Plastic-strain localization in barium fluoride single crystals at elevated temperatures

The strain distribution is studied in BaF₂ crystals subjected to compression tests along [110] and [112] at a constant strain rate in the temperature range T = (0.22–0.77)T _m. At T > 0.5T _m, the plastic strain in deformed samples is found to be strongly localized in narrow bands, where the shear strain reaches several hundred percent. The degree of localization increases with temperature. Localized-shear microbands are shown to be oriented along {001}〈110〉 slip systems. The phenomenon of serrated yielding is detected, and stress jumps (serrations) are established to correlate with the formation of shear zones.     

Indentation studies on alkaline earth fluoride crystals at elevated temperatures

Indentations have been made on the (111) planes of CaF₂, SrF₂ and BaF₂ at various temperatures up to 700°C. By etching the crystals, the indentation dislocation rosettes (IDRs) have been recorded and the relative hardness is estimated from the dimensions of the IDRs. The hardness of all these crystals falls with temperature according to the equationH=A exp(−BT). Dramatic changes are observed in the shape of the IDR at elevated temperatures indicating activation of new slip systems.     

Local heating and quasiathermal plastic deformation of crystals at low temperatures

A theoretical discussion is given of the relationship between local heating of a crystal by glide lines and bands and deviations observed in the temperature dependences of the flow stresses and their rate coefficients at low temperatures (<10 K) from the dependences characteristic of thermally activated plastic deformation. The appearance of plateaus in these dependences is currently explained by the onset of quantum-mechanical, athermal mechanisms for overcoming local barriers. In this paper it is shown that softening and the apparent athermicity of low-temperature deformation are caused by heating of sites where the strain is localized. Fiz. Tverd. Tela (St. Petersburg) 39, 2019–2022 (November 1997)     

Resistance of zinc crystals to shock deformation and fracture at elevated temperatures

Velocity profiles of the free surface of shock-loaded zinc crystals are measured in two different orientations. The test temperature is varied from room temperature to 410 °C. The results of the measurements show that the high-velocity deformation and fracture are athermal processes and that the fracture stresses are influenced by the preceding plastic deformation. Fiz. Tverd. Tela (St. Petersburg) 40, 1849–1854 (October 1998)     

Thermal expansion of cadmium fluoride crystals at high temperatures

The thermal expansion coefficient of cadmium fluoride crystals has been measured over the temperature range of 300 to 1070 K. The data fit well to measurements done by other authors in the range 80–300 K. At temperatures above 1070 K the thermal expansion coefficient shows an anomalous behaviour which has been attributed to the formation of cadmium oxide on the surface of the sample.     

The influence of quantum-mechanical athermal phenomena on the thermal instability of plastic deformation in crystals at low temperatures

The way in which athermal (quantum) mechanisms that allow dislocations to overcome local barriers affect the thermal instability of plastic deformation of crystals is discussed theoretically for the case of low and ultralow (<1 K) temperatures. Calculations show that increasing the athermal component of the dislocation activation leads to a considerable narrowing of the temperature/strain-rate region where discontinuous strains appear, and that further increases in the quantum component of the process by which dislocations surmount local barriers leads to the complete disappearance of these discontinuous strains. Experimental situations in which the effects of thermal instability and athermal effects are observed at the same time in a number of crystals under conditions of low-temperature strain are discussed in light of these results. Fiz. Tverd. Tela (St. Petersburg) 39, 1392–1398 (August 1996)     

Heating of slip lines and bands as a quasi-athermic mechanism of plastic deformation of crystals at low temperatures

We discuss theoretically a mechanism of violation of the Arrhenius law for the rate of plastic deformation, on the one hand, and of the appearance of plateau-like segments in the temperature dependence of the thermal-activation parameters, on the other, during deformation of crystals at low (<10 K) temperatures, which is associated with heating of the crystal by slip lines and bands. Via a self-consistent solution of the heat-conduction equation with allowance for variation of its coefficients and the rate of plastic deformation with temperature it is found that both a stable and an unstable regime (in the thermal sense) of propagation of slip lines and expansion of slip bands are possible depending on the ratio between the heating level and the level of strain hardening of the strain localization sites. The first regime is associated with the appearance of quasi-athermic plateaus in the temperature dependences of the thermally-activation parameters, and the second one leads to an instability (stepped) in the plastic deformation that is characteristic at low temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 1479–1485 (August 1998)     

Relationships and macroscopic mechanism of cyclic deformation of polycrystals at elevated temperatures

Systematic investigation of the development of deformation and fracture under cyclic stresses is conducted on a sample of lead alloy as a function of the type of alloying. A macroscopic mechanism for fatigue failure at elevated temperature is formulated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 27–31, November, 1987.     

