Showing papers on "Superplasticity published in 2022"

Using Severe Plastic Deformation to Produce Nanostructured Materials with Superior Properties

Abstract: The past decade was marked by significant advances in the development of severe plastic deformation (SPD) techniques to achieve new and superior properties in various materials. This review examines the achievements in these areas of study and explores promising trends in further research and development. SPD processing provides strong grain refinement at the nanoscale and produces very high dislocation and point defect densities as well as unusual phase transformations associated with particle dissolution, precipitation, or amorphization. Such SPD-induced nanostructural features strongly influence deformation and transport mechanisms and can substantially enhance the performance of advanced materials. Exploiting this knowledge, we discuss the concept of nanostructural design of metals and alloys for multifunctional properties such as high strength and electrical conductivity, superplasticity, increased radiation, and corrosion tolerance. Special emphasis is placed on advanced metallic biomaterials that promote innovative applications in medicine.

The role of grain boundary sliding and intragranular deformation mechanisms for a steady stage of superplastic flow for Al–Mg-based alloys

Abstract: The superplastic forming technique, which realized extremely high plastic deformation, is used for producing complex shaped lightweight constructions. Physical models and experiments with surface microstructure evolution indicate that superplastic deformation predominantly occurs owing to grain boundary sliding (GBS) that accommodated by dislocation slip/creep and diffusional creep. Unusually weak GBS and increased contributions of accommodation mechanisms during the initial stage of superplastic deformation are observed for the fine-grained commercial Al–Mg based alloys. In this study, microstructural evolution during the deformation at a temperature of 0.97Ti.m.with a constant strain rate of 4 × 10−3 s−1 was investigated by scanning and transmission electron microscopy for Al–Mg based alloys with a different Mg content. A strain-induced evolutions of the grain and dislocation structure and the surface structure with FIB-milled grids during elevated-temperature deformation were analyzed. A decrease in the strain rate sensitivity m-coefficient from 0.6 to 0.4, dynamic grain growth with significant grains elongation to the tensile direction accompanied by a pronounced dislocation activity, subgrains formation, and the development of the precipitated depleted zones were observed during deformation. The intergranular and intragranular strains were measured to estimate the contributions of superplastic deformation mechanisms. An increase in solute Mg from 4.8wt% to 6.5–7.6wt% inhibited dynamic grain growth and increased a mean GBS contribution from ~24% to ~40%. “Striation” zones developed near the transverse grain boundaries, and precipitation-depleted zones accumulated up to 50% of strain for the alloy with 4.8wt%Mg and 20–35% for higher Mg alloys. Grain body deformation via dislocation clip/creep provided 20–30% of total strain. The results confirmed the critical nature of the diffusional creep and dislocation slip/creep mechanisms for superplastic deformation of the studied alloys.

Superplastic formability of the developed Zr40Hf10Ti5Al10Cu25Ni10 high entropy bulk metallic glass with enhanced thermal stability

Abstract: The Zr55Cu30Al10Ni5 BMG as a base alloy was alloyed with Hf and Ti to develop a high entropy bulk metallic glass. The aim of the present work was to investigate the superplastic formability of HE-BMG, which can be used for micro-forming applications. Therefore, thermal stability and crystallization kinetics of the newly developed HE-BMG was first studied and then the results were used for the designed hot deformation experiments. It was demonstrated that HE-BMG has higher thermal stability than the base BMG, supposedly due to the sluggish diffusion and high entropy effect. Hot deformation behavior of the developed HE-BMG was investigated and it was found that at temperature of 753 K and the strain rate of 2 × 10–5 s-1, the alloy shows a good superplastic deformation with a plastic strain of at least 50% at compression stress around 500 MPa that is good enough for near net shape forming.

