Showing papers on "Superplasticity published in 2010"

Ferrous Polycrystalline Shape-Memory Alloy Showing Huge Superelasticity

Abstract: Shape-memory alloys, such as Ni-Ti and Cu-Zn-Al, show a large reversible strain of more than several percent due to superelasticity. In particular, the Ni-Ti-based alloy, which exhibits some ductility and excellent superelastic strain, is the only superelastic material available for practical applications at present. We herein describe a ferrous polycrystalline, high-strength, shape-memory alloy exhibiting a superelastic strain of more than 13%, with a tensile strength above 1 gigapascal, which is almost twice the maximum superelastic strain obtained in the Ni-Ti alloys. Furthermore, this ferrous alloy has a very large damping capacity and exhibits a large reversible change in magnetization during loading and unloading. This ferrous shape-memory alloy has great potential as a high-damping and sensor material.

Electron-beam-assisted superplastic shaping of nanoscale amorphous silica.

Abstract: Glasses are usually shaped through the viscous flow of a liquid before its solidification, as practiced in glass blowing. At or near room temperature (RT), oxide glasses are known to be brittle and fracture upon any mechanical deformation for shape change. Here, we show that with moderate exposure to a low-intensity ( 10(-4) per second. We show not only large homogeneous plastic strains in compression for nanoparticles but also superplastic elongations >200% in tension for nanowires (NWs). We also report the first quantitative comparison of the load-displacement responses without and with the e-beam, revealing dramatic difference in the flow stress (up to four times). This e-beam-assisted superplastic deformability near RT is useful for processing amorphous silica and other conventionally-brittle materials for their applications in nanotechnology.

Enhanced Sintering Rate of Zirconia (3Y‐TZP) Through the Effect of a Weak dc Electric Field on Grain Growth

Abstract: We show, for the first time, that a dc electric field of 20 V/cm shifts the densification curve to a lower temperature in constant heating rate experiments with yttria-stabilized tetragonal zirconia powder (3Y-TZP). The enhanced sintering rate is ascribed, at least in part, to the reduced rate of grain growth under the applied field, consistent with earlier experiments on the influence of such fields on grain size in superplastic deformation and isothermal grain growth in zirconia polycrystals.

High Temperature Deformation and Fracture of Materials

Abstract: Part 1 High temperature deformation: Creep behaviour of materials Evolution of dislocation substructures during creep Dislocation motion at elevated temperatures Recovery - creep theories of pure metals Creep of solid solution alloys Creep of second phase particles strengthened materials Creep of particulates reinforced composite material High temperature deformation of intermetallic compounds Diffusional creep Superplasticity Mechanisms of grain boundary sliding Multiaxial creep models. Part 2 High temperature fracture: Nucleation of creep cavity Creep embrittlement by segregation of impurities Diffusional growth of creep cavities Cavity growth by coupled diffusion and creep Constrained growth of creep cavities Nucleation and growth of wedge - type microcracks Creep crack growth Creep damage mechanics Creep damage physics Prediction of creep rupture life Creep - fatigue interaction Prediction of creep - fatigue life Environmental damage at high temperature.

Mantle superplasticity and its self-made demise

Abstract: The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics. Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth's lower mantle. In materials science, superplasticity is confirmed when the material deforms with large tensile strain without failure; however, no experimental studies have yet shown this characteristic in geomaterials. Here we show that polycrystalline forsterite + periclase (9:1) and forsterite + enstatite + diopside (7:2.5:0.5), which are good analogues for Earth's mantle, undergo homogeneous elongation of up to 500 per cent under subsolidus conditions. Such superplastic deformation is accompanied by strain hardening, which is well explained by the grain size sensitivity of superplasticity and grain growth under grain switching conditions (that is, grain boundary sliding); grain boundary sliding is the main deformation mechanism for superplasticity. We apply the observed strain-grain size-viscosity relationship to portions of the mantle where superplasticity has been presumed to take place, such as localized shear zones in the upper mantle and within subducting slabs penetrating into the transition zone and lower mantle after a phase transformation. Calculations show that superplastic flow in the mantle is inevitably accompanied by significant grain growth that can bring fine grained (≤1 μm) rocks to coarse-grained (1-10 mm) aggregates, resulting in increasing mantle viscosity and finally termination of superplastic flow.

