L. S. Fomenko

Plasticity and strength of metal oxide high-temperature superconductors (Review)

The results of research on the plasticity and strength of a wide class of metal oxide perovskite-like compounds which have the property of high-temperature superconductivity or which can be used as base compounds for making high-temperature superconductors (HTSCs) are systematized and presented from a unified point of view. The mechanical properties of materials with different morphology—single crystals, polycrystals, and composites,—measured by different methods of mechanical testing in the low-temperature, room-temperature, and high-temperature regions, are discussed. The characteristic defects of the crystal structure for these compounds are considered, the crystallography of two modes of plastic deformation—slip and twinning—is described, and the stress-induced structural rearrangement of the twin structure that appears at a high-temperature phase transformation is discussed. The features of plastic deformation and fracture of metal oxide materials due to structural microdefects (dislocations, impurit...

Micromechanical properties of nanocrystalline titanium obtained by cryorolling

It is shown by microindentation that the defect structure of nanocrystalline titanium obtained by rolling at low temperature is quite uniform. The presence of texture was manifested as a small difference of the average values of the hardness H¯V and the standard deviation in the measurements in the roll plane and in a plane perpendicular to the roll direction. The data on the temperature dependence of the microhardness in the temperature interval 77–300K are used to evaluate the activation volume and the activation energy that characterize the process of plastic deformation of nanocrystalline and coarse-grain titanium under a concentrated force. Evaluations attest to the dislocation nature of local deformation. The relation between the microhardness and the conditional yield stress HV≈3σ0.2 holds well for nanocrystalline titanium. The hardening of titanium with grain-size reduction is accompanied by appreciable decrease of its plastic compliance.

Influence of oxygen content and structural defects on low-temperature mechanical properties of high-temperature superconducting single crystals and ceramics

The data for the microhardness and fracture toughness of Y–Ba–Cu–O and Bi-based single crystals and ceramics in the temperature range 77–293 K are presented and analyzed. Our study reveals that the microhardness of high temperature superconductors is very sensitive to the oxygen stoichiometry, the phase composition, the temperature, and to the microstructural defects such as impurities, intergranular boundaries, and voids. Attention is drawn to the anisotropy of the micromechanical properties and to the features of the fracture in the vicinity of the indentation. The data available on the plasticity of Y–Ba–Cu–O and Bi–Sr–Ca–Cu–O from micro- and macrotests are compared.

Hydrogen absorption and desorption kinetics in fullerite C60 single crystals. Low-temperature micromechanical and structural characteristics of the interstitial solid solution C60(H2)x

The microhardness HV and lattice parameter a of C60 single crystals are measured at room temperature as functions of the hydrogen saturation time t for several values of the saturation temperature (250, 300, and 350°C) at a fixed hydrogen pressure p=30atm. According to the measurements of HV and a, the kinetics of hydrogen absorption is described by a simple exponential law with a single, temperature-dependent characteristic time. In highly saturated samples the microhardness is 4 times greater than for the initial C60 crystal, while the lattice parameter is 0.2% larger. The temperature dependence of the microhardness HV and lattice parameter a of C60(H2)x crystals is investigated in the temperature interval 77–300K. The introduction of hydrogen lowers the temperature of the fcc–sc phase transition, and the transition becomes strongly broadened in temperature. The dependence of the microhardness of the saturated sample on the hold time in air at room temperature is described by the sum of two exponentials...

Structural homogeneity of nanocrystalline VT1-0 titanium. Low-temperature micromechanical properties

The microhardness of samples of VT1-0 titanium with grain sizes ranging from 35 nm to 10 μm is measured at temperatures of 77-300 K. Nanocrystalline samples produced by rolling at low temperatures are found to be quite homogeneous, and their structure is stable with respect to thermal and mechanical interactions. The interrelationship between microhardness and grain size is well described by the Hall-Petch relationship, the parameters of which depend on temperature. Data on the temperature dependence of the microhardness and the Hall-Petch coefficient indicate that the microplastic deformation is of a thermally activated, dislocation character, regardless of grain size.

Micromechanical properties of VT1-0 titanium cryorolled to various degrees of strain

Multipass rolling of VT1-0 titanium at a temperature near that of liquid nitrogen to a strain e = −2 resulted in grain refinement from ∼10 μm to ∼35 nm and a nearly twofold increase in microhardness. The microindentation measurements showed that the cryorolled samples had a rather homogeneous structure. An increase in the microhardness with increasing the strain can be described by the modified empirical Voce equation. The grain size dependence of the microhardness in the Hall–Petch coordinates consists of two parts with the slopes kHP1 and kHP2 < kHP1 for the grain size smaller than ∼250 nm. The strong temperature dependence of the microhardness for the investigated samples suggests that their plastic deformation has a thermally activated character. Close values of the thermoactivation parameters presumably indicate a common deformation mechanism in this material placed under an indenter over the entire grain size range.

