Search

The ultra-fine grain Ti–6Al–4V alloy sheet was successfully achieved by friction stir processing (FSP). The low-temperature superplastic deformation mechanism of Ti–6Al–4V alloy under the strain rate of 3 × 10−4 s−1 at 600 °C was studied by scanning electron microscope, electron backscatter diffraction and superplastic tensile test. It is found that the grain size of Ti–6Al–4V alloy by FSP is refined from 7.48 μm to 0.82 μm, and there is no preferential crystallographic orientation of the uniform grain. After FSP, the β phase of the base material (BM) in the grain boundary is refined and homogenized. During the superplastic tensile processing, with the increase of strain, the grain is refined continuously, and the α→β phase transition is induced by the dislocations pile-up at the β grain boundary, and the content of β phase increases. The grain near the fracture was refined to 0.35 μm, and the stress concentration led to the formation of cavities near the β phase, which eventually caused the fracture. The dynamic recrystallization in superplastic tensile processing is mainly geometric dynamic recrystallization and discontinuous dynamic recrystallization. The elongation of superplastic tensile (1033%) is 19.7 times higher than that of BM (50%). The order of superplastic deformation mechanism in the low-temperature superplastic deformation of the ultra-fine grain Ti–6Al–4V alloy sheet is as follows: intragranular dislocation slip and diffusion, grain boundary sliding, and phase boundary sliding. Phase boundary sliding is the main deformation mechanism for low-temperature superplastic deformation.

An elongation of 880 % was obtained at a high initial strain rate of 1 × 10−2 s−1 at 470 °C in an Al–Cu–Li alloy produced by traditional thermomechanical-processing. The high-strain-rate superplastic deformation mechanisms during the whole deformation process were studied by SEM, EBSD, and FIB techniques. High-resolution surface studies employing focused ion beams reveal that grain boundary sliding accounts for up to 70 % of the overall strain. Furthermore, this phenomenon consistently maintains a high level, with a minimum value of 56 %, throughout the entire deformation process. Meanwhile, diffusion creep plays an accommodating role and stimulates the formation of striated bands.

Titanium alloys have high specific strength and corrosion resistance, which have promising applications in industry. However, the machinability of titanium alloys is limited due to their crystal lattice and physical properties. Thus, in recent years, the superplastic forming of titanium alloys has been intensively developing, in particular, forming at low temperatures and/or high strain rates. In this work, a tensile test of low-cost Ti-2Fe-0.1B alloys was carried out at a temperature of 550~750 °C and a strain rate of 1 × 10−3 s−1~1 × 10−2 s−1. The results showed that the alloy exhibited good superplasticity even at a high strain rate (1 × 10−2 s−1) and a low deformation temperature of 550 °C; the elongation of the alloy in this state reached 137.5%. The high strain rate sensitivity coefficient m (0.3) and the maximum elongation (452%) were obtained at a strain rate of 1 × 10−3 s−1 and a temperature of 750 °C. Characteristics of the microstructure showed that during superplastic deformation, the recrystallization and grain boundary sliding of the alloy phases were accelerated, which could be ascribed to the effect of the element Fe. At the same time, the TiB phase located around the primary elongated α grains could also induce dynamic recrystallization and dynamic globularization during deformation. [ABSTRACT FROM AUTHOR]

The present paper is concerned with the dynamic recrystallization of the Ti-6Al-4V alloy. Electron Backscatter Diffraction (EBSD) observations are performed after high-temperatures tensile tests, with the temperature ranging from 700 to ~950 °C, and the strain rates varying between 10−4 and 10−2/s. Based on the analysis of flow behavior, the dominant mechanism is identified, and a mechanism map is proposed. In particular, the conditions of 890 °C and strain rates ranging from 10−3 to ~10−2/s serve as the delineating boundary of dynamic recovery (DRV) and dynamic recrystallization (DRX). For superplastic deformation, the dominant softening mechanism is DRV. Consequently, the occurrence of continuous dynamic recrystallization (CDRX) can naturally be ascribed to the process of grain refinement. Then, a multi-scales physical-based constitutive model of CDRX is developed, demonstrating a good agreement is obtained between the experimental and calculated grain sizes, so the above model could be used to describe the grain growth for superplastic deformation. In conclusion, DRV and DRX in the superplastic forming of Ti-6Al-4V are studied in this study, the condition boundaries of their occurrence are distinguished, and a constitutive equation-based CDRX recrystallization mechanism is given, which might be employed in the fracture mechanism research. [ABSTRACT FROM AUTHOR]

This study investigates the hot deformation behavior and microstructural evolution of a novel medium entropy alloy subjected to low strain rates and temperatures ranging from 650 °C to 850 °C. The alloy was subjected to hot compression testing under various conditions to observe its response. At all deformation condition, discontinous dynamic recrystallization was the main restoratory mechanism. After full recrystallization, grain sizes ranged from 1 to 5 μm, accompanied by an average hardness of 250 HV. The flow curves demonstrated a typical three-stage work hardening behavior across all deformation conditions, indicative of dynamic recrystallization. It is found that the deformation activation energy (Q) exhibited a negative slope with increasing strain, with an average value of Q = 267.7 kJ/mol. Zener-Hollomon parameter was determined from the experimental results and effectively used to simulate deformation, demonstrating good agreement with experimental findings. Remarkably, following the initial cycle of recrystallization, the deformation process shifted towards grain boundary sliding instead of secondary recrystallization, displaying characteristics reminiscent of superplastic behavior. Further analysis revealed a high strain rate sensitivity (m > 0.3) at 750 °C and 850 °C, with grain sizes lower than anticipated values. In summary, this study provides a comprehensive understanding of the hot deformation behavior and microstructural evolution in the Fe50-(CoCrMnNi)50 alloy. The findings shed light on dynamic recrystallization, deformation activation energy, and the unexpected shift to grain boundary sliding. These insights are valuable for advancing the knowledge of medium entropy alloys and their potential applications in various industries.