Superplasticity, dynamic grain growth and deformation mechanism in ultra-light two-phase magnesium–lithium alloys

Abstract: The maximum superplasticity of 850% and 920% were demonstrated in Mg–7.83Li alloy and Mg–8.42Li alloy at 573 K and at an initial strain rate of 1.67 × 10−3 s−1 and at an initial strain rate of 5 × 10−4 s−1, respectively. Optical microstructural observations revealed that dynamic grain growth appears at 573 K. Different atomic mobilities of Mg, Li, alpha 5.7Li phase and beta(11Li) phase and different Gibbs free energies of alpha 5.7Li phase and beta(11Li) phase contribute to the obvious dynamic grain growth at 573 K. The dynamic grain sizes at 573 K were calculated by derived model. It is shown by comparing the stress exponent (n), grain size exponent (p), and deformation activation energy (Q) estimated experimentally with constructed deformation mechanism map incorporating dislocation quantities inside grains that the dominant deformation mechanisms in two alloys are grain boundary sliding accommodated by slip controlled by lattice diffusion.

Formability of Al 5xxx Sheet Metals Using Pulsed Current for Various Heat Treatments

Abstract: Previous studies have shown that the presence of a pulsed electrical current, applied during the deformation process of an aluminum specimen, can significantly improve the formability of the aluminum without heating the metal above its maximum operating temperature range. The research herein extends these findings by examining the effect of electrical pulsing on 5052 and 5083 Aluminum Alloys. Two different parameter sets were used while pulsing three different heat treatments (As Is, 398°C, and 510°C) for each of the two aluminum alloys. For this research, the electrical pulsing is applied to the aluminum while the specimens are deformed, without halting the deformation process (a manufacturing technique known as Electrically-Assisted Manufacturing). The analysis focuses on establishing the effect the electrical pulsing has on the aluminum alloy’s various heat treatments by examining the displacement of the material throughout the testing region of dogbone-shaped specimens. The results from this research show that pulsing significantly increases the maximum achievable elongation of the aluminum (when compared to baseline tests conducted without electrical pulsing). Another beneficial effect produced by electrical pulsing is that the engineering flow stress within the material is considerably reduced. The electrical pulses also cause the aluminum to deform non-uniformly, such that themore » material exhibits a diffuse neck where the minimum deformation occurs near the ends of the specimen (near the clamps) and the maximum deformation occurs near the center of the specimen (where fracture ultimately occurs). This diffuse necking effect is similar to what can be experienced during superplastic deformation. However, when comparing the presence of a diffuse neck in this research, electrical pulsing does not create as significant of a diffuse neck as superplastic deformation. Electrical pulsing has the potential to be more efficient than traditional methods of incremental forming since the deformation process is never interrupted. Overall, with the greater elongation and lower stress, the aluminum can be deformed quicker, easier, and to a greater extent than is currently possible.« less

Numerical simulation and experimentation of micro scale laser bulge forming

Abstract: Micro scale laser bulge forming is a high strain rate micro-forming technique, which employs the shock wave pressure induced by a laser pulse to deform thin metals to 3D configurations. The process holds promise for fast setup, well-controlled, high efficiency and precision of fabrication of complex miniaturized devices. In this paper, micro scale laser bulge forming of pure copper was investigated using both numerical and experimental methods. A finite element model was proposed to simulate the dynamic deformation of the shocked material, and the simulation results were validated by experiments. With the verified model, the deformation mechanism of the transient forming process was characterized through the evolution of equivalent plastic strain rate and the distributions of strains and residual stresses. In addition, the effects of laser energy, die diameter and sample thickness on the forming behavior of copper foils were studied. The experimental and numerical simulation results show that with an increase of sample thickness, the deformation depth decreases, while it becomes larger as the enhancement of laser energy. The shock forming process is not very sensitive to the diameter of the die, indicating that it is feasible for micro products in a wide dimension range.

Fundamentals and Engineering of Severe Plastic Deformation

Abstract: Continuum Approach to Severe Plastic Deformation (SPD) Processes of Severe Plastic Deformation Mechanics of Equal-Channel Angular Extrusion (ECAE) ECAE Processing Applications of ECAE Fundamentals of Poly-Crystal Modeling Models for Texture Development Modeling Single & Multiple Passes in ECAE Texture Characterization Texture Evolution during ECAE Deviation from Simple Shear Temperature Texture Evolution in Route C Nomenclature Deformation Twinning & SPD Mechanical Response of SPD'ed Metals Non-Equilibrium Grain Boundaries Dislocations in Nano-Microcrystalline(NMC) Metals Vacancies in NMC Metals Diffusion Properties of Grain Boundaries in NMC Metals Limit of Grain Refinement during ECAE Recrystallization & Grain Growth in Pure NMC Metals Recrystallization & Grain Growth in NMC Metals Optimum Grain Size for StructuralSuperplasicity Strain Hardening during Superplasticity Yield Stress of NMC Metals at RoomTemperature Strength & Ductility of NMC Metals at Room Index.

