T. V. Grigorova

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-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...

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.

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...

The plastic deformation of ultrafine grained aluminum at 0.52 K

The plastic deformation of ultra-fine-grained aluminum subjected to equal-channel angular pressing has been studied at a temperature below the superconducting transition point. The stress-strain curves σ(ɛ) for polycrystals and the effect of the superconducting transition on the jump of flow stress ΔσNS have been investigated. It is shown that the grain refinement, along with the increase in the flow stress, leads to a correlated change in the shapes of the dependences σ(ɛ) and ΔσNS(σ). The results obtained are explained by the features of dislocation accumulation in ultra-fine-grained polycrystals and by the manifestation of the inertial properties of dislocations under low-temperature plastic deformation.

Unstable plastic deformation of ultrafine-grained copper at 0.5 K

We investigate the relation between the strain-hardening rate and flow instability of polycrystalline Cu–OF deformed by tension at a constant rate in a liquid 3He atmosphere. The microstructure of the ultrafine-grained crystal, obtained by the equal-channel angular hydro-extrusion method, was varied by annealing at recovery and recrystallization temperatures and was monitored by x-ray diffraction. It is shown that that the flow instability, manifesting itself as macroscopic stress serrations on the tension curve, appears at a threshold tension sufficient for activation of a dynamic recovery that leads to a decrease of the strain-hardening coefficient. We discuss the effect of grain size and the initial dislocation density on the evolution of the dislocation structure that determines the scale and the statistical properties of the flow instability in the investigated crystals at low temperature.We investigate the relation between the strain-hardening rate and flow instability of polycrystalline Cu–OF deformed by tension at a constant rate in a liquid 3He atmosphere. The microstructure of the ultrafine-grained crystal, obtained by the equal-channel angular hydro-extrusion method, was varied by annealing at recovery and recrystallization temperatures and was monitored by x-ray diffraction. It is shown that that the flow instability, manifesting itself as macroscopic stress serrations on the tension curve, appears at a threshold tension sufficient for activation of a dynamic recovery that leads to a decrease of the strain-hardening coefficient. We discuss the effect of grain size and the initial dislocation density on the evolution of the dislocation structure that determines the scale and the statistical properties of the flow instability in the investigated crystals at low temperature.