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The last two decades have witnessed significant progress in the development of severe plastic deformation techniques to produce ultrafine-grained materials with new and superior properties. This review examines works and achievements related to the low-temperature superplasticity of ultrafine-grained aluminum alloys. The examples are provided of the possibility to observe low-temperature superplasticity in aluminum alloys at temperatures less than 0.5 Tmelt and even at room temperature, and herein, we demonstrate the cases of achieving high ductility and high strength in aluminum alloys from processing utilizing severe plastic deformation. Special emphasis is placed on recent studies of the formation of segregations of alloying elements at grain boundaries in UFG Al alloys and their influence on the development of grain boundary sliding and manifestation of low-temperature superplasticity. In addition, the current status and innovative potential of low-temperature superplasticity in aluminum alloys are observed. [ABSTRACT FROM AUTHOR]

One of the most effective ways to increase the processing plasticity of advanced superalloys (heat-resistant nickel-based alloys) is the formation of an ultrafine-grained (UFG) microstructure in bulk semi-finished products. Such a microstructure is a necessary condition for the manifestation of the structural superplasticity effect in the technological processes of manufacturing products from such superalloys. One of the most promising methods for producing UFG microstructures is thermomechanical treatment (TMT) according to the multiple isothermal forging scheme. It has been shown that the EK79 superalloy after TMT, with a gradual decrease in the processing temperature from 0.88 to 0.62 Ts (where Ts is the strengthening phase dissolution temperature) leads to the transformation of the initial microduplex fine-grained microstructure into a mixed UFG microstructure. Such a mixed UFG microstructure consists of: 1) relatively coarse (inherited from the fine-grain microstructure) particles – γ'-phase with a size of 3.0±0.8 μm; 2) γ-grains, and incoherent γ'-phase particles with a size of 0.3–0.5 μm; 3) strengthening coherent intragranular γ'-phase particles with a size of 0.05–0.1 μm, released upon cooling from the TMT temperature to room temperature. During uniaxial compression tests, the EK79 superalloy with such microstructure, demonstrates low-temperature superplasticity in the temperature range of 800–1000 °C. It has been found that an increase in the deformation temperature up to 1000 °C, leads to the increase of γ-phase grains to micron size. The maintenance of superplastic properties in the presence of relatively coarse incoherent particles in the microstructure of the second phase (γ'-phase) is apparently related to the fact that the deformation is localised in the UFG component.

A method for pressure welding of cylindrical workpieces in the solid state of an aluminum–zinc alloy in a vacuum at a temperature of 250°C under conditions of low-temperature superplasticity is developed. Tensile tests showed that the strength of the welding joints is 90–95% of the ultimate tensile strength of the base material. [ABSTRACT FROM AUTHOR]

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%. [ABSTRACT FROM AUTHOR]

Low-temperature superplasticity (LTS) is crucial to reduce manufacturing cost and to enhance the applications of superplastic forming. It is well known that grain refinement is the key point to decrease the temperature at which superplasticity is attained. Therefore, ultrafine-grained (UFG) materials have become attractive for achieving LTS. Severe plastic deformation (SPD) techniques provide abnormal grain refinement, and thus they have been used to achieve LTS in metallic materials. This paper overviews and examines the reports of LTS in the severely-deformed metallic materials. It provides fundamentals of grain refinement via different SPD techniques in various classes of metallic materials including Al-, Mg-, and Ti-based alloys. It also gives a brief summary about the effect of microstructural requirements on LTS with an emphasis on grain size, type and chemical composition of grain boundaries and microstructural alteration during the superplastic deformation. In the last section of the manuscript, the main deformation mechanism of LTS were also explained. [ABSTRACT FROM AUTHOR]

A new approach to solid-state pressure welding of Zn–22% Al aluminum–zinc alloy in vacuum at a temperature of 250°C under conditions of low-temperature superplasticity is proposed. An ultrafine equi-axial structure with an average size of both phases was obtained by annealing in an oven at 350°C followed by quenching in water. This treatment produces stable ultrafine grains of about 0.65 μm with a uniform distribution of Zn and Al grains. When welding high-height billets with pressure, barrel-like properties appear on the side surface of welded materials. It is shown that the sizing of the mold for pressure welding of cylindrical billets makes it possible to influence development of local flow and to provide the most favorable conditions of deformation by optimizing energy consumption for deformation in the zone of the welded joint. As a result of heating and precipitation, a joint superplastic deformation occurs and the main deformation is localized in the region of formation of the solid-phase joint, while most of the blanks are slightly deformed. Oxide films on the surfaces to be joined are broken by extrusion and removed by superplastic deformation in radial directions, and a new oxide-free layer appears on the surface of the workpieces to be welded. The welded area is formed due to the occurrence of metal bonds between the juvenile contacting surfaces of the welded workpieces. Mechanical tensile tests showed that the strength of the solid-phase joints was 90–95% of the ultimate strength of the base material. [ABSTRACT FROM AUTHOR]

