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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]

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]

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]