Papers by Author: Gennady A. Salishchev

Abstract: Characteristics of mechanical behavior during superplastic flow and associated microstructural evolution in the wrought AlCoCrCuFeNi high-entropy alloy were studied. The alloy had complex microstructure with fine grain/particle size of ≈2.1 μm. 4 different phases with volume fractions from 7% to 46% and different deformation characteristics were found in the alloy. Very high tensile elongations of up to 1240% were observed during deformation at temperatures of 800°C–1000°C and at strain rates of 10^-4 s^-1–10^-1 s^-1 despite presence of pronounced softening stage followed by steady state flow stage. Microstructure of the alloy after tensile testing was studied in detail. Phase transformations were analyzed employing thermodynamic modeling and their role in strain accommodation is discussed.

Abstract: Microstructure evolution and mechanical behavior of alpha/beta Ti-6Al-4V titanium alloy with initial α-colony microstructure during uniaxial compression at 600 and 800°C to a height strain of 70% were studied. It was shown that decrease in deformation temperature considerably influences on the kinetics of globularization of a lamellar microstructure. At the lower temperature stages of strengthening and softening extend that associates with inhibition of globularization. Deformation at 600°C is also associated with a smaller fraction of high-angle boundaries during deformation, smaller fraction of globular grains, increased contribution of shear deformation and more intensive rotation of α-lamellae towards the metal flow direction. In contrast to 800°C, the rate of thinning of α-lamellae at the lower temperature is noticeably higher. The results obtained are related to the change of the type of dislocation slip in α-lamellae due to inhibition of dynamic recovery with decreasing deformation temperature.

Abstract: The mechanical properties of two-phase Ti-6Al-4V titanium alloy with ultrafine grained microstructure were studied in the present work. Bulk ultrafine grained specimens of the alloy were produced by means of warm “abc” deformation. The final structure consisted of α/β particles with a size of 500 nm. Extensive studies of the mechanical properties of this material in comparison with conventionally heat-strengthened condition were conducted. A room-temperature strength and fatigue resistance of the ultrafine grained material was found to be 25-40% higher than that of heat-strengthened alloy. However such ductility related properties as tensile elongation and impact toughness noticeably decreased with decreasing grain size. Efficacy of ductility improvement and the strength/ductility balance optimization were analyzed.

Abstract: Mechanical behavior and microstructure evolution of commercial pure titanium during successive compressions of samples along three orthogonal directions (or so-called “abc” deformation) at 400°C and strain rate 10-3s-1 were studied. The cumulative S- curve demonstrates a steady state flow stage following the intensive strengthening. The microstructure evolution of titanium during first increments of “abc” deformation is associated with twinning and shear deformation. Further deformation results in microstructure refinement due to transformation of coincidence site lattice twin boundaries to high-angle arbitrary ones and formation of high-angle deformation induced boundaries. Another mechanism of new grains formation is continuous dynamic recrystallization.

Abstract: Commercial purity copper was subjected to ECAP and subsequent cold rolling. Structure and mechanical properties were studied using EBSD analysis, TEM and tensile tests. Effect of ECAP number passes on grain size and fraction of high angle boundaries after cold rolling was investigated. Rolling results in grain refinement and HABs fraction increase the more ECAP number passes. UTS increases significantly after rolling. Increase of strength is accompanied by loss of plasticity. Evolution of microstructure and mechanical properties is discussed.

Abstract: Evolution of micro- and macrostructure and mechanical properties of oxygen-free copper after MAF at room temperature was studied. MAF included sequential upsetting and drawing with total cycles number equal to 20 and maximum strain ≈50. MAF causes the formation of homogenous UFG structure with a grain/subgrain size of 0.3 m and fraction of high angle boundaries 50%, but macrostructure is heterogeneous. Rough shear macrobands areas of different orientation are observed. MAF results in significant strengthening from 280 MPa to 445 MPa, but samples remain very ductile even after large strains. Mechanisms of UFG structure formations during MAF are discussed.