The structure and mechanical properties of the titanium alloy VT6 joint diffusion welded in vacuum at 820°C using an ultrafine-grained interlayer made of alloy VT22 in are studied after annealing at 900°C for 2 h. The heating-induced activation of diffusion processes is found to halve the number of micropores in the weld zone and to increase the tensile strength of the welded joint from 975 to 1108 MPa. The impact toughness of this joint after heat treatment corresponds to the initial impact toughness of the VT6 alloy. [ABSTRACT FROM AUTHOR]

Superplasticity is characterized by an elongation to failure of >300% and a measured strain rate sensitivity (SRS), close to 0.5. The superplastic flow is controlled by diffusion processes; it requires the testing temperature of 0.5Tm or greater where Tm is the absolute melting temperature of metals. It is well established that a reduction in grain size improves the optimum superplastic response by lowering the deformation temperature and/or raising the strain rate. The low-temperature superplasticity (LTSP) is attractive for commercial superplastic forming, in view of lowering energy requirement, increasing life for conventional or cheaper forming dies, improving the surface quality of structural components, inhibiting quick grain growth and solute-loss from the surface layers, thus resulting in better post-forming mechanical properties. This paper will summarize the dependence of superplasticity on grain size and shape in various metallic materials, including ferrous and non-ferrous alloys, which has been considered as an effective strategy to enable the LTSP.

The superplastic tension and deformation mechanism of Ti-6Al-4V alloy at 923 K and a tensile speed of 10−3, 5 × 10−3, or 5 × 10−2 s−1 was studied on an AG 250KNE electronic tension tester. Through theoretical modeling, the unit dislocation count of this alloy during superplastic deformation was introduced into the Ruano–Wadsworth–Sherby (R-W-S) deformation mechanism map, and a new deformation mechanism map involving dislocation count was plotted. Thereby, the mechanism underling the low-temperature superplastic deformation of this alloy was predicted. It was found the superplastic tension of Ti-6Al-4V at the tested temperature was controlled by dislocation movement, and with an increase in strain rate, the deformation transited from the dislocation-controlled mechanism with a stress index of 4 to the dislocation glide mechanism with a stress index of 5 or 7. At the strain rate of 10−3 s−1, this alloy reached the largest tension rate of 790% and strain rate sensitivity index of 0.52 and had excellent low-temperature superplastic properties.

The rolled Ti–15V–3Cr–3Sn–3Al (Ti-15-3) alloy (metastable β titanium alloy) sheet with an average grain size of 44.0 μm was subjected to friction stir processing (FSP) at a tool rotation speed of 250 rpm and a tool traverse speed of 100 mm/min (250–100). Thereafter, a fine-grained (∼6.6 μm) and relatively equiaxed microstructure with a high angle grain boundary (HAGB) ratio of 74.5% was observed in the stir zone (SZ). Superplastic tensile tests were then conducted on this microstructure at the temperatures ranging from 600 °C to 800 °C and strain rates range of 1 × 10−4-1 × 10−2 s−1, and an excellent low-temperature superplasticity (LTSP) with the elongation of 463% was obtained at 650 °C and 1 × 10−4 s−1. In addition, the microstructure in the gauge section of the tensile specimens interrupted at different engineering strains of 20%, 50%, 200%, and 463% (tensile fractured) at the optimal superplastic tensile condition of 650 °C and 1 × 10−4 s−1 was studied. It was found that the precipitated α phase increased with the increasing strain, which contributed to the achievement of an enhanced LTSP by inhibiting the grain growth. Moreover, the α grains with a finer grain size of 4.4 μm was observed in the gauge section of the tensile fractured specimen and this was attributed to the occurrence of continuous dynamic recrystallization (CDRX). Therefore, the superplastic deformation mechanism of the Ti-15-3 alloy is recognized as grain boundaries sliding (GBS) accompanied with dislocation movement and CDRX at 650 °C and 1 × 10−4 s−1. • It is the first time to report the study on the superplasticity in the friction stir processed Ti-15-3 alloy. • The 6.6 μm-grained equiaxed microstructure was obtained in the friction stir processed Ti-15-3 alloy. • The largest elongation of 463% was achieved in the friction stir processed Ti-15-3 alloy at the low temperature of 650 °C. • The precipitated α phase during tensile deformation can effectively inhibit the severe grain growth. • The superior superplastic properties were mainly attributed to the precipitated α phase during tensile deformation. [ABSTRACT FROM AUTHOR]