In this paper, the superplastic behavior of SP-700 alloy (Ti–4.5Al–3V–2Mo–2Fe) was investigated by hot compression tests and hot tensile tests in the temperature range of 700–950°C and strain rates of 0.001 to 1 s–1. The microstructural analysis and mechanical behavior of the alloy were studied during superplastic deformation. Although the elongation became higher than 370% at 700°C, the alloy showed a relatively high work hardening. At 750 and 800°C, the typical superplasticity was occurred and the specimens endured a large elongation of 427–440%. Microstructure of the alloy in 700–800°C was mainly consisted of globular α phase, while the globular α phase was completely removed in the tip fracture of the specimen deformed at 850°C due to deformation-induced transformation. In the temperature range of 850–950°C, the elongation was decreased to 250–325% as a consequence of the removal of α phase, significant β grain growth, and crack formation along β/β grain boundaries. The maximum elongation of 440% was obtained at 800°C where the strain-rate sensitivity is equal to 0.38 which is accommodated by the m-value distribution map of the alloy. During superplastic deformation in the single β phase region, the Rachinger grain boundary sliding (GBS) mechanism is the dominant phenomenon. However, deformation in the dual-phase α/β region is encouraged by dynamic recrystallization (DRX) and decomposition of α colonies. [ABSTRACT FROM AUTHOR]

Titanium alloys have high specific strength and corrosion resistance, which have promising applications in industry. However, the machinability of titanium alloys is limited due to their crystal lattice and physical properties. Thus, in recent years, the superplastic forming of titanium alloys has been intensively developing, in particular, forming at low temperatures and/or high strain rates. In this work, a tensile test of low-cost Ti-2Fe-0.1B alloys was carried out at a temperature of 550~750 °C and a strain rate of 1 × 10−3 s−1~1 × 10−2 s−1. The results showed that the alloy exhibited good superplasticity even at a high strain rate (1 × 10−2 s−1) and a low deformation temperature of 550 °C; the elongation of the alloy in this state reached 137.5%. The high strain rate sensitivity coefficient m (0.3) and the maximum elongation (452%) were obtained at a strain rate of 1 × 10−3 s−1 and a temperature of 750 °C. Characteristics of the microstructure showed that during superplastic deformation, the recrystallization and grain boundary sliding of the alloy phases were accelerated, which could be ascribed to the effect of the element Fe. At the same time, the TiB phase located around the primary elongated α grains could also induce dynamic recrystallization and dynamic globularization during deformation.

The morphology of the fibrous structures formed in the working parts of the 01420T alloy samples with the initial bimodal grain structure, deformed to fracture under optimal conditions of superplastic deformation at a temperature Т = 520°С and flow stress σ = 4,5 MPa is investigated. The maximum elongation of specimens deformed to failure δ is 670%. It has been suggested that the specific type of fibrous structures found in the specimens of the investigated alloy 01420T probably depends on the volume of the metastable liquid-solid phase, which was concentrated in the form of inclusions at some grain boundaries and made a viscous flow during superplastic deformation, its shear viscosity , the characteristics of its surface tension, the degree of dynamic oxidation of the melt, and the kinetics of the development of this process. The final view of the fibers and their shape, likely, depends not only on the nature of the viscous flow of the liquid-solid material, but also on the process of its crystallization during the cooling of the specimen in air to room temperature after mechanical tests. It was found that in view, all fibrous structures found in the working parts of the specimens can be conditionally divided into the following: cylindrical fibers; tapered fibers; cylindrical fibers on which there is a thickening or one or more drop-like formations; ribbon-like fibers; fibers that look like stalactites or stalagmites. The reasons for the formation of cracks on ribbon-like fibers are considered. It is assumed that they were formed as a result of relaxation of internal stresses, which were not fully minimized in the course of recrystallization, which was carried out when the sample was cooled. The reasons for the formation of droplets on the fibers are considered. It has been suggested that fibrous structures similar to stalactites and stalagmites were formed from a viscous material, which, in the course of superplastic deformation, as a result of crystallization, occured in local microvolumes of fibers, gradually turned from liquid-solid to solid-liquid. This led to the fact that in the crystallized microvolume of this fiber, the viscous homogeneous flow of the material probably turned into a localized flow, which is characteristic of the plastic flow carried out as a result of displacement of dislocations in the solid phase, and leads to the formation of stalagmitic fibers.

