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.     

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.     

Some features of plastic deformation of LiF and NaCl single crystals at high temperatures

The deformation of the (001) plane of LiF and NaCl by indentation and scratching at high temperatures was performed. The dislocation patterns arising from deformation were sharply different from those observed at room temperature. It was proposed the explanation of these results. The model of plastic flow in (001) indentation and scratching were considered. Anisotropy of hardness by the scratch method was revealed clearly at high temperatures. It was proposed the explanation of the anisotropy of hardness. The correlation was established between the dislocation structures observed near the indentations and scratches.     

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.     

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.     

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.     

Influence of chemical composition on dislocation structure and its change by plastic deformation of V3Si single crystals

The dislocation structure and its change by plastic deformation of V₃Si single crystals has been studied by an etch technique. The solution hardening effect established elsewhere is interpreted in terms of the observed dependence of the dislocation density ϱ(r) and its increment with the plastic deformation on the chemical composition within the range of homogeneity.     

Mechanism of spatial ordering of dislocations during plastic deformation of crystals

Mechanisms of spatial ordering of dislocations during plastic deformation of crystals are considered. The system of evolution equations, which take into account the effects of elastic and correlated interactions of screw dislocations, is derived. The study is performed with due regard for the dynamics of spatial fluctuations of the dislocation density.     

Crystallographic aspects of macroinhomogeneous plastic flow in single crystals of metals

Various crystallographic aspects of the distribution of strain-localization zones have been studied in single crystals of metals subjected to tensile stresses at different orientations of the tension axes and characterized by different mechanisms of plastic flow (slip of dislocations and martensite transformations). It is shown that the crystallographic orientations of the strain-localization zones (interpreted as the patterns of plastic-flow self-organization) are preserved within the whole deformation process. Some characteristic features of the dynamics of the strain sites are considered.     

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.     

Macroscopic localization of plastic flow in zinc single crystals oriented for basal slip

The patterns of plastic-flow localization in hcp Zn single crystals oriented for slip in the {0001} 〈2110〉 systems are investigated. The main spatial and temporal regularities of the flow localization are established for all portions of the three-stage curve of such crystals. The relationship between the type of localization patterns and the regularities of strain-hardening of single crystals is traced for each stage. The role of the kinking in the formation of the observed macroscopic-flow distributions is estimated.     

Basal twinning as a room temperature plastic deformation mode of chromium- and titanium-doped sapphire crystals

The degree of basal twinning caused by indentation was determined quantitatively on a series of chromium and titanium doped sapphire crystals with variours dopant contents. It depends on the dopant content significantly. The effect of titanium ions in reducing the degree of twinning is more pronounced than that of chromium dopants. This “size effect” of the dopant ions is consistent with the mechanism of basal twinning as reported by KRONBERG .     

Preparation of Cu2ZnSnS4 single crystals from Sn solutions

We investigated the phase diagrams of the Cu₂ZnSnS₄ (CZTS)–Sn pseudobinary system in order to obtain knowledge useful for the growth of high-quality CZTS single crystals using a solution-based method. For Sn solutions saturated with less than ～60 mol% CZTS, the solutes are separated into two phases (CZTS phase+SnS_x phase+liquid phase). On the other hand, for solutions with more than 60 mol% CZTS, the solutes are single phase (CZTS phase+liquid phase). The CZTS single crystals were obtained from a 70 mol% CZTS solution (liquid temperature 850 °C) at 900 °C. The powder X-ray diffraction (XRD) pattern of the CZTS single crystal shows preferred orientations of (112), (220) and (312) planes, confirming the Kesterite structure of CZTS. The Raman spectrum shows three peaks at 287, 338, 371 cm^?1, which corresponded to CZTS peaks. The composition of the CZTS single crystal along the growth direction is found to be slightly Cu-poor, Zn-rich and S-rich. Therefore, it is assumed that the Cu vacancy is the dominant p-type conduction mechanism.     

Photoelastic and X-ray topographic studies of residual stress and lattice deformation in GaAs single crystals

The GaAs crystals, grown with three different methods were simultaneously studied by means of photoelastic and X-ray topographic methods. The methods provide complementary information about lattice deformation. The correspondence between interplanar distance changes and photoelastically measured stress values and the connection between cellular structure and high stress values were also found.     

Defect structure in nickel single crystals in dependence on the cyclic deformation prehistory

The development of the dislocation structure in nickel single crystals push-pull fatigued at room temperature is investigated for different deformation types using electrical resistivity measurements and results of TEM observations. The dislocation density in the bundles of the matrix is higher after changing the strain amplitude from region A to region B (CD_AB-type) of the cyclic stress-strain curve (N_dB ≈︁ 1.6 ≈︁ 10¹⁵ m⁻²) than after virginal deformation in region B (VD_B-type) especially at higher strain amplitudes (N_dB ≈︁ 0.75 ≈︁ 10¹⁵ m⁻²). Increasing the strain amplitude within region B (CD_BB-type) does not change N_dB. The farther development of the matrix structure after reaching the v_H-minimum during the VD_B-type of deformation suggests the action of the „formation and dissolution”︁ – mechanism, whereas the CD_BB-type causes the „cord”︁-mechanism of the PSB nucleation.     

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.     

Role of twinning in plastic deformation

A new concept of the structure of yttrium-aluminum garnet (Y₃Al₅O₁₂) has demonstrated the possibility of forming inversion twins in the (111) plane. The twinning processes are shown to be responsible for stress relaxation during the growth and plastic deformation of Y₃Al₅O₁₂ single crystals. This way of stress relaxation can be used to describe the plastic deformation of other crystals, at least cubic ones.     

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.     

Universal critical-like scaling of dynamics in plastic crystals

Many theoretical models for the glassy dynamics have been proposed so far describing the changes in molecular dynamics along the extraordinary slowing down in the vitrification process of a disordered phase on cooling. Many of these theories share the concept of cooperative rearranging regions firstly proposed by Adam and Gibbs. Among them, the dynamical scaling model (DSM) is based on the random diffusion of free volume which creates random walking clusters formed by cooperatively rearranging entities.Within this framework a critical phenomenon relating a hidden phase transition at T_C (below T_g) implies the divergence of the relaxation time (τ) or viscosity (η) τ, η ∝ (T − T_C) ^− ? with a universal scaling exponent φ → 9. In this work we apply the DSM model to orientational glasses, obtained from the quenching of orientationally disordered phases (plastic crystals) via the application of the linearized derivative-based transformation of dielectric spectroscopy τ(T) data.