Superplastic Deformation Conditions Research Articles

Rational Methods of Plastic Deformation Providing Formation of Ultrafine-Grained Structure in Large-Sized Products

Based on the peculiarities of plastic deformation mechanics and physical mesomechanics, the processes are considered and technological schemes for their realization are proposed to ensure the formation of ultrafine-grained structure in axisymmetric products of large sizes. Deformation of coarse-grained materials in temperature-velocity conditions of superplastic deformation and methods of severe plastic deformation are taken as methods of preparation of such a structure. All of these deformations include large strain, and they are carried out at low hydraulic press speeds, but at different temperatures: in the mode of superplasticity at hot temperatures, and in the mode of severe plastic deformation at warm or cold temperature deformation. The first mode allows grains to be refined to microcrystalline sizes of 1–10 microns, and such a material acquires the ability to deform in a state of superplasticity, i.e., in tension with low resistance to deformation and high elongation. The second mode (severe plastic deformation) refines grains to submicro- (1÷0.1 µm) and nanosizes (less than 0.1 µm), thus giving metal materials record structural strength, as well as the possibility to use processing in extended temperature-velocity conditions of superplastic deformation in comparison with microcrystalline structure.

Morphology of fibrous structures formed in the course of superplastic deformation of the 01420T alloy with the original bimodal grain structure

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.

Особенности моделирования измельчения зерен металлов в условиях сверхпластического деформирования

Особенности моделирования измельчения зерен металлов в условиях сверхпластического деформирования

Change in elastic properties of eutectic alloys under conditions of superplastic deformation

In this study, experimental data have been obtained which reveal taking place of complex physical processes under conditions of the structural superplasticity not only in the grain boundary, as it is commonly believed, but also in the grain body. It was established for the first time that superplastic deformation of the eutectic alloys is accompanied by significant changes in their Young’s moduli. It was demonstrated that under superplastic deformation the observed changes of the elastic properties of the eutectic Sn–38wt%Pb alloy under study are caused mainly by the decomposition of supersaturated solid solutions on the basis of the alloy components and the relaxation of internal stresses. It was also found that the viscous dislocation–diffusion non-conservative flow is actively developing in the eutectic alloys under conditions of superplasticity, in contrast to the existing ideas about grain boundary sliding as the main mechanism of the matter transport. The experimental results obtained are important for the deeper understanding of the physical nature of the structural superplasticity effect.

Changes in the diffusion properties of nonequilibrium grain boundaries upon recrystallization and superplastic deformation of submicrocrystalline metals and alloys

The effect of an increase in the coefficient of the grain-boundary diffusion upon recrystallization and superplastic deformation of submicrocrystalline (SMC) materials prepared by severe plastic deformation has been studied. It is shown that the coefficient of the grain-boundary diffusion of the SMC materials is dependent on the intensity of the lattice dislocation flow whose value is proportional to the rate of the grain boundary migration upon annealing of SMC metals or the rate of the intragrain deformation under conditions of superplastic deformation of SMC alloys. It is found that, at a high rate of grain boundary migrations and high rates of superplastic deformation, the intensity of the lattice dislocation flow bombarding grain boundaries of SMC materials is higher than the intensity of their diffusion accommodation, which leads to an increase in the coefficient of the grain-boundary diffusion and a decrease in the activation energy. The results of the numerical calculations agree well with the experimental data.

New Technologies Development and Equipment for Local Shape-Forming of the Complicated Parts Made of Heat-Resistant Alloys under Superplastic Deformation Conditions

The results of development of new resource-saving technologies of local shape-forming under superplastic deformation conditions (SP) by means of roll-forming and rotary swaging are presented. These technologies are efficient for fabrication of discs, shells, rings up to 800 mm in diameter, as well as hollow shafts made of heat-resistant nickel-, titanium-and iron-based alloys that are used in aircraft engines and ground power installations. In particular, to implement innovative technologies a universal SRZhD-800 roll-forming mill has been developed for discs roll-forming made of heat-resistant alloys under SP. An efficiency of using of semi-finished products with prepared ultrafine grained (UFG) structure for generating precise complex geometry profile parts both homogeneous and regulated structure that changing in the radial direction of the disc and for obtaining high mechanical properties that are optimized taking operational conditions into account is proved. The technological process of rotary swaging under SP conditions has been developed for fabrication parts made of high alloy heat-resistant alloys using demountable mandrels. This method allows to obtain precise complex geometry profile parts such as disc with a thin cone flange which is characterized by homogeneous structure and high mechanical properties.