Low-temperature microhardness of Xe-intercalated fullerite C60

The Vickers microhardness of Xe-intercalated polycrystalline fullerite C60 (XexC60, x≃0.35) is measured in a moderately low temperature range of 77 to 300 K. A high increase in the microhardness of the material (by a factor of 2 to 3) as compared to that of pure C60 single crystals is observed. It is shown that the step-like anomaly in the temperature dependences of the microhardness of pure C60 single crystals recorded under the orientational fcc-sc phase transition (Tc≃260 K) is also qualitatively retained for XexC60, but its onset is shifted by 40 K towards lower temperatures and the step becomes less distinct and more smeared. This behavior of HV(T) correlates with x-ray diffraction data, the analysis of which revealed a considerable influence of xenon interstitial atoms on the peculiar features of fullerite thermal expansion due to orientational phase transitions (see the paper by A.I. Prokhvatilov et al. in this issue).

Low-temperature micromechanical properties of annealed and hydrostatically extruded Al–3.8 at. % Li alloy

The structural state of as-received course-grained (CG) and extruded ultrafine-grained (UFG) billets of Al–3.8 at. % Li substitutional alloy as well as that of the samples of the same alloy subjected to tensile deformation until fracture at 4.2, 77, 295, and 350 K were studied by the microindentation method. The temperature dependence of microhardness was investigated in the temperature range of 77 to 295 K. The measurements revealed anisotropy of the micromechanical properties of the UFG samples due to the texture formed during the extrusion. It was shown that the samples which were pulled until failure exhibit correlation between the microhardness, which is a measure of local strain-hardening, and the magnitude of the local plastic shear deformation. This correlation breaks down for the UFG samples deformed at 295 and 350 K. In this case the microhardness was found to be independent on the magnitude of plastic shear deformation. The temperature dependences of microhardness for the CG and UFG samples wer...

Study of the structural nonuniformity and low-temperature micromechanical properties of ultrafine-grain aluminum

The optimal conditions for measuring the microhardness of aluminum, concerning the surface preparation of the samples (electropolishing) and the load on an indenter (at least 0.5N), are determined. The degree of structural uniformity of aluminum after deformation by equal-channel angular pressing (ECAP) is studied by the microindentation method. It is found that the microhardness of an extruded blank varies over the cross-section, and it reaches its maximum value in the central part. The nonuniformity decreases as the number of passes increases. The main structural changes giving rise to hardening occur during the first pass. The temperature dependence of the microhardness in the interval 77–295K intensifies as the number of ECAP passes increases. The Hall-Petch law describes the hardening of aluminum as result of grain-size reduction during ECAP well, and the Hall-Petch coefficient increases as temperature decreases. For ultrafine-grain aluminum the microhardness and yield stress with strain e=0.076 are ...

The influence of Peierls relief on low-temperature plasticity of CdTe single crystals

Abstract The following was investigated on CdTe single crystals: the kinetics of spontaneous elongation of dislocation arms arising near the indentor impression on the (111) face at room temperature; the kinetics of stress relaxation in the early stages of deformation in the vicinity of the yield point at 200, 225, 250, 273 and 300 K; the temperature dependence of the yield point in the temperature range 200–300 K. The experimental data for the temperature dependence of macroscopic plasticity parameters are well described by the model of dislocation movement in the Peierls relief by the kink pair mechanism for the case of low effective stresses τ∗ when the equation for the average dislocation velocity is ν∼ exp [ −H kp (τ∗) κT ] with the activation enthalpy H kp = 2H k −2α(τ∗) 1 2 . According to our estimates, the parameters of the theory have the following values: enthalpy of kink pair formation 2H k = 0.6 eV , α = 10 −23 N 1 2 m 2 , and Peierls stress τp = 21 MPa. To explain the temperature dependence of the yield point we suppose that it is determined not only by the natural contribution τ∗(T) but also by the temperature dependence of internal stresses τi(T). The empirical values of τi(T) obtained in the temperature region studied can be assigned to development of the superjogs structure on dislocations. The average distance between the superjogs impenetrable to kinks is estimated to be L⩽10 μm. The process of dislocation arm elongation can be described by the empirical formula l(t,P)∼P 1 3 t 1 3 (P is the indentation load, t is time. This dependence follows from the assumption that the velocity of a single dislocation in an array can be described by the equation ν∼(τ∗) m exp ( −H 0 κT ) at m = 1.