L. N. Pal-Val

Bio: L. N. Pal-Val is an academic researcher from National Academy of Sciences of Ukraine. The author has contributed to research in topics: Atmospheric temperature range & Young's modulus. The author has an hindex of 6, co-authored 17 publications receiving 103 citations.

Unusual Young׳s modulus behavior in ultrafine-grained and microcrystalline copper wires caused by texture changes during processing and annealing

Abstract: Effect of annealing on the dynamic Young׳s modulus E of ultrafine-grained (UFG) and microcrystalline (MC) copper obtained by combined severe plastic deformation (SPD) including repeated hydrostatic extrusion and drawing (UFG copper) or only repeated drawing (MC copper) is investigated. It is established that the Young׳s modulus in the SPD-prepared UFG and MC samples exceeds that in the coarse-grained fully annealed (CGFA) samples by 10% to 20%. Subsequent isothermal annealing at elevated temperatures between 90 and 470 °С leads to a sharp decrease of the Young׳s modulus for annealing temperatures above 210 °С. After annealing at 410 °С, the value of E reaches its minimal value that is 35% lower than E in CGFA samples (total change in E is about 50% of the initial value). Further annealing at higher temperatures leads to some increase in the Young׳s modulus. It is shown that the unusual behavior of the Young׳s modulus is caused by the formation of the 〈111〉 axial drawing texture in the SPD-treated samples which is replaced by the 〈001〉 annealing texture during the post-SPD heat treatments.

Effect of plastic deformation on the shape and parameters of the low-temperature peak of internal friction in niobium

Abstract: Temperature dependences of the decrement of longitudinal vibrations are studied at moderately low temperatures in polycrystalline niobium with the residual resistivity ratio RRR=60 at frequencies 78 and 363 kHz. A peak of internal friction is detected in the vicinity of 200 K. The height, width, and temperature of the peak change significantly upon a variation of vibrational frequency and as a result of changes in the defect structure of the sample under thermocycling, plastic deformation, or prolonged low-temperature recovery. It is shown that the absorption peak is due to the interaction of elastic vibrations of the sample with a system of identical thermally activated relaxators with an activation energy of 0.15 eV and an attack frequency of 1×1010 s−1 in a nearly perfect crystal. A theory is proposed for describing the variation of the shape and parameters of the internal friction peak due to statistical dispersion of the values of activation energy of the relaxators.

Low-temperature α peak of the internal friction in niobium and its relation to the relaxation of kinks in dislocations

Abstract: The influence of impurities on the parameters of the α peak of the internal friction in single-crystal and polycrystalline niobium is investigated; the internal friction is recorded in the temperature range 200–250 K at a vibrational frequency of ≈80 kHz. It is found that increasing the purity of the samples leads to an increase of the peak location temperature and to an increase of its width and amplitude. The structural sensitivity of this peak was observed previously in a study of the influence on this peak of a preliminary plastic deformation, thermocycling, and low-temperature recovery. A statistical description of the whole set of experimental data is proposed, which takes into account the dependence of the activation energy and attempt period on the impurity concentration and on the characteristic value and statistical scatter of the internal stresses. A new algorithm is developed for analyzing the experimental data to obtain empirical estimates for the values of the activation energy, its variance...

Statistical analysis of the low-temperature α peak of the internal friction in iron single crystals

Abstract: The effect of changes in the ultrasonic frequency or in the dislocation structure of the samples on the parameters of the low-temperature α peak of the internal friction and the Young’s modulus defect corresponding to it is investigated experimentally in single crystals of pure iron. A statistical interpretation of the temperature dependence and structural sensitivity of the characteristics of the dynamic relaxation in the neighborhood of the α peak is proposed, based on the assumption of a random scatter of the values of the activation energy of the elementary relaxators. Empirical estimates are obtained for the attempt period and for the mean value and variance of the activation energy of the relaxation process responsible for the α peak of the internal friction in iron.

Ultrasonic and related experiments in high-Tc superconductors

Abstract: The results of ultrasonic, sonic and infrasonic experiments in high-Tc superconductors (HTSC) are reviewed. Some other related fields of investigation are also mentioned.

Elastic behavior of a YBaCuO sample prepared by the MPMG method

Abstract: Ultrasonic velocity measurements have been undertaken by the pulse transmission technique using YBaCuO samples prepared by the melt-powder-melt-growth (MPMG) process over a temperature range of 80–300 K. It has been observed that the sample exhibits a very high value of Young's and rigidity moduli, thereby indicating that the material is an excellent one from the point of view of its mechanical strength. Further, the Young's modulus apart from exhibiting a hump at around 120 K, is found to increase continuously with decreasing temperature. In the superconducting phase, however, the modulus is found to remain almost constant. It has been concluded that the observed elastic behavior, to a large extent, is analogous to that of normal oxide materials. In order to understand how far the experimentally observed behavior deviates at 120 K, the elastic moduli of the sample were also estimated theoretically and compared with the empirical values.

Unusual Young׳s modulus behavior in ultrafine-grained and microcrystalline copper wires caused by texture changes during processing and annealing

Abstract: Effect of annealing on the dynamic Young׳s modulus E of ultrafine-grained (UFG) and microcrystalline (MC) copper obtained by combined severe plastic deformation (SPD) including repeated hydrostatic extrusion and drawing (UFG copper) or only repeated drawing (MC copper) is investigated. It is established that the Young׳s modulus in the SPD-prepared UFG and MC samples exceeds that in the coarse-grained fully annealed (CGFA) samples by 10% to 20%. Subsequent isothermal annealing at elevated temperatures between 90 and 470 °С leads to a sharp decrease of the Young׳s modulus for annealing temperatures above 210 °С. After annealing at 410 °С, the value of E reaches its minimal value that is 35% lower than E in CGFA samples (total change in E is about 50% of the initial value). Further annealing at higher temperatures leads to some increase in the Young׳s modulus. It is shown that the unusual behavior of the Young׳s modulus is caused by the formation of the 〈111〉 axial drawing texture in the SPD-treated samples which is replaced by the 〈001〉 annealing texture during the post-SPD heat treatments.

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

Abstract: 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, impurities, twin and grain boundaries) and macrodefects (voids, cracks, heterophase inclusions) are noted, and the role of heavy-cation diffusion in the kinetics of high-temperature deformation is discussed. The influence of structural phase transformations and the superconducting transition on the mechanical properties of metal oxides is considered. This review is a continuation of a review of the elastic and acoustic properties of HTSCs published earlier by the authors in Fiz. Nizk. Temp. 21, 475 (1995) [Low Temp. Phys. 21, 367 (1995)].

Cathodoluminescence and microRaman analysis of oxygen loss in electron irradiated YBa2Cu3O7−x

Abstract: The effect of oxygen loss on the luminescence of YBa_2Cu_3O_(7-x) has been investigated by cathodoluminescence (CL) in the scanning electron microscope (SEM) and by Raman microprobe measurements. The results herein indicate that a CL band at 530 nm is related to oxygen loss rather than to impurity phases such as Y_2O_3 formed in the material by electron irradiation in the SEM.