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.     

Investigation of the deformed region on the (111) plane of MgO crystals by cathodoluminescence method

The (111) plane of MgO single crystals was deformed by a concentrated load. It was observed that the number of rows of a cathodoluminescence rosette is a factor of two lower than that for the etch pit one. The explanation of this phenomenon is given. The polarity of plastic deformation was revealed using both an etch pit technique and cathodoluminescence method. It is shown that deformation causes a change of the cathodoluminescence spectra. The spectra, obtained from crystal deformed by indentation and from compressed crystal, are similar.     

On a delayed dislocation emission from hot hardness indentations

The change of the dislocation arrangements around microindentations after indentation test has been investigated by TEM and etching. The indentations were generated on {100} - surfaces of iron single crystals at temperatures between 800 K and 1000 K. The change of the dislocation structure was found to depend strongly on stress and dislocation density in the deformation field. The deformation field especially acts as a dislocation source operating over long periods. The results suggest that the propagation of emitted dislocations is controlled by dragging of Cottrell-atmospheres of carbon atoms. Conclusions regarding the hardness-temperature relation in pure iron are drawn.     

Application of X-ray diffraction topography for the study of Al single crystals plastically deformed at low temperatures

The Lang method has been used to investigate regularities and peculiarities of formation of Al single crystal dislocation structure in various stages of the stress-strain curve at 77.3 and 4.2°K up to high strains. The low temperature deformation of crystals has been found to cause a sharp localization of the deformation into slip bands that correspond to different glide systems. A comparison with metallography is added.     

Asymmetry of isolated dislocation mobility in Silicon single crystals

The main quantitative characteristics of asymmetry of isolated dislocation mobility in Si single crystals manifested in an essential difference (up to an order of magnitude) of the velocities of broadening and narrowing dislocation half-loops have been investigated. Transition from broadening to narrowing of the dislocation half-loop is found to result in the nonmonotonic temperature dependence of the 60° segments velocities as well as in changing the stress dependence of the velocity. The high temperature annealing of the sample befor the reversal of the dislocation glide is found to cause the asymmetry disappearance. The analysis of the results obtained is performed. It is concluded that the phenomenon observed may be due to a rearrangement of the point defect structure during the course of the dislocation glide.     

Microharness and dislocation mobility in ionic crystals

The correlation between microhardness and dislocation mobility was investigated for the series of ionic crystals in the wide temperature interval (77 – 400 K). It was shown that the dislocation mobility was not the main parameter responsible for the microharness change in the series of KCl—NaCl—NaF—LiF—MgO crystals or in temperature lowering from 293 to 77 K. A marked activation of the dislocation mobility can play a significant role in the range T > 293 K only. In the low temperature region the microhardness change may be connected with the parameters caracterising the dislocation multiplicatio and interaction.     

Plastic Deformation of Cesium Iodide Single Crystals at High Temperatures

The plastic deformation of CsI crystals of three crystallographic orientations 〈100〉. 〈110〉. and 〈111〉 at temperatures from 423 to 773 K (0.5 to 0.86T_m) and strain rates from 2 × 10⁻³ to 10⁻⁵ s⁻¹ is studied. Four-stage stres-strain curves were found, three stages being more and more distinct with an increase in temperature up to 623 K above which stage III prevails. For all the temperatures, strain rates and oreintations studied the superplasticity features (jerky flow, deformation localization, active dynamical recovery etc.) were observed. The thermal activatoin analysis has shown that the rate of high temperature deformation of CsI is controlled by dislocation climb limited in its turn by mobility of cation vacancies (quasiviscous creep).     

Influence of impurity and indentation time on dislocation rosettes

It has been shown that the impurity type and indentation time affect appreciable on dislocation rosettes of NaCl single crystals. It was established that the growth of indentation diagonal was not practically followed by the increase of the dislocation zone size. This phenomenon connected with the cross slip processes, which are perhaps more intensive for prolonged indentation time and high temperatures. The assumption is made that the two mechanisms of responsible for indentation formation the movement of dislocations and their multiplication by the cross slip.     

Mechanical properties of optical glass fibers damaged by nanoindentation and water ageing

Mechanical properties of optical glass fibers are strongly influenced by cracks on glass surface. Cracks may be generated during manufacture and handling. The chemo-mechanical effect due to water may decrease the response of glass fibers because of surface degradation. In this work glass fibers were aged in a wet chamber at humidity of 85% in temperatures lower than 90 °C, during seven weeks. After being aged, the fibers were submitted to tension tests. The fracture loads were investigated by Weibull statistics. An increase in dynamic fatigue parameter was observed. Surface analysis by AFM indicated smooth surfaces. Small cracks on optical fiber glass surface were made by Vickers indentation and nanoindentation using a Berkovich indenter. A decrease on applied load to fracture indicates that the fiber was damaged by indentation. The influence of indentation load was investigated. As the deformation rate was very low during tension tests, the cracks grow during the test and the fiber fracture at loads lower than for not indented fibers. The effect of water and small indentation fracture on glass is discussed considering the surface damage induced by both methods. The size of the cracks from the different indentations and ageing are estimated.     

Einfluß der Temperatur auf die Versetzungsstruktur in der Umgebung von Mikrohärteeindrücken in Eiseneinkristallen

The temperature dependence of microhardness on {100}-surfaces of iron single crystals was determined in the temperature range between 200 K and 1180 K. The dislocation structure around the microindentations is investigated by transmission electron microscopy, optical microscopy and etching. The results suggest that the separate ranges of the microhardness-temperature-dependence coincide with characteristic dislocation arrangements. Possible deformation processes are discussed.     

