

The mobility of dislocations in LiF has been measured at 4.2 and 10 K by means of an etch pit technique. The average velocity of screw dislocations is 2–5 times as large as edge dislocations. The results of the stress and temperature dependence of the velocities correlate well with the data of the critical shear stress in the same temperature region, and are well described by the theory of Peierls mechanism with thd Peierls stress of about 20 MPa for edge dislocations on {110} slip plane.     

Deformation mechanism and dislocation structure of high-purity molybdenum single crystals at low temperatures

High purity molybdenum single crystals were deformed in tension and compression along the symmetric double slip orientation [110] in the temperature range from 300 K down to 0.5 K with strain rates between 10⁻³ and 10⁻⁵ s⁻¹. The activation volume was measured by stress relaxation tests. The dislocation structure of the deformed crystals (T → 1.85 K) was examined by high voltage electron microscopy. It was established that the low temperature increase of the critical shear stress exhibits three distinct temperature regimes with different temperature dependences. These non-uniformities are discussed in terms of recently developed theories of kink-pair formation and kink-kink interactions on screw dislocations in bcc metals. The HVEM observations suggest that the mobility of screw dislocations at very low temperatures should be determined by the combined effects of the PEIERLS barriers and the jog dragging.     

Pulling CsI(Na) single crystals from melt by continuous method without plastic deformation

It is established that bromine impurity in CsI(Na) crystals not only facilitates the homogeneous incorporation of an activator into the lattice and prevents complex activator clusters from forming, but it also significantly hinders (at certain concentrations) the action of the primary and secondary dislocation slip systems. It is shown that the automatic pulling of large CsI-based crystals can be provided by the introduction of a single Br impurity into the charge; this impurity, to a large extent, strengthens only the top part of the crystal. The absence of plastic deformation in CsI-CsBr(Na) crystals with a diameter Ø300 mm and height h = 600 mm (grown by the continuous method) and Ø500 mm and h = 200 mm (grown by the automatic Kiropulos method) has been experimentally confirmed.     

Indentation deformation of magnesium orthosilicate single crystals

The variation in the microhardness of the magnesium orthosilicate crystals has been determined using Vicker's microhardness indentor. The length of the rays of dislocation rosettes around the indentation, obtained by etching the indented cleavages have been measured. It is observed that: (i) irrespective of the relative orientation of the indentor and the crystal, the median vents initiated at the sharp indentation edges, (ii) the microhardness of the crystals depends on the applied load, and (iii) the microhardness of the crystal is independent on the duration of loading. The implications are discussed.