This paper investigates the effect of thermomechanical treatments, including deformation by radial shear rolling or severe plastic deformation by abc pressing with subsequent aging at 773 K, on the structural-phase state, deformation behavior and mechanical properties of commercial near β titanium alloy VT22 (Ti-5Al-5Mo-5V-1Cr-1Fe). The structure of the alloy after radial shear rolling and subsequent aging consists of transformed β grains with a lamellar α + β structure and primary α-phase particles. Severe plastic deformation of the alloy followed by aging causes the formation of a grain-subgrain α + β structure with an average characteristic size of 0.23 µm. It is found that after the thermomechanical treatments, the strength characteristics of the alloy at room temperature increase by ~40% compared to the as-received alloy. The alloy after radial shear rolling and aging retains a 40-20% higher strength in the temperature range of 293-823 K. The strength of the alloy after severe plastic deformation and aging becomes lower than that of the as-received alloy already at a temperature of 773 K. Analysis of creep parameters at 743 K shows that the creep deformation of the alloy in the state after radial shear rolling and aging occurs by the motion of dislocations (glide + climb). The creep deformation of the alloy in the state after severe plastic deformation and subsequent aging is largely contributed to by grain boundary sliding.

The structural-phase state of an ultrafine-grained Ti-55511 alloy produced by radial shear rolling and subsequent aging was investigated after very high cycle fatigue (VHCF) tests with stress amplitudes close to the fatigue limit. The study revealed that a discontinuity region is formed in the central part of the specimen during VHCF testing at a stress amplitude below the fatigue limit (sigma(a) = 580 MPa) and a number of cycles without failure of 8.79 & sdot; 10(9). The size of the region (30-40 mu m) is in good agreement with the size of the fatigue crack initiation site. The selected VHCF conditions cause structural-phase transformations in the alloy, leading to a 52-75 % increase in the volume fraction of the alpha phase. This occurs with the accumulation of deformation defects and diffusioncontrolled redistribution of alloying elements in local regions at the particle/matrix interface, due to which the interfacial energy increases while the fatigue microcracking resistance of the interface decreases. A higher stress amplitude above the fatigue limit (sigma(a) = 660 MPa) intensifies the above processes, leading to internal microcracking and fatigue failure of the alloy at a much smaller number of cycles (6.(7) & sdot; 10(8)).

Studies have been carried out of the influence of annealing in the temperature range of 723–873 K on the evolution of the structural-phase state and mechanical properties of the VT22 titanium alloy depending on the method of its pretreatment (by all-round pressing and radial-shear rolling (RSR)). It has been shown that annealing of the alloy after all-round pressing leads to the development of phase transformations, formation of new grains with sizes less than 0.1 µm, slight grain growth, decrease in the degree of nonequilibrium grain boundaries, and recrystallization development. The preferential occurrence of certain processes, depending on the annealing temperature, causes a change in the mechanical properties of the alloy compared to the state after all-round pressing. It has been established that the RSR treatment of the VT22 alloy leads to a change in the nature of its structural-phase state evolution during annealing. The increase in the mechanical properties of the alloy in the case is due to the decomposition of the metastable β-phase with the formation of a fine-needle martensitic α phase and solid-solution strengthening. The coarsening of martensitic phase with increasing annealing temperature leads to a decrease in the mechanical properties of the alloy and an increase in the ductility.

Comparative studies are conducted on the structure and mechanical properties of the ultrafine-grained Ti-5Al-5V-5Mo-1Cr-1Fe alloy obtained by abc pressing and radial shear rolling with subsequent aging. It is shown that the ultrafine-grained structure formed by these methods provides increased strength properties under both tension and three-point bending compared to the initial coarse-grained state. At the same time, the alloy obtained by abc pressing demonstrates a higher fracture resistance during three-point bending compared to the alloy obtained by radial shear rolling + aging due to its enhanced ductility. This also determines the ductile fracture pattern of the ultrafine-grained alloy obtained by abc pressing during three-point bending in contrast to ductile-brittle fracture of the alloy obtained by radial shear rolling + aging.

