Papers by Author: Igor S. Golovin

Abstract: We report on experimental investigations of a Ni_2.36Mn_0.64Ga Heusler alloy, which transforms to tetragonal martensite at cooling below M_s ≈ 271°С. The evolution of lattice constants was tracked by in situ neutron diffraction measurements. It was found that the martensite tetragonality c/a gradually decreases during heating from room temperature to austenite transition start temperature A_s ≈ 272°С. The phenomenon of martensite stabilization was investigated by differential scanning calorimetry utilizing three different protocols of the martensite aging. It was found that the martensite aging at a constant temperature T = 255°С merely shifts the reverse transformation to higher temperatures, while the reverse transformation temperature interval (A_f – A_s) remains the same (≈ 30°C) independently of aging time. On the other hand, a multistep aging at different temperatures starting from T = 255°С not only shifts the reverse transformation temperature, but makes the transformation temperature interval narrower down to A_s – A_f ≈ 10°C.

Abstract: Magnetron sputtering with a chromium-containing Fe-19at.%Cr alloy is used to improve the corrosion resistance of Fe-20at.%Ga alloy. The structure of the 2 μm coated layer and distribution of the elements (Fe, Cr, and Ga) are investigated. The bcc phase (A2 structure) is observed in the sputtered sample by XRD analysis. The corrosion resistance in 3.5%NaCl solution increases 14 times in the sample with 2 μm Fe-Cr coated layer. At the same time, the magnetron sputtering leads to a 10% decrease in magnetostriction and a 20% decrease in damping. This difference is explained by schemes of loading during magnetostriction and damping tests.

Abstract: Diffusion couple technique is an efficient tool for the estimating the chemical diffusion coefficients. Typical experimental uncertainties of the composition profile measurements complicate a correct determination of the interdiffusion coefficients via the standard Boltzmann-Matano, Sauer-Freise or the den Broeder methods, especially for systems with a strong compositional dependence of the interdiffusion coefficient. A new approach for reliable fitting of the experimental profiles with an improved behavior at both ends of the diffusion couple is proposed and tested against the experimental data on chemical diffusion in the system Fe-Ga. An extension of the approach for reliable description of the up-hill diffusion phenomenon in multicomponent systems is presented.

Abstract: The microstructure of Al – Cu – Mg – Fe – Ni alloys with Mn and Zr additions was analyzed by optical and scanning electron microscopy, internal friction, X-ray and calorimetric analysis in order to optimize technology of superplastic alloy preparation. It is shown that the S (Al₂CuMg) phase precipitates during hot rolling and dissolves during annealing. This allows to create fine-grained recrystallized structure and to achieve elongation of 320 % at the strain rate of 1×10^-3 s^-1 during superplasticity testing. It is shown that annealing in saltpeter before superplastic deformation improves the superplastic behavior: at the constant strain rate of 4×10^-3 s^-1 elongation is 500 %.

Abstract: A detailed study of anelastic effects in submicrocrystalline copper using resonance (~70 kHz, 2 K to 320 K) and sub-resonance (0.05-100 Hz, 300 K to 675 K) techniques was carried out. Several relaxation processes were found in the temperature range of 2 K - 675 K: the relaxation loss peaks (Q^-1) near 35 (P1) and 90K (P2) with the activation energy and the pre-exponential factor (H₁ ≈ 0.02 eV, τ_ο1 ≈ 10^-9 s and H₂ ≈ 0.09 eV, τ_ο2 ≈ 10^-11 s) similar to those of the Bordoni and the Niblett-Wilks peaks in coarse-grained Cu. This suggests that the peaks are due to the thermally activated motion of dislocation kinks in the primary and secondary Peierls relief. The mean values of activation parameters (H₃ ≈1.4-1.6 eV, τ_ο3 ≈10^-17 s) of a third thermally activated peak (P3), which was significantly broadened, can be interpreted as a grain boundary peak with uncoupled activation parameters H₃^*≈0.45 eV and τ_ο3^* ≈10^-14 s. A pseudo peak P_R is associated with irreversible recrystallization processes. The influence of annealing on the observed effects is also discussed.