At present, titanium alloy is widely used in the field of materials. Because of its high specific strength and good corrosion resistance, it is also widely used in sports equipment. To overcome the shortcomings of traditional sports equipment, such as low yield and weak strength, the superplastic deformation effect of titanium alloy was tested and analyzed in this paper. The superplastic strength of titanium alloy was adjusted by adjusting the structure of the material. In this paper, by adjusting the proportion of Ti, Mo, alloy, CD, and Fe, the inductively coupled parameters of sports equipment were improved by vacuum consumable melting. The macro high temperature tensile test of sports equipment was carried out, and the temperature and strain rate under different states were recorded. Finally, the mechanism of superplastic deformation of titanium alloy for sports equipment is analyzed. The results show that the superplastic titanium alloy has higher elongation after fracture at high temperature and lower strain rate. The elongation ratio is more than 90% between 500 °C and 800 °C. At high temperature, the corrosion resistance of titanium alloy is high α, The number of phases decreases, and the alloy particles change from short rod to equiaxed. When the strain rate is reduced, the mechanical properties of titanium alloy are improved α, phase increases, the β the number of phases decreased. From its mechanism, the elongation of titanium alloy is caused by the dislocation movement caused by its structure. Therefore, titanium alloy in sports equipment can generally increase the short axis shape to improve the superplasticity of titanium alloy. [ABSTRACT FROM AUTHOR]

Ti–6Al–4V fine-grained plates were manufactured using a rolling method and then subjected to superplastic tensile tests at varying temperatures and strain rates on an AG 250KNE electronic tensile testing machine. The superplastic behaviours of the plates were also tested. A cavity-growth model was established and the changing laws of energy during cavity growth and microstructure evolution of superplastic deformation were predicted. The Ti–6Al–4V alloy possessed the maximum elongation rate of 886% at 840°C and a strain rate of 5 × 10−4 s−1. The strain-rate sensitivity index m for this alloy was 0.54. The mechanism of superplastic deformation was established to be strain-induced grain-boundary slip, and the mechanism of cavity growth to be plasticity-controlled cavity coalescence and growth. [ABSTRACT FROM AUTHOR]

Body centered cubic (BCC) refractory metals exhibit excellent high-temperature strength, but intergranular brittle fracture often occurs after thermal services. In this work, a fully-recrystallized pure molybdenum (Mo) material with room temperature (RT) superplasticity after annealing is prepared by powder metallurgy and Y-type hot rolling. After annealing at 1200 °C, the total elongation of Mo at RT reaches a maximum of 113.5%. The deformation mechanism of RT superplasticity is studied through in-situ EBSD. The grain shape is significantly elongated, while the 〈110〉//RD texture component remains dominantly along with the deformation. The RT superplasticity is mainly attributed to the excellent plastic deformation ability of <110>//RD oriented grains, which mainly due to the single slip system of dislocations within the grains reducing dislocation accumulation at grain boundaries, increasing the probability of dislocation transmission between adjacent grains, and improving overall coordinated deformation. [Display omitted] • The RT superplasticity is mainly attributed to the excellent plastic deformation ability of <110>//RD oriented grains. • Other oriented grains coordinate deformation by undergoing orientation rotation and forming high misorientation gradients internally. • Dislocations are dominated by single slip system within the <110>//RD oriented grains, reducing the accumulation of dislocations at GBs, achieving coordinated deformation between grains. [ABSTRACT FROM AUTHOR]

Foreign works containing the results of new studies in the field of alloying and deformation of modern magnesium alloys are reviewed. Attention is shown to be paid to studying the effect of increasing the properties of the alloys containing various rare earth metals (REM), including alloys with a low REM content. Attention is also paid to LPSO-phase-containing alloys. In such alloys, a specific mechanism of deformation kink bands in LPSO phases during deformation is operative, and it promotes an increase in the strength properties of a deformed semifinished product. The processes accompanied by superplastic deformation (SPD) are being actively studied. Many studies are aimed at developing new complex schemes of preliminary deformation to facilitate SPD. The application of a unique scheme of preliminary deformation of a serial ZK60 alloy was shown to give rise to a very high strain-rate sensitivity (m = 0.57–0.62). Magnesium wrought alloys are still in the focus of research as a modern significant structural material. [ABSTRACT FROM AUTHOR]

Owing to excellent strength, toughness and corrosion resistance, duplex stainless steels (DSS) are widely used in constructional and petrochemical applications. Sigma phase, which has detrimental impact on the properties, is readily precipitated during hot working of DSS. However, precipitation behavior of sigma phase during superplastic deformation, which is the most significant processing method of DSS, is still unclear. In the current study, the effect of superplastic deformation on the precipitation behavior of sigma phase was investigated in 3207 duplex stainless steel. The result shows that superplastic deformation could prevent sigma phase precipitation generally by increasing mobility of grain boundaries and decreasing misorientation of the sigma phase boundaries, resulting in some sigma phase precipitated on the twin boundaries. Most of the sigma phase precipitated on ferrite-austenite interface with misorientation of 20–25°, while it precipitated in ferrite or austenite with the misorientation of 40°–45°. The orientation relationship between sigma phase and matrix matched well in austenite and on the ferrite/austenite interfaces, while it showed a small misfit in ferrite. The prevention effect of the superplastic deformation on the sigma phase precipitation was beneficial to quasi stable deformation stage, resulting in longer elongation. • σ precipitation is systematically investigated in superplastic deformation. • σ precipitatation on twin boundaries is observed and the mechanism is discussed. • Orientation relationship between σ and δ, γ, γ/σ interfaces is clarified. • Effect and mechanism of superplastic deformation on σ precipitation is clarified. [ABSTRACT FROM AUTHOR]

