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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]

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]

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]