Commercial 5083Al rolled plates, 2.8 mm thick, were subjected to friction stir welding (FSW) with the aim of achieving low-temperature superplasticity (LTSP) in the nugget zone (NZ). Fine-grained microstructure with average grain sizes of 1.6 and 1.8 µm was obtained in the upper and lower parts of the NZ, respectively. The NZ was subjected to superplastic investigation at 250 and 300 °C. It was indicated that the upper and lower parts of the NZ exhibited similar LTSP values of 550–570% at 300 °C, much higher than those reported previously (< 300%) in friction stir processed 5083Al. This excellent LTSP was attributed to the extremely fine-grained microstructure and predominant high angle grain boundaries (> 84%). Grain boundary sliding was determined to be the dominant deformation mechanism, with grain boundary diffusion as the rate-controlling step. [ABSTRACT FROM AUTHOR]

An ultrafine microstructure consisting of α grains (~ 0.51 µm) and a small amount of β phase was successfully achieved in a friction stir-processed (FSPed) Ti-6Al-4V alloy. The fraction of high angle grain boundaries (HAGBs) with random crystallographic orientations reached 89.3% revealed that dynamic recrystallization was responsible for the ultra-grain refinement mechanism during friction stir processing (FSP). Low-temperature superplasticity (LTSP) of such an ultrafine microstructure was demonstrated in the temperature range of 550–650 °C and strain rates of 1 × 10 −4 –3 × 10 −3 s −1 . Specifically, an extremely superior LTSP of 1130% was achieved at 600 °C and 3 × 10 −4 s −1 , which was explained by means of the ultrafine equiaxed grains, a large proportion of HAGBs with random orientations as well as the presence of β phase. The predominant superplastic deformation mechanism was considered as grain boundary sliding associated with grain boundary diffusion. [ABSTRACT FROM AUTHOR]

Superplastic steels with high elongation above 300% are expected to be used to manufacture complex-shaped mechanical parts without joining. However, their practical application is difficult due to high energy consumption and low productivity caused by high deformation temperature and low strain rate. In the present study, we newly developed an Fe-Mn-Si-Ni steel, which exhibited superplasticity at a low temperature of 1023 K and a high strain rate of 1 × 10−1 s−1. This steel also had remarkable room-temperature tensile strength (∼1.3 GPa) and total elongation (38%) after a simulation of superplastic forming. The mechanism of excellent superplasticity is grain boundary sliding occurring at the boundaries of fine γ grains, whose coarsening was suppressed by both Fe 5 (Mn,Ni) 3 Si 2 and (Fe,Mn,Ni) 3 Si precipitates. • Fe-Mn-Si-Ni steel showed superplasticity at 1023 K with a strain rate of 1 × 10−1 s−1. • The steel had high tensile strength (∼1.3 GPa) and elongation (38%) at 293 K. • The steel had a γ matrix embedded with Fe 5 (Mn,Ni) 3 Si 2 and (Fe,Mn,Ni) 3 Si precipitates. • Outstanding superplasticity was due to grain boundary sliding of ultrafine γ grains. [ABSTRACT FROM AUTHOR]