Al–Li alloy possesses remarkable advantages such as low density, high specific strength and specific elastic modulus, making it an exceptional aerospace structural material with considerable application potential. However, Al–Li alloy suffers from poor formability, which restricts its broader engineering applications. Alternatively, superplastic forming at high temperatures is widely applied for producing such components. Temperature is a key factor affecting the thermal deformation properties of Al–Li alloy. In this study, tensile tests of 2198 Al–Cu–Li alloy were conducted at different temperatures, and dedicated characterizations were performed to reveal the underlying mechanisms responsible for the hot deformation behavior. Results reveal that three distinct stages are observed in the superplastic deformation process of this alloy. Specifically, in the initial low-temperature range (400 °C–450 °C), incomplete dynamic recrystallization (DRX), accompanied by grain rotation and elongation plays a dominant role in the hot deformation mechanism. In the middle temperature stage (450 °C–500 °C), both DRX and grain boundary sliding (GBS) occur. At the high-temperature stage (500 °C–550 °C), diffusion creep emerges as the prominent mechanism. Moreover, optimum superplasticity is achieved at 500 °C, which results from a synergistic combination of several factors, including grain refinement by DRX, an increased fraction of high-angle grain boundaries (HAGBs), reduced dislocation density and weak texture. [ABSTRACT FROM AUTHOR]

In the present study, the effect that superplastic deformation exerts on the microstructures, texture, and mechanical properties in 2A97 Al–Cu–Li alloy was investigated. The as-received material was deformed to a series of strains for 50%, 100%, 150%, and 200% at 430 °C with an initial strain rate of 2 × 10−3 s−1, and the grain structure, dislocation, secondary phase particles, cavities, texture, and tensile properties of deformed alloys were demonstrated and discussed comprehensively. Results show that with the increase of superplastic strain, the grain size of the alloy decreased gradually due to the dynamic recrystallization, and the Cu-rich secondary phase particles partially dissolved and partially coarsened. The initiation and growth of the cavities mainly depended on the grain boundary sliding and grain rotation, the fraction of cavities increased rapidly at higher strains (100%–200%). Superplastic deformation weakened the rolling texture (Brass, Copper, and S) and enhanced the recrystallization texture (Goss and ND-Rotated-Cube). The strength of the material decreased with increasing the superplastic strain firstly, after the strain exceeded 100%, the strength started to increase. The fracture elongation varied oppositely. The strengthening mechanism was discussed in detail. The coarsening of Cu-rich secondary phase particles and the rapid formation of cavities resulted in the drastic loss in ductility after the strain of 100%. • The microstructures and mechanical properties of an Al–Cu–Li alloy with different superplastic strains are studied. • The evolution of grain structure, secondary phases, cavities, and texture are demonstrated systematically. • The mechanical properties of the post-deformed materials are strongly related to the dramatically changed microstructures. • The formation of cavities and coarsening of secondary phase particles exert an adverse influence on the ductility. [ABSTRACT FROM AUTHOR]

Fine-grained Ti-6Al-4V titanium alloy plates were manufactured using a direct rolling approach, and a series of superplastic tensile tests were conducted at varying temperatures and strain rates. The maximum tensile elongation of 816% was obtained at 810 °C and 5 × 10−4 s−1, and the superplastic flow behavior was dominated by a strain-induced grain boundary slip. In addition, the superplastic behaviors of the alloy were investigated, and the α-and β-phase Gibbs free energy was calculated. The vital role of phase β in the transformation was discussed from a thermodynamical perspective. A power dissipation rate model of Ti-6Al-4V during the superplastic deformation was built and used to predict the energy changing laws in the dynamic recrystallization and grain boundary sliding during the superplastic deformation.

The effect of the initial powder particle size on the hot workability of Ni-based superalloys was investigated. Herein, four powders with different particle size ranges (106 μm) exhibited superior hot workability than that of the sample fabricated using the powder with finer particles (

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. [ABSTRACT FROM AUTHOR]

Due to attractive strength, toughness, and corrosion resistance, duplex stainless steels (DSS) are widely used in various applications. The current study performs a systematic investigation on the microstructure and orientation of austenite in DSS during superplastic deformation. Results show that austenite grains became uniform, equiaxed, coarse, and some were elongated. The superplastic deformation could weaken textures in Gauss {011}<100>, Brass {011}<211> and Copper {211}<111> orientations. Dynamic recovery followed by continuous dynamic recrystallization occurred in austenite during superplastic deformation. Concentrated strain in softer ferrite phase leads to a slightly lower dislocation density in austenite. This paper provides new insight into the deformation mechanism of the austenite matrix during hot deformation in duplex stainless steels. [ABSTRACT FROM AUTHOR]