Relevance. Titanium and its alloys are the most durable and corrosion-resistant metal materials that makes them widely used in mechani-cal engineering, aircraft and machinery, chemical apparatus construction, medicine. The use of such construction materials with increased resistance in aggressive environments makes it possible to increase the efficiency of technologies for processing natural raw materials (high-pressure reactors, centrifuges, separators, high-speed pumps, heat exchangers, communications), the production of chlorine and al-kalis (evaporators, electrolysis equipment), organic synthesis (equipment for the production of halogen derivatives), the production of nitric acid, ammonia and nitrogen fertilizers. The combination of mechanical and thermal treatment of aloys, including the use of severe plastic deformation, allows modifying their structure and obtaining materials with improved physical and mechanical characteristics. Purpose: to determine the effect of various modes of the deformation and heat treatment of titanium alloys VT6 and VT22 on their corrosion resistance in aqueous solutions of acids, alkalis and salts. Objects: samples of VT6 and VT22 alloys with different structures, aqueous solutions of acids, alkalis and salts. Methods: DC voltammetry, gravimetry under conditions of free corrosion, optical microscopy, scanning electron microscopy, X-ray diffrac-tometry, atomic emission spectroscopy. Results. Under conditions of severe plastic deformation, samples of titanium alloys VT6 (triaxial pressing in the temperature range from 550 to 800 degrees C) and VT22 (radial shear rolling in the temperature range from 750 to 850 degrees C, additional cold rolling of hot-rolled samples, ag-ing at 550 degrees C for 3...6 hours) with ultrafine-grained structure have been produced. It was found that VT6 alloy with ultrafine-grained struc-ture has less corrosion resistance under conditions of free corrosion in solutions of sulfuric acid of a high concentration (5 M), as well as in 3,5 % NaCl solution at anodic polarization compared to the initial alloy with coarse-grained structure. The main types of corrosion damage to the surface of the ultrafine-grained samples were ulcers and pitting, whereas for samples with a coarse-grained structure, continuous corrosion was characteristic. The increase in the rate of corrosion and morphological features of the destruction of the surface are ex-plained by structural-phase heterogeneity and segregation of alloying elements as a result of the deformation and thermal effects. The cor-rosion resistance of VT22 samples with an ultrafine-grained structure correlates with the content of the /3-phase and with the features of the interphase distribution of alloying elements resulting from the polymorphic transformation of alpha-Ti reversible arrow beta-Ti under conditions of the defor-mation and heat treatment. It is shown that VT22 samples after hot rolling are the most stable in acid solutions (0,1 M HCl, H2SO4) and the least stable in alkali solution (0,1 M NaOH) compared to samples treated under other regimes. In a 3,5 % NaCl solution, the ultrafine-grained sample VT22 showed the least resistance after additional cold rolling due to a large concentration of structural defects that con-tribute to increased corrosion cracking of the material. The interpretation of the experimental results was carried out using theoretical cal-culations of the equilibrium composition of corrosion products formed in the multicomponent systems under study.

The influence of low-temperature superplastic deformation on the structural-phase state and mechanical properties of ultrafine-grained titanium alloy VT22 has been studied at room temperature. It is shown that the tensile strain of alloy samples has an insignificant effect on their mechanical properties at strain rates of 2·10 –3 and 6.9·1 –3 s –1 and a temperature of 823 K. It has been established that this effect is due to the preservation of the ultra fine grade (UFG) structural-phase state of the alloy during superplastic deformation formed as a result of its processing by the method of all-round pressing. An increase in the deformation temperature to 873 K at the same strain rates leads to a decrease in the mechanical properties of the alloy by about 6–8%.

Studies of the low-temperature superplasticity (SP) of ultrafine-grained (UFG) near beta alloy Ti-5Al-5V-5Mo1Cr-1Fe at a temperature of 823 K (similar to 0.42 Tm) in the range of strain rates (2.0-6.9).10(-3) s(-1) have been carried out. It is shown that for an UFG alloy with an average grain size of d similar to 0.17 mu m, obtained by the method of multi-axial pressing, the value of the relative elongation to failure exceeds 950% at a strain rate of 2.10(-3) s(-1). Annealing of the UFG alloy at a temperature of 873 K for 1 h does not cause a noticeable increase in the average grain size (d similar to 0.23 mu m), but leads to the transition of a part of grain boundaries to a more equilibrium state and a decrease in elongation to failure by more than 3 times in the range of strain rates. The study of the evolution of the microstructure under tension at a rate of 2.10(-3) s(-1) showed that in both states of the alloy up to an elongation of 150%, the average size of the elements of the grain-subgrain structure practically does not change, which is due, among other things, to the appearance of new grains less than 100 nm in size during the SP deformation. It is shown that the significantly lower elongation to failure of the UFG Ti-5Al-5V-5Mo-1Cr-1Fe alloy subjected to annealing may be due to the hindered accommodation of grain boundary sliding during the SP deformation by intragranular dislocation slip because of the transition of individual grain boundaries to an equilibrium state. The preservation of the UFG structure of the alloy with high strength characteristics under conditions of low-temperature superplasticity creates good prerequisites for obtaining high-strength products of complex shape in the regime of SP forming. (C) 2021 Elsevier B.V. All rights reserved.