Enhanced superplasticity for (α + β)-hot rolled Ti–6Al–4V–0.1B alloy by means of dynamic globularization

Thermo-mechanically processed Ti–6Al–4V alloy, with (0.1wt.%) and without boron addition, has been subjected to tensile test under superplastic deformation conditions (Temperature, T=850°C and initial strain rate, ε̇=3×10-4s−1). The boron added alloy exhibited higher elongation (∼430%) in comparison to the base alloy without boron (∼365%). Superior ductility of the boron added alloy has been attributed to an enhanced α/β interfacial boundary sliding. This was caused by riotous dynamic globularization leading to the abundant presence of equiaxed primary α grains with refined sizes and narrow distribution in the deforming microstructure. Cavities do occur around TiB particles during deformation; the cavities are, however, extremely localized and do not cause macroscopic cracking.

Scientific fundamentals of high-efficiency roll forming technology for axially symmetrical parts of a gas-turbine engine rotor of high-temperature alloy

Fundamentals of the roll forming technology for disks of gas-turbine engines under super-plasticity conditions are stated. Because of deep underlying reasons of materials science, the technology provides a high process efficiency, equipment universality, the possibility of obtaining workpiece blanks with a high accuracy approaching that of the final part, and the development of structural states corresponding to the optimum complex of operational properties of parts [1, 2]. It is shown that a substantial role in roll-forming technology is played by superplastic deformation conditions, which, in contrast to the simple isothermal deformation, allow us to obtain in disks, either the homogeneous structure and isotropic properties, or the regulated gradient structure corresponding to actual temperature-force conditions of the operation of different parts of gas-turbine rotor disks.

APPLICATION OF ARTIFICIAL NEURAL NETWORK FOR PREDICTION OF SUPERPLASTIC BEHAVIOUR IN NiAl ALLOYS

Chemical composition, grain size, and processing conditions such as temperature and strain rate have important influence on superplasticity of NiAl alloys, which would allow the optimization of these parameters in order to achieve the desired combination of properties. In this work, the optimal superplastic deformation conditions of NiAl alloys were studied by using artificial neural networks (ANN). The standard multilayer feedforward networks were trained and tested using comprehensive datasets from previous experimentally works on the as-extruded NiAl, NiAl-25Cr, NiAl-20Fe-Y(Ce), NiAl-30Fe-Y, NiAl-9Mo, NiAl-P alloys. Different effects are modeled, including the influence of the alloying elements on the superplastic, and the influence of deformation temperature, strain rate and grain size on the elongations during the superplastic tensile tests. The artificial neural network models are combined with computer programmers for optimization of the inputs in order to achieve desirable combination of outputs. Good performances of the neural networks are achieved. Results of this research propose a range of strain rate and temperature within which the NiAl alloy possesses superplasticity with larger elongations, although the deformation temperature and strain rate of superplastic alloys alternately influence each other within the range. These models are convenient and powerful tools for practical applications in superplastic prediction in NiAl alloys.

Micro structure and texture evolution during super plastic deformation of Mg–Re extruded alloy

Microstructure and texture evolution during super plastic deformation of Mg–Gd–Y–Zr extruded rods were investigated in the study. For the purposes, the tensile tests under super plastic deformation condition were conducted at different strain levels. The static annealing tests were performed to reveal the micro structural evolution only influenced by thermal load. Grain structure analysis proves that initial grain refinement during the preheating stage is driven by store energy inherited from extrusion deformation. Microstructure and texture analysis suggests that deformation mechanisms are initial grain boundary sliding and subsequent slip creep. Moreover, the β phases (Mg5 (Gd, Y)) on (sub) grain boundaries hinder grain boundary sliding and continuous dynamic recrystallization. Macro-texture evolution shows that texture components varied according to the transition of deformation mechanisms. Micro-texture measurement using EBSD proves that β phase has undergone severe deformation before elongation-to-failure.

X-ray diffraction study of structural and phase states of a superplastic Sn-38 wt % Pb alloy and their variations under the effect of external mechanical stresses and aging

Structural and phase states of the eutectic alloy Sn-38 wt % Pb, which exhibits superplastic properties at room temperature, have been studied using samples that were cast, forged in a hydraulic press to ∼73%, aged in the as-cast state, as well as preliminarily forged samples and samples deformed in the regime of superplasticity. It has been shown that the phase state of the alloy in as-cast condition does not correspond to the equilibrium tin-lead phase diagram. The superplastic flow of the alloy occurs against the background of processes of decomposition of the supersaturated solid solution. In the preliminarily forged samples, the aging is accompanied by an increase in the concentration of lead in the surface layers. This is related, in particular, to the existence of tensile stresses in the tin-based phase and to their relaxation in the process of aging. In the structure of cast and preliminarily forged samples, an amorphous-crystalline component has been revealed, which most likely is located at grain boundaries. The experimental results agree well with the previously obtained results that are indicative of the nonequilibrium character of the initial structural phase state of the Sn-38 wt % Pb alloy and of the occurrence of structural-phase transformations under conditions of superplastic deformation, and substantially supplement them.

