I. S. Braude

Low-temperature plastic strain of ultrafine-grain aluminum

The microstructure and mechanical properties of ultrafine-grain (UFG) commercial-grade Al obtained by equichannel angular pressing (ECAP) are study in the temperature range 4.2–295K. Transmission electron microscopy and x-ray diffraction methods are used to show that as the number of passes increases, the grain size decreases, the grain shape becomes increasingly equiaxial, and the dislocation density inside a grain and the character of the intergrain boundaries change. An increase of the coherent scattering region and a decrease of the level of microdeformations indicate that pressing decreases the total density of imperfections of the crystal structure inside grains. As temperature decreases, the yield stress, plasticity, and strain hardening rate of UFG and coarse-grain polycrystals increase substantially. The deformation of UFG polycrystals at 4.2K becomes unstable (abrupt). The temperature dependences of the yield stress σy(T) of UFG and coarse-grain polycrystals, where the form of these dependences ...

Low-temperature plastic deformation of AZ31 magnesium alloy with different microstructures

Yu. Z. Estrin, P. A. Zabrodin, I. S. Braude, T. V. Grigorova, N. V. Isaev et al. Citation: Low Temp. Phys. 36, 1100 (2010); doi: 10.1063/1.3539781 View online: http://dx.doi.org/10.1063/1.3539781 View Table of Contents: http://ltp.aip.org/resource/1/LTPHEG/v36/i12 Published by the American Institute of Physics.

Microstructure and low-temperature plastic deformation of Al–Li alloy

Features of the plastic deformation of solid Al–Li solutions with microstructures formed by direct and angular hydroextrusion are studied under tension at temperatures of 4.2–350 K. It is found that the grain size reductions, increases in the average density of defects, and changes in the orientational textures during combined hydroextrusion lead to increased strength and reduced plasticity of the microcrystalline alloy relative to initially large-grained samples. The high yield stress of the microcrystalline alloy is explained by a higher grain density and the evolution of an orientational texture. The strong temperature dependence of the yield stress is typical of thermally activated interactions between dislocations and local obstacles in the form of deformation defects produced during hydroextrusion. The low plasticity of the microcrystalline alloy, which already shows up as a localization of plastic deformation with small deformations, is caused by a low rate of work hardening owing to enhanced dynam...

Features of the low-temperature plasticity of Pb–In single crystals

The temperature dependence of the plasticity parameters of Pb–In single crystals with indium concentrations of 1–20 at. % are investigated under tensile deformation at a constant strain rate in the temperature interval 4.2–295 K. From an analysis of the experimental data, empirical estimates are obtained for the main parameters of the dislocation–impurity interaction and the dynamic drag coefficient of the dislocations. The calculated values of these parameters are consistent with the idea of a gradual transition on cooling, from a thermally activated motion of dislocations through local impurity barriers to a thermal–inertial motion. With increasing indium concentration (to 20 at. %) the low-temperature mechanisms of dislocation motion begin to be affected substantially by regions of short-range order (clusters), the presence of which is detected by the diffuse x-ray scattering method.

Features of the microstructure and low-temperature yield stress of quenched Al–Li alloys

The features of the microstructure of quenched Al–Li alloys with lithium concentration below the solubility limit (3.8 at. %) and above the solubility limit (7.0 and 10.4 at. %) are studied by the x-ray diffractometry. In addition to the standard structural reflections, the diffraction patterns of all the alloys show diffuse halos that fall off in intensity as the lithium concentration is increased. As a result of an analysis of the intensity of the diffuse scattering of x rays it is established that the observed halos are due to short-range order of the layered type in regions with a characteristic radius of 1.5 nm. The decrease in the intensity of the diffuse halo on the diffraction pattern of the alloy Al–10.4 at. %Li and the appearance of new structural reflections are explained by the precipitation of disperse particles of the δ′ phase. It is shown that these particles have a substantial influence on the mechanism of plastic deformation in these alloys in the temperature region 40–170 K.

Strain hardening and jump-like deformation of ultrafine polycrystalline Al-Li solid solutions at 0.5 K

This is a study of the effect of microstructure created by severe plastic deformation (SPD) and annealing on strain hardening and jump-like deformation in Al-Li alloys. It is shown that under tension at 0.5 K, SPD processed polycrystals retain a significant strain hardening rate and have high strength and ductility. SPD also simulates unstable (jump-like) flow of the polycrystals owing to dislocation dynamics that shows up as stress jumps in the tension curve. The average amplitude of the jumps increases with strain, while the dislocation amplitude distribution corresponds to collective motion of dislocation avalanches with a distinctive scale. Jump-like deformation is partially suppressed by high-temperature annealing, while the distribution of the jump amplitudes is described by a power law. The relationship established between the coefficient of strain hardening and the average stress jump amplitude suggests a common dislocation dynamic for strain hardening and jump-like deformation at low temperatures...