Superplastic deformation behavior plays a significant role in the manufacturing of light and complex shaped components, and particularly, the superplastic behavior of Ti-6Al-4V alloy has different fields of applications such as hollow fan blades used in a gas turbine engine and high-performance heat exchangers. To study this, uniaxial tensile tests have been conducted within a temperature range of 700 to 900 °C at different strain rates, 0.01/s, 0.001/s, and 0.0001/s. The test results show more than 50% elongation in general and more than 200% elongation from 750 to 900 °C at 0.0001/s strain rate, representing the superplastic deformation behavior in Ti-6Al-4V alloy. The fractured specimens have been characterized by means of an optical microscope, scanning electron microscope, and X-ray diffraction techniques. Microstructure analysis confirms coarsening of grain size and variation in volume fraction of β with temperature, while SEM study clearly indicates ductile fracture with improved amount of dimples and flow lines at elevated temperatures. X-ray diffraction results indicate that the basic peaks position remains the same, but parameters vary due to superplastic deformation behavior. To accurately estimate the flow stress behavior, modified Arrhenius model has been developed and found to have the correlation coefficient (R) as 0.9939 when compared with experimental flow stress. Furthermore, by using the flow stress data, processing maps have been developed for analyzing the superplastic deformation behavior based on the efficiency and flow instability region at different elevated temperatures and strain rates. Processing maps clearly show excellent efficiency of power dissipation without any presence of flow instability in the superplastic deformation domain, i.e., from 770 to 900 °C temperature range and at 0.01–0.0001/s strain rate. [ABSTRACT FROM AUTHOR]

The superplastic tensile behaviors of Ti-6Al-4V (TC4) titanium alloy were tested at temperature ranges from 850 ℃ to 950 ℃ and tensile velocities in the range of 0.3–6.0 mm/min. The microstructures of the specimen under various deformation parameters were evaluated by scanning electron microscope (SEM) and transmission electron micrograph (TEM). The results indicate that the maximum tensile force decreases with the increase of temperature and the decrease of tensile velocity. The optimum tensile elongation of 1120.5 % is attained at 900 ℃ and 0.6 mm/min. α and β phase grain sizes increase with the increase of temperature and the decrease of tensile velocity. β phase volume fraction plays an important role in superplasticity, and the best elongation is attained under the condition of ~7.0 µm α phase and ~4.0 µm β phase and β phase volume fraction is about 31 %. High deformation velocity leads to finer grains and results in larger tensile force.

The aim of the study is to determine the correlation between the structure evolution and features of submicrocrystalline VT6 alloy deformation behavior under superplastic deformation. in the alloy was obtained by the multi-directional pressing method changing the deformation axis and gradual decrease of deformation temperature in the interval of 823-1073 К. Deformation behavior of VT6 alloy submicrocrystalline state was studied under tensile tests of 10-2 Pa in vacuum with the initial stretching rate of 6,9 10-3 sec.-1 and in the temperature range of 773-1073 К. The TEM method has been used to investigate the alloy structure in the initial state as well as in the submicrocrystalline state after the elongation of 40, 60 and 200 %. Submicrocrystalline structure formation in the alloy is found to lead to decreasing the temperature interval of superplastic flow beginning from 1173 to 873 K. It was shown that the increase in temperature of superplastic deformation of submicrocrystalline VT6 alloy samples from 873 to 973 K results in appearance of strain hardening stage in the stress-strain curves in the coordinates of the "true stress - true strain". Structure evolution of the submicrocrystalline VT6 alloy during superplastic deformation depends on the test temperature. At 973 K one can observe the change in the character of size distribution histogram of grain-subgrain structure elements from unimodal to bimodal. The strain hardening of the alloy under superplastic flow is caused by bimodal structure formation in it and growth in volume fraction of relatively large grains at deformation.

Abstract The zirconia ceramic was flash joined in 1 s by applying an electric field. The joining was performed at temperatures as low as 600 °C, with a joint three-point bend strength of ca. 418 MPa and a smaller grain size than that of the parent material. The joining strength was further enhanced with the increase in bonding temperature, reaching ca. 595 MPa at 900 °C. Ultrafast superplastic deformation of the ceramic under the applied electric field and the subsequent accelerated mass transport are proposed as the underlying mechanism for flash joining. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The paper presents research into the behavior of high-temperature plastic deformation of titanium alloy Ti-4.74 wt.% Al-5.57 wt.% Mo-5.04 wt.% V alloy (VT22) depending on its structure and phase composition. It is shown that the formation of fine-grain structure in this alloy is not a sufficient condition for realizing superplastic deformation. It is found that the formation of the ultra-fine grain structure in VT22 alloy leads to the temperature decrease down to 823 K at the beginning of plastic deformation. This allows achieving the percent elongation of the alloy specimens over 1300% within the temperature range of 973-1073 K and at a 6.9·10-3 s-1 initial tensile rate. The grain boundary sliding is considered to be the main deformation mechanism. [ABSTRACT FROM AUTHOR]