Abstract: A comparative investigation of mechanical properties of Ti–6Al–4V titanium alloy with coarse-grained (400 m), microcrystalline (10 µm) and submicrocrystalline (0.4 µm) structures in the temperature range 20–500°C has been carried out. The submicrocrystalline structure was obtained by multiaxial isothermal forging. The alloys with the coarse-grained and microcrystalline structures were used in a heat-strengthened condition. The microstructure refinement increases both the strength and fatigue limit of the alloy at room temperature by about 20%. The strength of the submicrocrystalline alloy is higher than that of the microcrystalline alloy in the range 20 - 400°C. Long-term strength of the submicrocrystalline specimens below 300°C is also considerably higher than that of the other conditions. However, the creep strength of the submicrocrystalline alloy is slightly lower than that of the heat-strengthened microcrystalline alloy already at 250°C. The impact toughness in submicrocrystalline state is lower especially in the samples with introduced cracks. Additional surface modification of submicrocrystalline alloy by ion implantation gives a considerable increase in the fatigue limit. Advantages of practical application of submicrocrystalline titanium alloys produced by multiaxial isothermal forging have been evaluated.

Abstract: The method for production of a structure with a grain size of 30-40 nm in two-phase titanium alloys is proposed. It is shown, that the nanostructure can be formed in billets of 150×70×15 mm, and sheets of 250×150×1 mm. The method consists of several steps including hydrogen alloying of the alloy, heat treatment, warm deformation and finally dehydrogenating vacuum annealing. α-, α+β and β-titanium alloys have been investigated. Hydrogen content varied in the range 0.1– 30 at. %. Microstructure was examined using optical, scanning, transmission electron microscopy and X-ray analysis after every step of the treatment. The investigations have shown that a specific character of phase transformations in hydrogenated titanium alloys plays a leading role in formation of nanostructure. The effect of dissolved hydrogen on dynamic recrystallization in α- and β- phases is of a secondary importance. Additional refinement in structure is observed in the deformed alloys after vacuum annealing, if its temperature is less than the temperature of their deformation. The work was focused on the optimization of hydrogen content and deformation conditions with the aim to create the nanostructure in titanium alloys and to enhance their mechanical properties.

Abstract: The structure and properties of oxygen-free copper (99,98%) were studied after different types of severe plastic deformation (SPD): equal-channel angular pressing (ECAP), multiaxial deformation (MD), and accumulative roll bonding (ARB) as a function of the strain at room temperature (to a true strain of 30-40). The SPD facilitates the formation of submicrocrystalline structure with a grain size of 200-250 nm and predominantly high angle boundaries (83-94%). ECA pressing leads to the formation of the most uniform submicrocrystalline structure.The strength characteristics increase with increasing strain and reach the steady stage at ε ≈ 5. At the steady stage, UTS = 460-480 MPa at ARB, and MD, while UTS at ECAP is somewhat lower, 430-440 MPa. The smallest "steady" values EL = 4 - 5% were obtained in the case of ARB, and the maximum EL = 18% was obtained at MD.

Abstract: A novel approach to fabrication of globularized fine-grained structure in γ+α2 titanium aluminide alloys has been proposed. The approach included the use of a specially designed alloy Ti-43Al-X(Nb,Mo,B) and heat treatment. It was found that the ingot structure of the alloy might be partially globularized on a scale of bulk material using only globularization anneal excluding any hot working procedure. The microstructure and tensile mechanical properties of the alloy in the cast + heat treated condition were investigated. The tensile mechanical tests were performed in air in the temperature range of T=900-1130°C at an initial strain rate of ε′=1.7×10-4 s-1. High elongation (δ=160-230%) and low flow stresses (σ=36-100 MPa) typical of superplastic behavior were measured at T=1050-1130°C. It was demonstrated that the sheet material produced by spark cutting of the cast + heat treated alloy might be successfully hot formed.