Erholung und Rekristallisation des stabilen Austenits bei der Verformung durch Walzen

For the explanation of the processes which take place during a high-temperature thermomechanical treatment (HTTT) in the stable austenite hardness, grain structure and dislocation structure of a Fe 24Ni 0,5C alloy are examined in dependence of the degree and temperature of rolling deformation (ϵ = 13 to 52%; T_v = 830 and 1150°C). The observed structures can be represented by a recovery-recrystallization-diagram, that describes the influence of technological parameters on the recovery and the recrystallization of stable austenite.     

Unstable internal friction in Bismuth and Antimony single crystals

In the temperature range of 6 to 300 K an effect of the high amplitude ultrasound waves on amplitudedependences of decrement and modulus defects in the antimony and bismuth single crystals of different orientation is studied. The character of changes of the decrement and modulus defect amplitude dependences under the influence of the high amplitude ultrasound indicates that dislocation multiplication, obstacle break-through by dislocation pile-ups or deformation twinning may be observed in crystals depending on the ultrasound propagation direction and the temperature interval investigated.     

On the initial stage of plastic deformation of sulphur-doped gallium phosphide

The overall deformation behaviour between room temperature and 1000 K has been investigated by bending and uniaxial compression at small strains ≦ 0.1%. Ductile-to-brittle transition temperature was found at 44% of the melting point. Dislocations are generated mainly at surface irregularities. Whereas screw dislocations predominate at the beginning 30° and 60° segments become more important after cross slip and interaction of different slip systems.     

Dislocation internal friction and temperature dependence of yield stress in high-purity molybdenum single crystals

High purity molybdenum single crystals (residual resistivity ratio ≈ 6 · 10⁴) were studied by means of computer-controlled internal friction technique at frequencies of about 100 kHz in the temperature range 6 to 300 K. The amplitude dependences of decrement were measured within the vibrational strain amplitude 10⁻⁷ to 10⁻⁴. It was established that the temperature dependence of ultrasonic amplitude providing a constant level of reversible dislocation deformation coincides well in relative units with the temperature dependence of the critical resolved shear stress at 20 to 180 K.     

Impedance Measurements on Sintered Discs of Sodium Metavanadate

Impedance measurements of sintered discs of sodium metavanadate (NaVO₃) are investigated in the frequency range 10 KHz to 1 MHz and in the temperature region from room temperature (≈ 30 °C) to 450 °C. A.C. conductivity is calculated from the data. D.C. Conductivity in measured in the temperature range 150 to 450 °C. Debye type of relaxation effects are observed in the loss measurements. The activation energy of the dipoles involved in the relaxation is estimated to be 0.76 eV. The activation energy for A.C. conduction in the temperature range 380 to 450 °C is about 0.78 eV. The relaxation phenomenon observed is explained in terms of space charge polarisation due to defects.     

Thermally activated deformation of Alpha Zirconium

The effect of temperature on the yield stress of polycrystalline α-zirconium has been examined at temperatures between 77 and 900 K. A strong temperature dependence of the yield stress (interrupted in the temperature ranges 300 to 400 K and 500 to 700 K where the yield stress is independent of temperature) indicates the occurence of the thermally activated processes during deformation. The activation volume at the onset of deformation has been determined as a function of temperature in the range 77 to 523 K.     

Plastische Verformung von Siliziumeinkristallen unterschiedlicher Ausgangsversetzungsdichte im Streckgrenzenbereich

Floating-zoned Silicon crystals orientated for single slip both dislocation-free and with grown-in dislocations (EPD₀ = 6,5 · 10⁴ cm⁻²) were dynamically deformed in compression in the temperature range 1120 … 1300 K up to the lower yield point. The dislocation structure of plastically deformed crystals (T = 1138 K; a₀ = 1,45 · 10⁻⁴ s⁻¹) was examined by etching and high-voltage electron microscopy. The crystals exhibit essential differences in deformation behaviour and in the resulting dislocation structure. Typical for the dislocation-free basic material are (i) an inhomogeneous deformation in the range of the upper yield point (ii) a greater portion of sessile lomer dislocations at the lower yield point in relation to the crystal with grown-in dislocations. The measured stresses were related to those values calculated from structure data by employing plasticity theory of diamond-like semiconductors.     

Dislocation arrangements in cyclically deformed polycrystalline molybdenum

The present paper deals with the cyclic stress-strain behaviour of polycrystalline molybdenum at room temperature and the dislocation structures built up within this material during the fatigue process. A cyclic stress-strain curve of molybdenum deformed in a strain-controlled and symmetrical push-pull test is shown. At strain amplitudes e_a < 3 × 10⁻³ arrangements of relatively homogeneously distributed dislocations are observed in the stage of the stabilization of mechanical properties. The characteristics of these dislocation arrangements are similar to those of dislocation structures of unidirectionally deformed molybdenum single crystals. At strain amplitudes e_a > 3 × 10⁻³ dislocation structures are developed with an inhomogeneous dislocation distribution (bundles structures). The dislocation density in the surface layers of fatigued specimens shows larger values than within the material. The cyclic deformation after a change from a small deformation amplitude to a larger one, or vice versa, is connected with characteristic changes of dislocation density.     

Correlation between microhardness and dislocation mobility in crystals with a different type of bond

The complex investigations of microhardness and dislocation mobility in temperature interval 77–873 K have been performed on ionic crystals, semimetals and covalent crystals. It is shown that the correlation between the microhardness and dislocation mobility exists as a whole only. The temperature change of microhardness is not often connected with the change of dislocation mobility. An “anomalous” behaviour of semimetals has been established – they have a small microhardness and a low dislocation mobility at a time.     

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.