Papers by Author: Yuri Estrin

Abstract: The article presents the results of an investigation of microstructure, mechanical properties and corrosion resistance of magnesium alloy WE43 processed by rotary swaging. The resulting microstructure is characterized by an average size of structural elements of 0.5 – 0.8 μm. The grain refinement leads to an increase in the strength of the alloy to 393 – 416 MPa while the tensile elongation stays at a level of 7 – 12.5%. The microstructure produced by rotary swaging does not lead to deterioration of the resistance of the alloy to electrochemical and chemical corrosion.

Abstract: Mg-30Ca and Mg-14Li (wt %) master alloys were melted successively in the induction furnace to obtain a Mg-Li-Ca ternary alloy containing 3.99 % Li and 1 % Ca. The as-cast material of thickness 4 mm was homogenised at 350° C for 120 mins and subsequently rolled to 62.5 % reduction in thickness at 300 °C to get 1.5 mm thick sheet. The microstructures of hot rolled samples were examined in as-rolled condition as well as after annealing at 350° C for various lengths of time. The presence of deformation twins was clearly seen in the as-rolled structure, whereas equiaxed twin-free grains were observed in the annealed condition. The average grain size was found to increase from 10 μm to 18 μm by annealing, according to the kinetics that follows a parabolic law. Tensile samples taken from rolled plate were deformed to failure at room temperature and a strain rate of 10^-4 s^-1. Ultimate tensile strength of as-rolled material increased to 213 MPa, while tensile elongation dropped to 6.5 % from the initial values of 134 MPa and 8.5 %, respectively. Annealing after rolling offered a good compromise between the enhanced tensile strength (160 MPa) and tensile ductility (9 %) suggesting viability of the proposed thermomechanical treatment as a means for enhancing both strength and ductility of Mg-4Li-1Ca alloy.

Abstract: In the present study, ECAP compaction was used to produce Ti-6Al-4V billet from CP Ti powder mixed with two different Al-V master alloys. It was demonstrated that ECAP at 400 °C with applied back-pressure of about 210 MPa permits consolidation of the powder mixtures to relative green densities as high as 99.3 %. A combination of temperature, high hydrostatic pressure and plastic shear deformation gave rise to excellent green densities and good compositional homogeneity due to enhanced self-diffusion. It was shown that the temperature of sintering required after direct compaction of BE powders can be reduced by 150-250°C if compaction is conducted by ECAP with back-pressure. This becomes possible due to high green density and the presence of a large number of fast diffusion paths associated with dislocations and grain boundaries. It is suggested that the ECAP-based processing route may offer a significant saving of production cost.

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) 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: Densification of metallic powders by means of extrusion is regarded as a very attractive processing technique that allows obtaining a high level of relative density of the compact. However, the uniformity of the relative density depends on that of strain distribution and on the processing parameters. Several variants of extrusion can be used for compaction of metal particulates, including the conventional extrusion (CE) and equal channel angular pressing (ECAP), often referred to as equal-channel angular extrusion. Each of these processes has certain advantages and drawbacks with respect to compaction. A comparative study of these two extrusion processes influencing the relative density of compacts has been conducted by numerical simulation using commercial finite element software DEFORM2D. The results have been validated by experiments with titanium and magnesium powders and chips.

Abstract: Among the known severe plastic deformation (SPD) techniques, there is a special group of processes involving friction-induced shear. One of the sample or work-piece dimensions used in such processes, namely the thickness, is much smaller than the other two dimensions. The well-known process of High Pressure Torsion (HPT) and the relatively new Cone-Cone (CC) method applied to thin conical samples fall into this category of SPD techniques. Wrought aluminium alloy 2124 was used to study the effect of CC processing on microstructure and mechanical properties. The influence of the processing parameters, including the rotation speed and thickness of the conical strip specimens on the microstructure and the mechanical properties of the CC-processed material was investigated.

Abstract: High Mn steels demonstrate an exceptional combination of high strength and ductility due to their high work hardening rate during deformation. The microstructure evolution and work hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uni-axial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanism : slip, twinning and dynamic strain aging. Constitutive modeling was carried out based on the Kubin-Estrin model, in which the densities of mobile and forest dislocations are coupled in order to account for the continuous immobilization of mobile dislocations during straining. These coupled dislocation densities were also used for simulating the contribution of dynamic strain aging on the flow stress. The model was modified to include the effect of twinning.

Abstract: The effect of equal channel angular pressing (ECAP) on the structure and mechanical properties of Al-4% Mg-1.5% Mn-0.4% Zr and Al-4% Mg-1.5% Mn-0.4% Zr-0.4% Sc alloys in the initial as-cast state was studied. The ECAP processing was shown to lead to the formation of predominantly submicrocrystalline structure with an average grain size of 850 nm in the Al-Mg-Mn-Zr-Sc alloy and 1060 nm in the Al-Mg-Mn-Zr alloy. It is remarkable that both strength and ductility of the two alloys were enhanced by ECAP. The highest strength was observed in the Al–Mg–Mn–Zr–Sc alloy (UTS = 425MPa), in combination with elongation to failure of EL=17 %.

Abstract: Friction stir processing (FSP) combines frictional heating and severe plastic deformation to produce microstructural modification, either locally targeted at the near-surface regions or through the bulk, of metallic components fabricated by conventional processing routes. In this paper, we highlight the capabilities of this process by applying it to a high-pressure die cast Al-Si-Mg-(Cu) alloy and examining the resulting microstructure and mechanical properties.