The effect of severe plastic deformation by multiple pressing and subsequent annealing on the structure and mechanical properties of VT35 titanium alloy is studied. It is shown that the formation of ultrafine-grained state results in a 40–60% increase of its room-temperature mechanical properties compared to the initial coarse-grained state. A subsequent annealing of the ultrafine-grained VT35 alloy at 773 K improves homogeneity of the structure formed by multiple pressing; the average size of the grain-subgrain structure elements does not increase and there is no precipitation of additional fine α-phase particles in the bulk of β-grains. These structural changes are expected to improve room-temperature mechanical properties of the alloy after its annealing at 773 K. A further increase in the annealing temperature to 873 K is followed by recrystallization, an increase in the average size of the grain-subgrain structure elements and, as a consequence, a degradation of the mechanical properties. It is found out that an increase in the total imposed strain degree within the temperature range of 773–823 K during multiple pressing of VT35 alloy provides an additional 65–70% improvement of its room-temperature properties compared to the initial coarse-grained state.

The features of the development of superplastic deformation and the evolution of structural-phase states of ultrafine-grained VT22 and VT35 alloys under tension in the temperature range of 823−973 K and strain rate 6.9·10−3 s−1 are studied. It is shown that for the VT35 alloy, the structural-phase state evolution under tension at the temperatures of 873 and 973 K has a qualitatively different character. After deformation at 873 K, a stable microduplex α-β structure with an average grain-subgrain size of about 0.2 μm is formed in the alloy. In this case, the main mechanism of deformation under the conditions of the experiment is grain boundary sliding. At the same time, after deformation of the VT35 alloy at 973 K, there is a significant increase in the grain size with an increasing strain degree and an increase in the β-phase volume fraction to 95%. In this case, the α-phase is observed mainly in the form of individual particles along the grain boundaries of the β-phase. It is found out that the structural state evolution of the VT22 alloy at the deformation temperature of 973 K is qualitatively similar to that of the structural state of the VT35 alloy at 873 K. Based on a comparative analysis of the structural-phase state evolution during the deformation of VT22 and VT35 alloys at 973 K, it is hypothesized that the relatively low ductility of the VT35 alloy samples under the conditions is due to the rapid grain growth, the alloy transition to an nearly single-phase state, which results in the hindered development of grain boundary sliding.

The effect of aging at 723 K on the structural-phase state and mechanical properties of VT35 titanium alloy after beta quenching and various treatments using severe plastic deformation has been studied. It is shown that annealing at 1073 K followed by quenching and aging leading to the precipitation of large alpha-phase lamellae with volume fraction of 20 % doesn't have a strengthening effect on the alloy. After processing by radial shear rolling and subsequent aging fine alpha-phase precipitations in the form of plates and nanometer-sized particles with a volume fraction of 24 % are observed in beta-phase grains. However, large areas of beta-phase without alpha-phase precipitates are observed. The strength properties of the alloy after the treatment increase by 40 - 45 %. The processing by the method of multiple pressing leads to the formation of a mixed alpha / beta UFG structure with a grain / subgrain size of 0.11 mu m. Subsequent aging leads to the decomposition of the residual beta-phase and to a homogeneous distribution of alpha and beta phases in the alloy with volume fraction of alpha-phase 51 %. The formation of such an UFG structural -phase state leads to an increase in the strength properties of the alloy by a factor of almost two compared to the initial state.

The fatigue and fracture behavior of an ultrafine-grained near ? Ti-5Al-5V-5Mo-1Cr-1Fe alloy produced by radial shear rolling and subsequent aging was investigated by very high cycle fatigue (VHCF) testing with the stress ratio R = -1. It was shown that the formed structure contributes to a high fatigue strength of the alloy in tests with a stress amplitude of 850 MPa and the number of cycles to failure 2.78 ? 107. Fractographic analysis revealed an internal mode of fatigue crack development with ?K values higher than 27 MPa m1/2. The fatigue crack initiation region was an area about 25 ?m in diameter showing the presence of up to 10 ?m long microcracks and micron-sized microvoids. The fatigue microcracks initiated at the interfaces between the primary ? (?") phase particles and the transformed ? matrix. A fatigue fracture model of the ultrafine-grained titanium alloy under VHCF loading was proposed which agrees well with the obtained experimental data.

The development of plastic deformation in an ultrafine-grained titanium alloy (Ti-Al-V-Mo; hereinafter VT16 alloy) under tension with a rate of 6.9.10(-3) s(-1) in the temperature range of 293-973 K is investigated depending on the structure and phase composition of the alloy. It has been established that the deviation of the phase composition from the equilibrium one toward the increasing beta phase enhances the resistance of the UFG VT16 alloy to the localization of plastic deformation during tension at low temperatures (293-673 K). Under tension of the UFG VT16 alloy in the temperature range of the superplastic flow (823-973 K), the indicated deviation of the phase composition from the equilibrium one contributes to a greater elongation to failure.

