N. V. Isaev

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.

Strain hardening of metals and alloys in the superconducting state

The strain hardening of single crystals of pure Al and a Pb–In alloy in the normal (N) and superconducting (S) states is investigated. It is found that the strain hardening coefficients satisfy the inequality θS>θN irrespective of whether the SN transition is brought about by an external magnetic field or temperature.

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...

Strain-rate sensitivity of the flow stress of ultrafine-grain aluminum at temperatures 4.2–295K

The influence of the grain size on the dislocation interaction mechanisms that control the deformation of aluminum in the temperature range 4.2–295K is studied. For this, samples of coarse-grain (CG) and ultrafine-grain (UFG) aluminum obtained by equi-channel angular pressing were deformed by stretching at constant rate e as well as in the rate-cycling regime along the deformation curve. The effects of temperature on strain hardening and the stain-rate sensitivity of the flow stress of CG and UFG material are compared. It is shown by means of thermal-activation analysis of the experimental data that the dependence of the rate sensitivity parameter m=[∂lnσ∕∂lne]T on the grain size and temperature of the aluminum is explained by a change of the dislocation mechanisms controlling the plastic deformation of the aluminum. In the temperature interval 4.2–40K the plastic deformation of the UFG and CG aluminum is due to a single mechanism of intersection of forest dislocation. In the interval 120–295K for CG an...

A low-temperature plasticity anomaly of concentrated fcc solid solutions: the Pb–In system

This paper discusses the regularities of the plastic strain of single crystals of solid solutions of the Pb–In system at low and very low temperatures, 0.5 K<T<30 K. For Pb alloys with 5, 10, and 20 at. % In, the temperature dependences of the yield strength, τ0(T), and of the increment of the deforming stress, Δτ(T), are measured after a tenfold increase of the strain rate. Specific features (anomalies) of these dependences are detected that do not correspond to concepts of thermally activated motion of dislocations through impurity barriers. It is established that the character of the anomalies substantially changes as the indium concentration varies from moderate values (5 and 10 at. %) to high values (20 at. %). The low-temperature plasticity anomaly of moderately concentrated alloys is interpreted on the basis of the concepts of thermal-inertial and quantum-inertial motion of dislocations through a system of single impurity atoms. In the case of the highly concentrated alloy, the anomaly is interpret...

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.

Jumplike deformation in normal and superconducting states: The solid solution Al–Li

Low-temperature jumplike deformation (LTJD) of the solid solution Al–3.8at.% Li is studied at 0.52K in the normal (N) and superconducting (S) states. The magnitude of the plasticization effect at the NS transition is used to estimate the local heating of the sample. It is shown that the local heating is insufficient to change the sign of the temperature sensitivity of the deforming stress that explains the influence of the electronic state of the sample on the development of the LTJD on the basis of the thermal concept. Analysis of the statistics of the stress jumps showed that the amplitude density distribution of jumps into the N state is described by a power law with exponent α=1.3±0.2. The power law is considered to be an indication of self-organization of criticality in dislocation dynamics and the development of LTJD a manifestation of avalanche-like motion of dislocation pileups.

Anomalous diamagnetism in aluminum-lithium alloys

In the region of the equilibrium solid solutions of lithium in aluminum an anomalous low-temperature peak of diamagnetism is observed which is due to the presence of a boundary point on a line of band degeneracy directly below the Fermi level. The position and structure of the peak in the average valence function are analogous to those studied previously in the aging systems Al–Mg and Al–Zn under suitable heat treatment. The value of the lithium impurity scattering parameter for the electron states in the vicinity of the point of degeneracy mentioned is estimated, and the linear relation of that parameter with impurity-related electrical resistance in aluminum alloys is established.