The superplastic deformation of a hot-extruded GH4151 billet was investigated by means of tensile tests with the strain rates of 10−4 s−1, 5 × 10−4 s−1 and 10−3 s−1 and at temperatures at 1060 °C, 1080 °C and 1100 °C. The superplastic deformation of the GH4151 alloy was reported here for the first time. The results reveal that the uniform fine-grained GH4151 alloy exhibited an excellent superplasticity and high strain rate sensitivity (exceeded 0.5) under all experimental conditions. It was found that the increase of strain rate resulted in an increased average activation energy for superplastic deformation. A maximum elongation of 760.4% was determined at a temperature of 1080 °C and strain rate of 10−3 s−1. The average activation energy under different conditions suggested that the superplastic deformation with 1 × 10−4 s−1 in this experiment is mainly deemed as the grain boundary sliding controlled by grain boundary diffusion. However, with a higher stain rate of 5 × 10−4 s−1 and 1 × 10−3 s−1, the superplastic deformation is considered to be grain boundary sliding controlled by lattice diffusion. Based on the systematically microstructural examination using optical microscope (OM), SEM, electron backscatter diffraction (EBSD) and TEM techniques, the failure and dynamic recrystallization (DRX) nucleation mechanisms were proposed. The dominant nucleation mechanism of dynamic recrystallization (DRX) is the bulging of original grain boundaries, which is the typical feature of discontinuous dynamic recrystallization (DDRX), and continuous dynamic recrystallization (CDRX) is merely an assistant mechanism of DRX. The main contributions of DRX on superplasticity elongation were derived from its grain refinement process.

MgLiAl alloy containing 9 wt% Li and 1.5% Al composed of hexagonal α and bcc β phases was cast under protecting atmosphere and hot extruded. Various methods of severe plastic deformation were applied to study their effect on structure and grain refinement. Rods were subjected to 1-3 passes of Twist Channel Angular Pressing TCAP (with helical component), cyclic compression to total strain ε = 5 using MAXStrain Gleeble equipment, both performed at temperature interval 160-200 °C and, as third SPD method, KOBO type extrusion at RT. The TCAP pass resulted in grain refinement of α phase from 30 μm down to about 2 μm and that of β phase from 12 to 5 μm. Maxstrain cycling 10 × up to ε = 5 led to much finer grain size of 300 nm. KOBO method performed at RT caused average grain size refinement of α and β phases down to about 1 μm. Hardness of alloy decreased slightly with increasing number of TCAP passes due to increase of small void density. It was higher after MAXStrain cycling and after KOBO extrusion. TEM studies after TCAP passes showed higher dislocation density in the β region than in the α phase. Crystallographic relationship (001) α|| (110) β indicated parallel positioning of slip planes of both phases. Electron diffraction technique confirmed increase of grain misorientation with number of TCAP passes. Stress/strain curves recorded at temperature 200 °C showed superplastic forming after 1st and 3rd TCAP passes with better superplastic properties due to higher elongation with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.29 after 3rd TCAP pass for strain rate range 10−5 to 5 × 10−3 s−1. Deformation by MAXStrain cycling caused much more effective grain refinement with fine microtwins in α phase. Superplastic deformation was also observed in alloy deformed by KOBO method, however the value of m = 0.21 was obtained at lower temperature of deformation equal to 160 °C and deformation rate in the range 10−5 to 5 × 10−3. Tensile samples deformed superplastically showed grain growth and void formation caused by grain boundary slip. Summarizing, all methods applied resulted in sufficient grain refinement to obtain the effect of superplastic deformation for alloys of two phase α + β structure. [ABSTRACT FROM AUTHOR]

This article presents the results of investigation of the influence of holding temperature during the quenching process on the microstructure and superplasticity of the Co-Ni-Nb alloy. Temperature-strain rate intervals of the deformation of the superplasticity effects are stated. The optimal regimes of the preliminary treatment by quenching and rolling as well as the routine of the superplastic deformation of the Co-Ni-Nb alloy are defined. The interval of the temperatures of the precipitation, morphology, composition, type and parameters of the lattice of the secondary phase, which appears after the annealing + rolling (to 90%) Co-Ni-Nb alloy, are determined. [ABSTRACT FROM AUTHOR]

The carbon nanotube reinforced aluminum matrix composite (CNT/6061Al) fabricated by flake powder metallurgy was cold rolled to reduce grain size, and was examined by high temperature tensile experiments within temperature from 325 to 450 °C and strain rate from 4.17E−2 to 2.09 s -1 to explore the effect of grain refinement on the superplastic deformation behavior. The average grain size was reduced from 580 nm to 300 nm with better homogeneity after cold rolling, and elongation to failure at 400 °C and 4.17E−1 s -1 was improved by 40% compared to that without cold rolling. The temperature of 375 °C and intermediate strain rate turned out to be appropriate deformation conditions for the refined microstructure, in which both large elongation to failure and uniform elongation was obtained. Stress exponent was changed to 2 and activation energy was determined to be 76.7 kJ/mol, indicating grain boundary sliding was the main deformation mechanism. [ABSTRACT FROM AUTHOR]