Superior superplasticity and multiple accommodation mechanisms in TiB reinforced near-α titanium matrix composites

Abstract: Superplastic forming is a promising technique for producing complex-shaped components of hard-to-deform titanium matrix composites (TMCs). Superplastic tensile tests were performed on hot-rolled TiB/near-α Ti matrix composites with different reductions to investigate the effects of matrix microstructural characteristics and whisker orientation on superplastic deformation behavior. The results showed that the composite with initial BT-type texture and finer matrix grains at a rolling reduction of 80% exhibited lower flow stress and larger elongation than the composite with initial T-type texture at a rolling reduction of 40%, and superior elongation of 682% was achieved in the former. Significant randomization of crystal orientation and dynamic grain growth phenomenon occurred in the matrix alloys during deformation. The originally misaligned TiB whiskers gradually rotated to the tensile direction and simultaneously rotated circularly around the [010] axis during superplastic tensile deformation, which would accommodate the matrix deformation and share more load transferred from the matrix. Another important accommodation mechanism of TiB whiskers was to accelerate the dynamic recrystallization of the surrounding matrix by particle-stimulated nucleation. TEM and EBSD observations together with GND-distribution analysis confirmed that the primary superplastic deformation mechanism of TiB/Ti composites is grain/phase boundary sliding accommodated by dislocation slip and dynamic recrystallization.

Effect of grain size on the yield stress and microscopic mechanism of a near-α titanium alloy during non-superplastic hot deformation

Abstract: In this paper, the effect of grain size on the yield stress of a near-α TA15 titanium alloy during the non-superplastic hot deformation was modelled, and the microscopic mechanism was analyzed through the crystal plastic finite element simulation. The effect of grain size on the yield stress changes from the refinement strengthening to refinement softening with the increase in temperature. To model this transformative effect, the grain boundary region is separated to consider the deformation mechanism of grain boundary sliding, and a concise relationship was obtained with a max prediction error of 3.7%. The distribution of microscopic strain is determined by the critical grain size Dc, when the strength of grain boundary regions is equal to α phase. When the grain size is greater than Dc, there are two high strain regions in the α phase, which are the intragranular deformation band caused by the prismatic slipping and the deformation band near grain boundaries caused by the multiple slipping. The continuous and discontinuous dynamic recrystallization appears simultaneously. When the grain size is less than Dc, the strain in the α phase is concentrated near the grain boundary due to the stimulant grain boundary sliding, and the discontinuous dynamic recrystallization is promoted.

Ultralow-temperature superplasticity of high strength Fe–10Mn-3.5Si steel

Abstract: Superplastic steels with high elongations above 300% are expected to be used for manufacturing complex-shaped mechanical parts without joining. However, their practical application is difficult due to high energy consumption and surface oxidation caused by a high deformation temperature (≥873 K). Here, we propose a new high strength Fe–10Mn-3.5Si steel, which is superplastically deformed at 763 K. This steel exhibits different microstructural and deformation features from previous superplastic steels, such as single phase before deformation, coarse elongated grains, low strain rate sensitivity, and strong texture. The superplasticity of the steel results from both dislocation creep and grain boundary sliding due to dynamic reverse transformation. Because the steel has a low material cost and is produced by conventional rolling, it is suitable for practical application. • 10MnSi steel had superplasticity at the lowest temperature (763 K) reported to date. • 10MnSi steel also revealed ultrahigh tensile strength (1336 MPa) at room temperature. • Outstanding superplasticity was caused by the GBS of dynamically reverted γ grains.

Superplasticity of bulk metallic glasses (BMGs): A review

Abstract: Under specific deformation conditions, thermoforming in the supercooled liquid region (SLR) might lead to Newtonian flow characterized by high strain-rate-sensitivity-index (m) of ∼1 and achieving superplastic ductilities. Both incipient deformation-induced crystallization and rapid increase in the stress-assisted free volume lead to decreased m-values and transition to non-Newtonian flow. The maximum elongations can be achieved near the transition strain rate from Newtonian to non-Newtonian flow. The effects of heating rate to the thermoforming temperature, total processing time, and the extent of the SLR are significant for structural stability against the in-situ crystallization during elevated-temperature hot deformation. Increasing the deformation temperature normally accentuates the superplastic behavior, but high temperatures might promote crystallization and loss of superplastic ductility . Future prospects for research have been recognized as optimization of the volume fraction of the crystalline (reinforcement) phase in BMG composites, superplasticity of high-entropy BMGs, thermoplastic micro-formability, ultrasonic-assisted forming, improved thermoplastic formability by alloying method, and superplasticity of BMG parts fabricated by additive manufacturing processes.