Anisotropy of the hopping magnetoresistance of antiferromagnetic La2CuO4+δ single crystals

The anisotropy of the hopping conductivity of antiferromagnetic La2CuO4+δ single crystals with TN≈188 K is investigated in the temperature range 5–295 K and the anisotropy of their magnetoresistance, in the temperature range 5–55 K. The resistance is measured by the Montgomery method for different combinations of directions of the transport current and magnetic field relative to the crystallographic axes. For the case when the field and transport current are directed parallel to the CuO2 layers, a transition from negative to positive magnetoresistance is observed when the temperature is raised to T≈20 K. For fields perpendicular to the CuO2 layers, only negative magnetoresistance is observed. The nature of the positive magnetoresistance is discussed. It is shown that the effect is most likely due not to the interaction of the spin of the charge carriers with the surrounding magnetic medium but to the orbital motion of these carriers. The corresponding values of the positive magnetoresistance and its behav...

Anisotropy of the yield strength and structural parameters of nanocrystalline titanium obtained by cryodeformation

Anisotropy of the yield strength of nanocrystalline VT1-0 technical-grade titanium with grain size ∼45 nm, obtained by the cryomechanical grain fragmentation method, was studied. The experiments were carried out using the uniaxial compression regime at a nominal plastic deformation rate of 3.5 × 10−4 s−1 in the temperature range 4.2–300 K. The temperature dependences of the macroscopic yield strength of the samples with the compression axis parallel and perpendicular to the cryorolling direction were obtained. The anisotropy coefficient of the yield strength was calculated, and its temperature dependence was established. The parameters of the deformation microstructure in mutually orthogonal planes were determined: the crystallite (coherent scattering region) sizes and the magnitudes of microdeformations. It was observed that the crystallites are morphologically anisotropic. The calculated and experimental values of the yield strength of the samples deformed parallel and perpendicular to the cryorolling d...

Influence of low temperature on deformation changes in the structure of the polyimide film PMA

ABSTRACT The polyimide films PM-A were investigated by X-ray diffractometry after deformation at temperatures of liquid nitrogen and liquid helium. As a result of deformation the temperature of liquid nitrogen in the films two regions with different densities were formed. It was found that the deformation of films at liquid-helium temperatures, does not lead to significant changes in the structure of the film. Assuming that the film consists of quasi-linear chain molecules, two «mechanism» has been proposed. These «mechanisms» are qualitatively describe the processes taking place in the polyimide films PM-A after deformation at low temperatures.

Microstructure anisotropy of nanocrystalline titanium produced by cryomechanical grain fragmentation

Using X-ray diffraction analysis, a systematic study was undertaken of the parameters of the deformation microstructure formed in commercially pure VT1-0 titanium as a result of cryogenic rolling at a temperature of 77 K at different degrees of compression. In order to ascertain the anisotropy of the microstructure, a comparative analysis of diffraction patterns, dimensions of crystallites (coherent scattering regions) L and microdeformation values e 2 1 / 2 in the rolling plane and in a plane perpendicular to the rolling direction was performed by comparison with the relative activity of deformation modes. As a result, anisotropy was detected in the distribution of integral intensities of diffraction peaks for mutually perpendicular planes. The established difference in the dimensions of crystallites in the rolling plane and in the plane perpendicular to the rolling direction indicates the shape anisotropy of the crystallites. The effect of morphological anisotropy of crystallites/grains is most pronounced for the nanocrystalline state. The observed complex variation in the microdeformation values e 2 1 / 2 ( e ) with compression deformation is well correlated with relative slip and twinning activity, which affect the level of local internal stresses and the possibility of their relaxation. The observed anisotropy with respect to the magnitude of microdeformations may be attributed to the presence of oriented grain boundaries associated with the shape anisotropy of crystallites/grains.Using X-ray diffraction analysis, a systematic study was undertaken of the parameters of the deformation microstructure formed in commercially pure VT1-0 titanium as a result of cryogenic rolling at a temperature of 77 K at different degrees of compression. In order to ascertain the anisotropy of the microstructure, a comparative analysis of diffraction patterns, dimensions of crystallites (coherent scattering regions) L and microdeformation values e 2 1 / 2 in the rolling plane and in a plane perpendicular to the rolling direction was performed by comparison with the relative activity of deformation modes. As a result, anisotropy was detected in the distribution of integral intensities of diffraction peaks for mutually perpendicular planes. The established difference in the dimensions of crystallites in the rolling plane and in the plane perpendicular to the rolling direction indicates the shape anisotropy of the crystallites. The effect of morphological anisotropy of crystallites/grains is mos...