Localization of plastic deformation because of self-heating

Conclusion The influence of the heat dissipated by plastic deformation on the mechanical behavior of the crystalline specimen being loaded is examined in this paper. It is shown that the plastic deformation can be localized because of the heat dissipation. The characteristic scales describing the thermal processes in this case depend exponentially on the applied stress. The heat being dissipated contributes, on the one hand, to the formation of a crack of critical size, and on the other hand, the thermoelastic energy being formed during the plastic deformation assists its spontaneous growth.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 78–81, July, 1981.     

Acceleration of atoms in crystals under plastic deformation

It is shown that a double-well potential can appear under mechanical loads for atoms in rows parallel to the axis of a screw dislocation and located near the nucleus of a dislocation and under dislocation slip conditions atoms can be accelerated up to energies much higher than the binding energy of atoms in a crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 499–504 (March 1997)     

Spontaneous excitation of discrete breathers in crystals with the NaCl structure at elevated temperatures

The density of phonon states at various temperatures has been calculated for crystals with the NaCl structure with equal and strongly differing weights of anions and cations. It has been shown that, in the crystal with a considerable difference in the weights of components, a wide band gap exists in the spectrum of phonon states, which leads to spontaneous excitation of nonlinear localized vibrational modes??gap discrete breathers having frequencies inside the band gap, if the temperature is sufficiently high. It has been found that the peak of the density of phonon states lying above the spectrum of linear vibrations appears at elevated temperatures, which can indicate the existence of discrete breathers with corresponding frequencies. It has been noted that, previously, the existence of gap discrete breathers in the NaI crystal at 555 K was shown experimentally. The presented results bring up the question of the theoretical justification and experimental observation of the breathers with frequencies above the phonon spectrum.     

Rotational instability of filamentary crystals undergoing plastic deformation

We investigated the kinetics of spontaneous torsion in filamentary crystals during plastic deformation by a tensile load. The nature of the observed effect is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 76–81, August, 1988.     

Spectral-kinetic characteristics of transient absorption in calcium fluoride crystals

The method of absorption spectroscopy with nanosecond resolution is used to study transient absorption in nominally pure crystals of CaF₂ irradiated by a pulse of fast electrons. Optical absorption caused by self-trapped excitons appears in the form of a superposition of bands with half-widths of no more than 0.2 eV in both the electron and hole components. These groups of bands are assigned to absorption of F, H pairs in various configurations. We observe a regular change in the spectral-kinetic characteristics depending on the distance between the components of the pair. Fiz. Tverd. Tela (St. Petersburg) 39, 1197–1201 (July 1997)     

Thermal depolarization in crystals of calcium fluoride doped with oxygen

The study of thermally induced depolarization (TD) in crystals of calcium fluoride doped with oxygen reveals the existence of nearest-neighbour (nn) dipolar complexes comprising substitutional oxide ions (O_s^2?) and fluoride ion vacancies (F_v^?) on nn sites. Evidence for this relaxation is seen in TD experiments both on pure calcium fluoride doped with oxygen and on Na⁺ doped CaF₂ crystals that had been heated in air. Similar measurements on CaF₂: Y³⁺, O^2? reveal six separate relaxations, two of which are due to Y_s^3??F_i^? complexes that do not involve oxygen, one is due to O_s^2??F_v^? dipoles, and one is t the T₁ complex, Y_s³⁺ (O^2?)₄(F_v^?)₃. The remaining two relaxations were not identified but are probably d larger defect clusters.     

Electronic excitations owing to plastic deformation of ionic crystals

Electron emission and luminescence accompanying plastic deformation of alkali halide crystals are studied. It is shown that the intersection of dislocations can cause electronic excitation. Deformation electron emission and luminescence are produced by relaxation of these excitations. Fiz. Tverd. Tela (St. Petersburg) 41, 900–902 (May 1999)