The α+β titanium alloy Ti-6Al-4V having different structure and phase composition is investigated in this paper under the tensile deformation in the temperature range from 773 to 1223 K. The transmission and scanning electron microscopy observations show that superplastic deformation of the Ti-6Al-4V alloy with the ultra-fine grain structure produces the formation of the secondary phase particles along the grain boundaries. It is identified that this process is connected with the vanadium redistribution during deformation. The obtained temperature dependence of the fracture elongation of the alloy samples is nonmonotonic and exceeds 700% at 973 K. It is supposed that the reduction in the fracture elongation within 1073–1173 K range is associated with the deformation-induced micron-sized grains and the formation of lamellar β- and α″-phases along the grain boundaries.

In the paper, we study the evolution of the structural phase state, deformation behavior, and fracture of ultrafine-grained near-β titanium alloy Ti–5Al–5Mo–5V–1Cr–1Fe after annealing in the temperature range 773–1073 K (0.4–0.55 Tm). The duration of the strain-hardening stage under tensile conditions is shown to be almost independent of the annealing temperature. The character of the strain-softening stage is largely determined by the alloy structure formed after annealing. It is found that annealing at the temperatures 773 and 873 K does not change the deformation behavior of the ultrafine-grained alloy under tension at room temperature. Deformation and fracture of the specimens localize in shear bands. Recrystallization occurring at the annealing temperature 973 and 1073 K leads to the transition of grain boundaries to a more equilibrium state and to a sharp decrease in strength and softening rate of the titanium alloy. It also affects the neck formation prior to fracture, giving a developed neck. Fracture surfaces are indicative of ductile dimple fracture of the alloy in all states. The dimple size depends on the size of structural elements after heat treatments. Based on the experimental data, the σ0 value and Hall–Petch coefficient k are determined. In the investigated grain size range (0.17–1.25 μm) the values are found to be, respectively, 680 MPa and 0.36 MPa m1/2.

This study explores the superplastic behavior of ultrafine-grained Ti-5Al-5V-5Mo-1Cr-1Fe alloy in the temperature range 773-1023 K depending on its structural-phase state after severe plastic deformation and additional 1-h annealing at temperatures 773, 873, and 973 K. It is shown that the formation of a more equilibrium structural-phase state after the annealing treatments significantly reduces the elongation to failure and causes a shift in the superplasticity temperature to higher deformation temperatures (-100 degrees after 1-h annealing at 973 K). A possible reason for the high elongation to failure (over 1500%) during superplastic deformation of ultrafinegrained Ti-5Al-5V-5Mo-1Cr-1Fe alloy is thought to be the formation of a deformation-induced grain structure which is a mixture of small (<1 mu m) and large grains, wherein small grains form interlayers between large grains. The structure evolves in this way at least until 500% strain due to the continuous formation of new less than 1 mu m grains. After preliminary annealing of the ultrafine-grained alloy at 973 K under similar superplastic deformation conditions, the fine-grained interlayers do not form. This factor, along with the annealing-induced changes in the state of grain boundaries and density of deformation defects in the grain bulk, is assumed to be the cause of ductility reduction for Ti-5Al-5V-5Mo-1Cr-1Fe alloy under the considered conditions.

The effect of oxide (alpha-case) layer on room temperature deformation behavior of near beta titanium alloy Ti-5Al-5Mo-5V-1Cr-1Fe produced by radial-shear rolling with subsequent aging was studied. Tensile tests revealed the simultaneous decrease in ultimate strength (from 1500 to 1460 MPa) and ductility (from 6.5 to 3.3%) of specimen with oxide layer formed by aging in air at 550 degrees C. The study of the deformation relief and the character of the fracture revealed that the reason for the decrease in ductility of alloy samples with the oxide layer in tension is the development of surface microcracks which thickness and depth increases with strain. The spread of the microcracks into a depth exceeding the thickness of the alpha-case layer leads to the development of brittle fracture of oxidized specimen.

The effect of mechanical-thermal treatment using the radial shear rolling method on the structural-phase state and mechanical properties of the near beta titanium alloy was studied by using the methods of scanning electron microscopy and X-ray diffraction analysis. It was shown that this treatment makes it possible to decrease grain structure in the alloy. The thermal stability of the obtained structure was investigated. It was shown that the grain size does not change up to the annealing temperature of 750 degrees C. Subsequent aging at 420-500 degrees C leads to the formation of a hierarchically organized structure and allows a significant (by more than 40%) increase in the ultimate strength of the rolled alloy from 860 to 1200 MPa with satisfactory plasticity (similar to 7%).