Novel titanium-based alloys are required to increase the efficiency of superplastic forming technology and decrease energy consumption. For this purpose, the superplastic deformation behavior, strain-induced microstructure evolution in a temperature range of 625–775 °C, and post-deformation mechanical properties of Ti–4Al–1V–1Fe–1Ni-0.1B- x Mo alloys (x = 1, 2.5, or 5 wt%) were investigated. The studied alloys demonstrated a stable flow with a high strain rate sensitivity coefficient m of 0.50–0.65 and an elongation-to-failure δ of 700–1000% at temperatures of 700–775 °C. Molybdenum insignificantly influenced superplastic deformation behavior at high temperatures due to a high fraction of the β phase of 22–62%. The Mo effect was significant at a low deformation temperature of 625 °C. At this temperature, an increase in Mo content from 1 to 5% provided the β -phase fraction above the critical value of ∼20% and increased the m- value from 0.4 to 0.5 and δ from ∼200 to ∼700%. Increasing Mo from 1 to 5% enhanced the post-deformation yield strength at room temperature by 180 MPa, the ultimate tensile strength by 170 MPa, and the ductility by 2.5%. Consequently, alloying Ti–4Al–1V–1Fe–1Ni-0.1B with 5% Mo provided an excellent combination of superplasticity at a low temperature of 625 °C and post-forming tensile mechanical properties. • Increase in Mo content from 1 to 5 wt% improved low temperature superplasticity. • 5Mo and 20% of the β phase are required for low-temperature superplasticity at 625 °C. • Alloys exhibit a stable fine-grained structure during deformation at 625–700 °C. • Increase in Mo content improved the post-forming mechanical properties. • 5 wt%Mo provided a good combination of the superplastic and mechanical properties. [ABSTRACT FROM AUTHOR]

The present study reports an equilibrium ultrafine equiaxed dual-phase microstructure to regulate and achieved the low deformation temperature and high hot ductility balance in a novel 3.6 Al medium Mn steel. The cold-rolled specimens were subjected to high-temperature tensile tests and microstructure characterization with deformation temperature as a variable. The results showed that during the hot tensile process, the initial ultrafine martensite grains were transformed into ferrite + austenite equiaxed dual-phase grains, accompanied by dynamic recovery and recrystallization. At the deformation conditions of 625°C and 5 × 10-4 s-1, the accumulation of dislocations is obvious and the grain refinement is caused significantly. The reason for its failure is the development of the neck. The maximum elongation of 880% was obtained at 675°C and 5 × 10-4 s-1 due to the dynamic balance between 'the grain refinement and strain hardening caused by dislocation creep and accumulation' and 'the grain coarsening and softening caused by dynamic recovery and recrystallization'. When the deformation temperature continues to increase to 775 °C, the elongation sharply decreases up to 491%. The stress concentration at the phase boundary induces cavity behavior, which is the main reason for the premature failure at 775 °C. [ABSTRACT FROM AUTHOR]

The influence of microstructure evolution on the low-temperature superplasticity of ultrafine Ti 6Al 4V was established. For this purpose, the static and dynamic coarsening response and plastic-flow behavior of the alloy with a mean size of α (sub)grains and β particles of 0.1–0.4 μm were determined via a series of tension tests at temperatures between 450 and 700 °C. Both static and dynamic coarsening exhibited diffusion-controlled ( r 3 -vs-time) kinetics. However, dynamic coarsening was approximately two orders of magnitude faster than the corresponding static behaviors due to enhanced diffusion through the deformed β matrix. A total elongation of 1000% and strain-rate-sensitivity exponent m = 0.48 were obtained at 550 °C and 2 × 10 −4 s −1 . Very limited cavitation was observed in the specimens after superplastic deformation under optimal conditions. [ABSTRACT FROM AUTHOR]

Abstract: Microstructural influence on low-temperature superplastic behavior of ultrafine-grained Ti–6Al–4V alloy fabricated by equal channel angular pressing (ECAP) was investigated. The deformed structures were analyzed with the increment of strain by transmission electron microscopy. Also, a series of tensile tests were carried out on ultrafine-grained (UFG) samples to measure elongation at temperature of 973K and at strain rates of 10−4 to 10−2 s−1. The results indicated that elongation was significantly increased with increasing ECAP straining from 4 to 8 revealing more high-angle grain boundaries. Deformation mechanisms for UFG structure were analyzed in the context of inelastic deformation theory, which consisted of dislocation glide and grain boundary sliding. [Copyright &y& Elsevier]