• The high-strain rate superplasticity in Al-Cu-Li alloy was processed via a thermos-mechanical processing. • A thermos-mechanical processing was exploited to prepare fine-grained with 3.1 μm grains. • The calculated deformation activation energy suggest that the grain boundary sliding mechanism is active. In order to obtain high strain rate superplasticity, it is usually necessary to prepare fine-grained aluminum alloy by severe plastic deformation such as equal channel angular pressing. A traditional thermo-mechanical processing was exploited to prepare fine-grained Al-Cu-Li alloy with 3.1 μm grains. A maximum elongation of 1220% was obtained at a high strain rate of 1 × 10−2 s−1 and 490 °C. The high-stain rate superplasticity is attributed to the existence of stabilized fine grains and nano-scaled Al 3 Zr particles which inhibit the dynamic growth of grain. Furthermore, the calculated deformation activation energy and m values suggest that the grain boundary sliding mechanism is active. [ABSTRACT FROM AUTHOR]

The superplastic deformation and microstructure evolution of a new near-α TNW700 titanium alloy with short-term service temperature up to 700 °C were systematically investigated in the temperature range of 900–975 °C and strain rate range of 0.0005–0.01s−1. The results illustrate that TNW700 alloy has excellent superplasticity with elongation above 300% and m value above 0.41 at 900–950 °C, and the maximum elongation of 523% is obtained at 950°C-0.001s−1 due to the appropriate grain size and phase ratio. In addition, TNW700 alloy exhibits high temperature softening and strain rate hardening behaviors. The increase of β phase volume fraction induced by dynamic phase transformation is a crucial cause for flow softening, and this phenomenon is accelerated with increasing temperature and decreasing strain rate. The refinement of primary α grains and the inconspicuous growth of β grains at lower temperatures are favorable for grain boundary sliding and superplasticity enhance, whereas the overgrowth of β grains at 975 °C results in superplasticity absence. The decrease of dislocation density in primary α grains caused by strain partitioning, dynamic recovery (DRV) or dynamic recrystallization (DRX) is another important factor for high temperature softening; the increase of dislocation density and velocity, and the decrease of annihilation time contribute to strain rate hardening. The appearance of random texture indicates the activation of grain boundary sliding (GBS) and grain rotation. GBS is the overarching mechanism of superplastic deformation of TNW700 alloy, accompanied by grain rotation, dislocation movement, dynamic phase transformation and grain refinement or coarsening. [Display omitted] • Insignificant grain growth and appropriate phase ratio at 900–950 °C facilitate elongation and m value increase. • Excessive growth of β grains at 975 °C leads to the absence of superplasticity. • Dynamic phase transformation decreases stress, increases α/β interface density and promotes phase boundary sliding. • The superplasticity is stimulated by grain boundary sliding, grain rotation and dislocation movement. [ABSTRACT FROM AUTHOR]

In this study, the compressive deformation of zirconia toughened alumina (ZTA) ceramics doped with different amounts of TiO2 dopants were investigated in the temperature range of 1300−1400 °C to evaluate the stress exponent (n value) and apparent deformation activation energy (Q value). With 0−8 wt.% TiO2 dopants, the n values and Q values of the TiO2-doped ZTA ceramics were calculated as 2−3 and 605−749 kJ/mol, respectively. Moreover, three grain boundary features were observed in these deformed materials, named the clean grain boundary, thin liquid phase grain boundary, and thick liquid phase grain boundary. Based on the deformation behavior and microstructure evolution, it was found that the lower apparent activation energy and higher strain rate of TiO2-doped ZTA ceramics are intensively related to the grain boundary feature.

Determining a desirable strain rate-temperature range for superplasticity and elongation-to-failure are critical concerns during the prediction of superplastic forming processes in α + β titanium-based alloys. This paper studies the superplastic deformation behaviour and related microstructural evolution of conventionally processed sheets of Ti-6Al-4V alloy in a strain rate range of 10−5−10−2 s−1 and a temperature range of 750−900 °C. Thermo-Calc calculation and microstructural analysis of the as-annealed samples were done in order to determine the α/β ratio and the grain size of the phases prior to the superplastic deformation. The strain rate ranges, which corresponds to the superplastic behaviour with strain rate sensitivity index m ˃ 0.3, are identified by step-by-step decreasing strain rate tests for various temperatures. Results of the uniaxial isothermal tensile tests at a constant strain rate range of 3 × 10−4−3 × 10−3 s−1 and a temperature range of 800−900 °C are presented and discussed. The experimental stress-strain data are utilized to construct constitutive models, with the purpose of predicting the flow stress behaviour of this alloy. The cross-validation approach is used to examine the predictability of the constructed models. The models exhibit excellent approximation and predictability of the flow behaviour of the studied alloy. Strain-induced changes in the grain structure are investigated by scanning electron microscopy and electron backscattered diffraction. Particular attention is paid to the comparison between the deformation behaviour and the microstructural evolution at 825 °C and 875 °C. Maximum elongation-to-failure of 635% and low residual cavitation were observed after a strain of 1.8 at 1 × 10−3 s−1 and 825 °C. This temperature provides 23 ± 4% β phase and a highly stable grain structure of both phases. The optimum deformation temperature obtained for the studied alloy is 825 °C, which is considered a comparatively low deformation temperature for the studied Ti-6Al-4V alloy.