Gradient Microstructure of FeCr30Co8 Hard Magnetic Alloy Subjected to Plastic Deformation by Complex Loading

Magnetically hard Fe-Cr-Co-based alloys are distinguished by their good ductility, excellent magnetic properties and low cost. Their superior magnetic properties are obtained by magnetic treatment and multistage tempering, which results in spinodal decomposition of the solid solution into the isomorphous α1 and α2 phases. However, the α1+α2 microstructure causes a reduction in the plasticity and strength of the material. It can often be advantageous for permanent magnets to maintain fine magnetic properties throughout their volume along while retaining good mechanical properties only in the subsurface layer. To improve the mechanical properties of the latter, FeCr30Co8 samples were deformed in tension combined with torsion. Loading was applied at 750°C, which ensured that the conditions for superplastic deformation were fulfilled. Here, we present the results of microstructure investigations of the samples treated in the aforementioned manner. Observations of the longitudinal section of the samples showed the formation of a gradient microstructure with the maximum grain refinement in the surface layer and the characteristic rotation of the elongated α phase grains from positions nearly perpendicular to the tension axis at the surface to positions tilted at approximately 45º to the tension axis inside the material. Deformation at superplastic conditions also activated precipitation of the σ intermetallic phase, particularly in the areas of highest deformation. The refinement of the microstructure and precipitation of the σ-phase resulted in a significant increase in hardness at the surface of the FeCr30Co8 samples.

Compression Superplasticity of Ultrahigh Carbon Steel in Electric Field

Taking 1.4%C and 1.6%C ultrahigh carbon steels (UHCSs) with ultra-fine structures developed through thermo-mechanical processing as research objects, the influences of superplastic deformation conditions and electric field on compression superplasticity were conducted without electric field and with electric field. The experimental results indicate that 1.4%C and 1.6%C UHCSs have respectively superplasticity or electro-superplasticity under the conditions of deformation temperature of 800°C, the initial strain rate of 0.75-3.75×10-4 s-1 and deformation temperature of 780 °C, the initial strain rate of 0.5-5.0×10-4 s-1 and the suitable external electric field. Its strain rate sensitivity are respectively 0.32 and 0.46.When the electric field intensity of 3 kV/cm in the manner of the specimen connected to positive terminal, the steady flow stress of 1.6%C UHCS decreases more than 10.5% as compared with electric field intensity of 0 kV/cm.

High temperature compressive deformation behavior of an extruded Mg–8Gd–3Y–0.5Zr (wt.%) alloy

The deformation behavior of an extruded Mg–8Gd–3Y–0.5Zr alloy has been characterized by compression tests in the temperature range of 400–490°C and strain rate range of 10−4s−1 to 10−2s−1 with the objective to evaluate the superplastic deformation conditions as well as the rate-controlling mechanisms. The strain rate sensitivity index m decreases with increasing temperature in the strain rate range of 10−4s−1 to 5×10−4s−1, which was attributed to grain growth. Optimum superplasticity characterized by grain boundary sliding accommodated by dislocation creep was obtained at 400°C and at 10−4s−1. Dynamic precipitation leading to precipitates distributed mainly in the grain boundaries was responsible for the stable microstructure and the increase of the stress during deformation under this condition. Moreover, at strain rate higher than 5×10−4s−1, it remained relatively constant of about m≈0.3 and the activation energy Q was found to be equal to 131kJ/mol which was close to the activation energy for lattice diffusion of Mg.

Superplastic failure mode in ultrafine grained magnesium alloy AZ31

Abstract The paper addresses the failure mode of ultrafine grained magnesium alloy AZ31 under superplastic deformation conditions. The ultrafine grain structure and the resulting exceptional superplastic properties were produced by Equal Channel Angular Pressing with an imposed hydrostatic pressure conducted at a relatively low temperature of 150 °C. The mechanism of the superplastic ductility was studied by examining the fracture surfaces and internal cavities in superplastically deformed specimens by scanning electron microscopy. The presence of Mg17Al12 particles was identified as the controlling microstructural feature giving rise to porosity formation and limiting tensile ductility. Fine details of the failure process depending on the particle and precipitate structure as well as textural effects were described.