Effect of static annealing on superplastic behavior of a friction stir welded Ti-6Al-4V alloy joint and microstructural evolution during deformation

Abstract: • A new combination process of FSW + static annealing + superplastic deformation was proposed for eliminate strain localization and the welded structure • Similar superplastic abilities were obtained by the BM and NZ with totally different structures • An optimized view on the classical Langdon theory was put forward in the field of superplasticity • The mechanisms of the spheroidization in the NZ and the equiaxed structure fragmentation in the BM were discussed • This study provides a new way to fabricate large-scale complex Ti alloy components Structural integration is one of the most critical developing directions in the modern aerospace field, in which large-scale complex components of Ti alloys are proposed to be fabricated via the method of welding + superplastic forming. However, the undesired strain localization appeared during superplastic deformation of the entire joint has largely hindered the development of this method. In our study, a combination process of friction stir welding (FSW) + static annealing + superplastic deformation was first time proposed to eliminate severe local deformation. To achieve this result, a fully fine lamellar structure was obtained in the nugget zone (NZ) via FSW, which was totally different from the mill-annealed structure in the base material (BM). After annealing at 900 °C for 180 min, the BM and NZ then exhibited the similar elongation of >500% and similar flow stress at 900 °C, 3 × 10 −3 s −1 , which was the precondition for achieving uniform superplastic deformation in the entire joint. Moreover, the different microstructures in the BM and NZ tended to become the similar equiaxed structure after deformation, which was the result of different microstructural evolution mechanisms in the NZ and BM. For the NZ, there was a static and dynamic spheroidization of the fully lamellar structure during the process, which could largely reduce the flow softening of the fully lamellar structure. For the BM, a new view of “Langdon-CRSS theory” (CRSS, critical resolved shear stress) was proposed to describe the fragmentation of the coarse equiaxed structure, which established the relationship between grain boundary sliding and intragranular deformation during deformation.

Influence of Fe on the microstructure, superplasticity and room-temperature mechanical properties of Ti–4Al–3Mo–1V-0.1B alloy

Abstract: Decreasing the superplastic forming temperature of Ti alloys is currently an important issue. This study investigated the impact of modifying the Ti–Al–Mo–V alloy with different percentages of Fe (0–2 wt.%) on the microstructure, superplasticity and post-forming mechanical properties. The results revealed that during superplastic deformation, the increase in Fe content facilitated the recrystallisation and fragmentation of the phases and grain boundary sliding due to the accelerating diffusivity by Fe. In contrast, the presence of Fe increased the susceptibility to grain growth. A high strain rate sensitivity coefficient m (0.45–0.5) and maximum elongation to failure (500–1000%) were achieved for these alloys at a constant strain rate of 1 × 10 −3 s −1 and in a low temperature range (625–775 °C). Alloying the investigated alloy with Fe increased the post-forming room-temperature tensile strength by 90–220 MPa and decreased the ductility by 1–2%. Alloying with 0.5% Fe provided a good combination of the superplastic and room-temperature mechanical properties for the studied alloys. • 0.5-2 wt%Fe addition substantially improved the superplasticity of Ti-based alloy. • Fe facilitated the recrystallisation/fragmentation. • Alloying with Fe accelerated grain growth in Ti-based alloy. • Fe alloying increased the post-forming room-temperature tensile strength. • 0.5%Fe provided a good combination of the superplastic and mechanical properties.