An ultrafine grained 5083 Al alloy was prepared by equal channel angular pressing with route Bc and an effective strain of ∼4 or ∼8. The changes of the size and distribution of second phase particles and voids pre-existing in the alloy during equal channel angular pressing (ECAP) were examined in detail. Under the present ECAP conditions, a portion of particles of 1.5–3.5 μm substantially increased. However, the further break-up of particles with these sizes became less significant and they were to be distributed more uniformly throughout the sample with increasing the ECAP pass from 4 to 8. In addition, these particles were likely to act as the preferential cavity nucleation sites during low-temperature superplastic deformation of the alloy. The population of voids with the size of 0.5–1.5 μm remarkably increased after ECAP. An analysis revealed that these voids were difficult to be sintered out at low-temperature superplastic deformation temperatures prior to deformation and might grow to larger size cavities. From the present results and analysis, it is suggested that the number of the ECAP pass less than that resulting the largest elongation may be preferable in the viewpoint of cavitation damage and commercial application of low-temperature superplasticity of ultrafine grained Al alloys, if moderate superplastic elongation is ensured. [Copyright &y& Elsevier]

Excellent low-temperature superplasticity (<300 °C) was observed in the ECAP-processed materials: elongations to failure are 810% and 570% at a strain rate of 3 × 10−3 s−1 for ZK60 and AZ91 alloys, respectively. Internal friction can be practically used to determine the optimum temperature for superplasticity. [Copyright &y& Elsevier]

Studies of the low-temperature superplasticity (SP) of ultrafine-grained (UFG) near β alloy Ti-5Al-5V-5Mo-1Cr-1Fe at a temperature of 823 K (∼0.42 T m) 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 ∼ 0.17 µm, 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 ∼ 0.23 µm), 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. • UFG near β titanium alloy demonstrates elongation above 950% at a strain rate 2·10−3 s−1. • Low elongation of annealed UFG alloy is caused by hindering grain boundary sliding. • UFG structure with high strength is retains in alloy after low-temperature superplasticity. [ABSTRACT FROM AUTHOR]

The rolled Ti–15V–3Cr–3Sn–3Al (Ti-15-3) alloy plates were processed by friction stir processing (FSP) using an ultra-low heat input parameter of 100 rpm- 100 mm/min and 1.6 μm grain-sized microstructure was obtained. Subsequently, tensile tests were conducted in this microstructure within the temperature range of 625–725 °C with an interval of 25 °C at three different strain rates of 1 × 10−4 s−1, 3 × 10−4 s−1 and 1 × 10−3 s−1. The largest elongation of 842% was achieved at 675 °C and 3 × 10−4 s−1 and the present superplastic deformation is dominantly controlled by grain boundary sliding (GBS) accompanied by phase transformation and continuous dynamic recrystallization (CDRX). To the best of our knowledge, the elongation of 842% is the largest one for Ti-15-3 alloy at the temperature as low as 675 °C. The increasing temperature supplied more energy for GBS, which was beneficial to the achievement of excellent superplasticity. However, when the temperature increased over 675 °C, the α phase volume fraction severely decreased which led to the significant β grains growth, the high angle grain boundaries (HAGBs) remarkably reduced, all of which impeded the GBS hence the achievement of excellent superplasticity. Therefore, the largest elongation, most stable and uniform superplastic flow and lowest peak stress were achieved at the temperature of 675 °C for all the three testing strain rates of 1 × 10−4 s−1, 3 × 10−4 s−1 and 1 × 10−3 s−1. • A fine and equiaxed Ti-15-3 alloy was obtained by FSP using an ultra-low heat input parameter. • The maximum elongation of 842% for Ti-15-3 alloy was achieved in the present fine-grained microstructure. • The optimal temperature for the enhanced superplasticity in the fine-grained Ti-15-3 alloy was found as 675 ℃. • The reasons for the optimal temperature of superplasticity appearing at 675 ℃ were revealed. [ABSTRACT FROM AUTHOR]

The low-temperature superplastic tensile behavior and the deformation mechanisms of Ti-6Al-4V alloy are investigated in this paper. Through the experiments carried out, elongation to failure (δ) is calculated and a set of values are derived that subsequently includes the strain rate sensitivity exponent (m), deformation activation energy (Q) at low-temperature superplastic deformation, and the variation of δ, m and Q at different strain rates and temperatures. Microstructures are observed before and after superplastic deformation. The deformation mechanism maps incorporating the density of dislocations inside grains at temperatures of 973 and 1123 K are drawn respectively. By applying the elevated temperature deformation mechanism maps based on Burgers vector compensated grain size and modulus compensated stress, the dislocation quantities and low-temperature superplastic deformation mechanisms of Ti-6Al-4V alloy at different temperatures within appropriate processing regime are elucidated. [ABSTRACT FROM AUTHOR]