Structure and mechanical properties of strips and shapes from ultrafine-grained aluminum alloy 1421

The structure and mechanical properties of high-strength shapes from ultrafine-grained aluminum alloys for the aircraft industry are studied. The initial material is commercial alloy 1421 in coarse-crystal state. An ultrafine-grained structure is formed in preforms 40 mm in diameter and 160 mm long by equichannel angular pressing at 370°C with degree of accumulated strain e ≥ 10. Strips with ultrafine-grained structure with a thickness of 2 mm are fabricated by rolling with e = 2.7 at 350°C. The strips are used for making shapes with Π-section under conditions of superplastic deformation.

Influence of Microstructure Change on the Superplastic Properties of Ti<sub>3</sub>Al Base Alloy

The superplastic properties of Ti3Al base alloy have been investigated in the range of 900~1020°C and strain rate range of 3.3×10-4~3.3×10-2s-1 .The largest elongation of 1570% was obtained under the optimal superplastic deformation condition of 980°C and 3.3×10-4 s-1. The m-value varied from 0.46 to 0.59, its maximum value was obtained at 980°C.The microstructure of the alloy after heat treatment was composed of two phases of small spherical α2 phase particles distributing in βtransus matrix. Microstructure change has an obvious effect on the superplastic properties of Ti3Al-based alloy. At 980°C,when theα2 phase grain size changed from large to small then large and the cavitations in the fracture surface were larger and deeper with decreasing strain rate, the alloy showed the larger elongation and the lower flow stress.

Superplastic deformation of Ti3Al based alloy

Superplastic deformation behaviours of Ti3Al based alloy at high temperature have been investigated by using the non-constant strain rate electronic tensile testing apparatus. The results show that the effect of strain rate on the deformation ability of the alloy at high temperature is more significant than that of the temperature. When the alloy is deformed at lower temperature and higher strain rate, the large grain occurs necking and the small grain occurs sliding along grain boundary. Under the optimal superplastic deformation condition, the main superplastic deformation mechanism is the grain boundary sliding accompanying the dynamic recrystallisation process, which can supply the sliding and deformation sources for the deformed alloy. At higher temperature (1020°C), the superplastic deformation of the alloy consists of grain boundary sliding and the homogeneous extensions of the soft phase as well as the hard phase during the superplastic deformation.

The Activation Energy of Superplastic Flow above the Critical Temperature for Steel CrWMn

By means of superplastic tensile test above the Ac1 temperature (γ→α transformation temperature), superplastic deformation activation energy of ultrafine-grained commercial die steel CrWMn is investigated on the basis of the Arrhenius theory equation, exp( / ) 1 ε& = Aσ m −Q RT , which indicates the resistance of the superplastic deformation. According to the Arrhenius equation, the activation energy is estimated from the log σt vs 1/T relationship at a constant of sensitivity index of strain rate. The results show that the strain rate sensitivity index is a constant and rather high at the conditions of superplastic deformation for the CrWMn steel, the activation energy for superplastic deformation of steel CrWMn above the critical temperature is 187KJ/mol, and the superplastic deformation activation energy is approached to the grain boundary diffusion activation energy of γ-Fe. This indicates that the grain boundary sliding (GBS) in superplastic deformation of CrWMn steel is controlled by grain boundary diffusion. The characters of superplastic deformation of the steel above the critical temperature, on the other hand, are also analyzed in this paper.

Isothermal Compressive Superplastic Deformation and its Constitute Equation of Structural Steel and Tool Steel

The microstructure of structural steel (40Cr) and tool steel (T10A) samples and its surface are respectively fined through salt-bath cyclic quenching (SCQ) and high frequency hardening (HFH), then the superplasticity is studied under isothermal compressive superplastic deformation condition. The experimental results indicate that the true stress-strain curves of 40Cr and T10A steel are separately shown to take place obvious superplastic flow characteristic at the temperature of 730-770 °C 710-770°C and at the initial strain rate of 1.7-50×10 -4 s -1 2.5-8.3×10 -4 s -1 , its strain rate sensitivity are respectively 0.30-0.38 and 0.32-0.50, the steady superplastic flow stress are respectively 60-70 MPa and 35-45 MPa, the superplastic flow activation energy are respectively 198-217 KJ/mol and 182-195 KJ/mol, and they are close to α-Fe grain boundary self-diffusion activation energy. The compressive superplastic constitute equations of both steels are correspondingly set up. Due to the finer microstructure of HFH, it appears higher strain rate sensitivity value, smaller the steady superplastic flow stress and the superplastic flow activation energy, so it has better capability of superplastic deformation.