Abstract: Temperature and amplitude dependent internal friction (TDIF and ADIF) in ultrafine-grained copper (99.95% Cu) specimens processed by equal channel angular extrusion by route BC in 1, 4, and 8 passes and then subjected to annealing is investigated by means of dynamical mechanical analyzer DMA Q800 in the temperature range from -100 to 550 °C, amplitude range from 10-6 to 10-3, and frequency range from 0.05 to 100 Hz. Two IF peaks were registered and explained by structural relaxation due to the recrystallisation process and by thermally activated grain boundary relaxation with broad distribution of relaxation times. Increase in amplitude dependent damping in ultrafine-grained copper is due to dislocation but not grain boundary contribution.

Abstract: Temperature and amplitude dependent internal friction (TDIF and ADIF) in ultrafine-grained copper (99.95% Cu) processed by 1, 4, or 8 passes of equal channel angular pressing (route BC) and then subjected to annealing was investigated by means of mechanical spectroscopy. A dynamical mechanical analyzer DMA Q800 was used. The tests covered the temperature range from -100 to 550 °C, the strain amplitude range from 10-6 to 10-3, and the frequency range from 0.05 to 100 Hz. Two internal friction peaks were observed. They were explained by structural relaxation due to the recrystallisation process and by thermally activated grain boundary relaxation with a broad distribution of relaxation times. Increased amplitude dependent damping in ultrafine-grained copper is believed to be associated with a dislocation mechanism, rather than a grain boundary mechanism.

Abstract: The simultaneous influence of both thermal and mechanical treatment was applied to produce a geometrically complex shaft from 51CrV4 steel. This special treatment led to the formation of adjacent microstructures which were significantly different from each other. It was found that these microstructural changes were accompanied by a change of mechanical properties in terms of hardness, electrical resistivity and especially internal friction. Specimens for structural studies and study of mechanical and physical properties were taken out from different places of the produced shaft and tested in order to verify and understand the obtained gradation. The significant variations in properties could be explained in terms of structure and dislocation behaviour under applied cyclic stress using mechanical spectroscopy technique. The cold-work (Snoek-Köster) peak was recognised and analysed in the structure of this steel.

Abstract: Study of anelasticity in AZ31 magnesium-based alloy in initial (hot-rolled at 370 оС) condition and after one and four passes of equal channel angular pressing is carried out. An internal friction peak was found at 170 °C at a frequency f of about 1 Hz. Two possible mechanisms of this peak are discussed: it is suggested that the peak origin is the grain boundary relaxation.

Abstract: The influence of heat treatment on the amplitude dependence of internal friction in Fe - 11 at. % Al alloys with carbon contents in the range 0.005 - 0.2 at. % has been studied using an inverted torsion pendulum in the temperature range 300 – 950 K and a vibrating reed apparatus at room temperature. The specimens were annealed at 1273 K in vacuum and cooled down with different cooling rates in order to obtain different degrees of order. It was found that ordering is hardly avoidable in Fe - Al alloys with Al contents > 11 at. %. Ordered alloys are characterised by lower damping capacity due to higher coercivity caused by additional pinning of magnetic domain walls by antiphase boundaries. X-ray diffraction investigations indicate that water-cooling suppresses ordering in Fe - 11 at. % Al alloys while cooling in air or in furnace provokes D03–type ordering. Slowly cooled specimens are characterised by higher damping capacity due to lower coercivity than water cooled or plastically deformed specimens. The amplitude dependent magneto-mechanical damping was determined as the difference between amplitude dependent damping without and with saturating magnetic field (~ 20 kA/m). Magneto-mechanical damping was found to be proportional to the strain where the amplitude dependent damping is maximum and reciprocal to the coercivity and saturation polarisation. Cold rolling increases the coercivity and therefore decreases the magneto-mechanical damping. An increase of the grain size in the investigated samples by heat treatment leads to a qualitatively expected decrease of coercivity and therefore to an increase of magneto-mechanical damping.