This paper introduces an approach for modelling the flow behaviour of different titanium alloys (VT6, OT4-1 and VT14 alloys by Russian specifications) in superplastic deformation temperature and strain rate ranges. The initial microstructure parameters ( ${d}_{\alpha ,\beta },{V}_{\alpha ,\beta }$ ) before starting the deformation test were included in the constructed model for each alloy. The investigated alloys have different initial microstructures and flow behaviour characteristics. The isothermal uniaxial tensile deformation tests were performed at the superplastic deformation temperature and strain rate ranges of each alloy. The VT6, OT4-1 alloys were characterized by strain hardening effect during the deformation test, while VT14 alloy was characterized by strain softening effect. A comparison study between the experimental and modelled data was performed. The general equation of the constructed models was affected by the flow behaviour of the investigated alloys. The comparison results proved the good capability of the constructed models to predict the flow behaviour of the studied alloys. The correlation coefficient R was 0.98, 0.95 and 0.97 for VT6, OT4-1 and VT14, respectively. The predictability of the constructed model was assessed by the cross-validation technique, which ascertained the quality of the constructed model.

In this paper, the equiaxed superfine/nanocrystalline duplex PM-TiAl-based alloy with (γ + α2) microstructure, Ti-45Al-5Nb (at %), has been synthesized by high-energy ball milling and vacuum hot pressing sintering. Superplastic deformation behavior has been investigated at 1000 °C and 1050 °C with strain rates from 5 x 10-5 s-1 to 1 x 10-3 s-1. The effects of deformation on the microstructure and mechanical behaviors of high Nb containing TiAl alloy have been characterized and analyzed. The results showed that, the ultimate tensile strength of the alloy was 58.7 MPa at 1000 °C and 10.5 MPa at 1050 °C with a strain rate of 5 x 10-5 s-1, while the elongation was 121% and 233%, respectively. The alloy exhibited superplastic elongation at 1000 and 1050 °C with an exponent (m) of 0.48 and 0.45. The main softening mechanism was dynamic recrystallization of y grains; the dislocation slip and γ/γ interface twinning were responsible for superplastic deformation. The orientation relationship of γ/γ interface twinning obeyed the classical one: (001)γ//(110)γ. [ABSTRACT FROM AUTHOR]

Superplastic deformation of a carbon nanotube reinforced aluminum alloy matrix composite (CNT/6061 Al) fabricated through Flake powder metallurgy (Flake PM) was investigated using uniaxial tensile tests under different temperatures and different strain rates. The result showed that the maximum elongation at 853 K was 106% at strain rate of 4.17E−1 s −1 , while the maximum elongation at 673 K was 89% obtained at 4.17E−1 s −1 . Threshold stress was evident at lower temperature but insignificant at higher temperature. Strain rate sensitivity and apparent activation energy was determined to be 0.33 and 87.3 kJ/mol, respectively, indicating the grain boundary diffusion controlled mechanism was dominant for this material. With the increase of temperature, partial liquid phase appeared and the reaction of CNT was intensified, which indicated the superplastic deformation of CNT/6061 Al should be conducted at lower temperature (673 K). [ABSTRACT FROM AUTHOR]

High-strength and hard-deformable 7075 aluminum alloy in T6 state underwent KOBO extrusion at a temperature of 400°C and room temperature. After the rod's exit from the die, 2 cooling variants were applied, i.e. in water and in air. The influence of KOBO deformation (classified as an SPD method) on grain size was investigated using the EBSD technique, which additionally made it possible to determine the disorientation angle between grains. Test samples were prepared by the mechanical or electrochemical polishing technique. It was demonstrated that both methods of preparing metallographic specimens are adequate as they afford a clear diffraction picture. Nevertheless, preparation is somewhat more complex than in the case of preparing specimens for light microscopy. Samples for strength testing were made from material in its original state as well as from extruded rods, and these samples underwent tension at room temperature and 400°C. In the case of material after the KOBO process, elongation amounted to 247%, which suggests that a superplastic state was achieved. Despite having similar grain size, the material in its original state did not exhibit superplastic deformation, and elongation equaled 75%. This indicates other factors and microstructural elements other than grain size that enable achievement of superplastic deformation. After superplastic deformation of the alloy, its microstructure still contains grains with an approximately equiaxial shape despite the high deformation. The alloys microstructure after deformation at room temperature has a completely different appearance - grains are elongated, and colonies of approximately equiaxial grains are found between them. [ABSTRACT FROM AUTHOR]

This study theoretically investigates superplastic deformation and two typical coordination mechanisms of fine-grained magnesium (Mg) alloys. The interaction among grain boundary sliding, grain boundary migration and the fracture of particles during superplastic deformation is extensively discussed. The process of cooperative grain boundary sliding and migration is modelled with double wedge disclination dipoles. The whole-region stress field and the critical crack length are attained using the complex potential method for plane elasticity. The results show that the cracking of the particle itself is an effective coordination mechanism that can relax the intense stress concentration resulting from superplastic deformation based on grain boundary sliding; the farther the grain boundary slides and the higher the misorientation angle is, the more energetically the particle fractures; the higher the shear modulus ratio of the particle to Mg matrix and the smaller the particle radius, however, the larger is the resistance to crack nucleation and the weaker is the coordination effect based on the particle cracking; as a competitive coordination mechanism, cooperative migration consumes the local energy accumulated by the grain boundary sliding, toughening the particle-reinforced alloys for some relative orientations between the moving boundaries and the particle. [ABSTRACT FROM AUTHOR]