Investigation on hot deformation behavior of P/M Ni-base superalloy FGH96 by using processing maps

Abstract: The hot deformation behavior of a powder metallurgy (P/M) Ni-base superalloy FGH96 was investigated at the temperature (T) of 1050–1140 °C, strain rate ( e ˙ ) of 0.002–1.0 s−1, and the height reduction of 20–50% via isothermal hot compression experiment. The flow behavior and the microstructural mechanism of the synthesized superalloy were systematically studied. The processing map was constructed based on the experiment data to evaluate the efficiency of power dissipation and recognize the instability regimes for forging process determination. The processing map with the strain of 0.15 exhibits a smooth and small variation of η and predicts a little instability regime confined at (T: 1050 °C, e ˙ : 1.0 s − 1 ), attributed to the previous particle boundary (PPB) cracks. The map obtained at the strain of 0.65 reveals a large domain with the efficiency above 40% and predicts two instability regimes at around (1050 °C, 1 s−1) and located in the regime of (T: 1080–1140 °C, e ˙ : 0.002 − 0.01 s − 1 ), respectively. The optimum processing condition is identified as (T: 1140 °C, e ˙ : 1.0 s − 1 ), which presented the highest efficiency in hot working process, as well as the obtained fine microstructure after the hot processing. Via microstructure characterization and observation, it is concluded that the dynamic recrystallization (DRX) occurred at the PPBs in the P/M superalloy. The DRX and the strain rate sensitivity exponent m are both affected by true strain. The local DRX presents a relatively low m in a small strain compression. With an increase of strain, the fraction of the recrystallized grains increases with m. The global DRX, however, presents a relatively high m in a large strain compression. It is thus indicated that the superplasticity of FGH96 superalloy can be realized at a slow strain rate such as 0.001 s−1 and the deformation temperature of 1110 °C, near to the γ′-transus temperature, by using the uniformly fine-grained superalloys as well as the high temperature superplastic forming equipment.

Laser-Induced High-Strain-Rate Superplastic 3-D Microforming of Metallic Thin Films

Abstract: Microforming of metals has always been a challenge because of the limited formability of metals at the microscale. This paper investigates an innovative microforming technique: microscale laser dynamic forming (?LDF), which induces 3-D superplastic forming in metallic thin films. This forming process proceeds in a sequence of laser irradiation and ionization of ablative coating, shockwave generation and propagation in metallic thin films, and conformation of metallic thin films to the shape of microscale molds. Because the deformation proceeds at ultrahigh strain rates, it is found that materials experience superplastic deformation at the microscale. In this paper, experiments are systematically carried out to understand the deformation characteristics of ?LDF. The topologies and dimensions of the deformed samples are characterized by scanning electron microscopy and optical profilometry. The thickness variations are characterized by slicing the cross section of the deformed material using the focused ion beam. The magnitude of deformation depth in ?LDF is determined primarily by three critical factors: film thickness, mold geometry, and laser intensity. The relationships between these factors are explored in process maps to find suitable processing conditions for ?LDF. Nanoindentation tests are conducted to show that the strength of the thin films is increased significantly after ?LDF.

Low-temperature superplasticity and coarsening behavior of Ti–6Al–2Sn–4Zr–2Mo–0.1Si

Abstract: The low-temperature superplasticity and dynamic coarsening behavior of Ti–6Al–2Sn–4Zr–2Mo–0.1Si were established and interpreted in the context of inelastic-deformation theory. The starting microstructure with an equiaxed-alpha particle size of 13 μm was refined to 2.2 μm by a treatment comprising beta annealing/water quenching followed by warm rolling at 775 °C. A series of tension and load-relaxation tests were carried out for the coarse and fine microstructures at strain rates of 10 −4 to 10 −2 s −1 in the temperature range of 650–750 °C. The fine microstructure exhibited enhanced superplasticity (382–826% elongation) compared to that of the coarse microstructure (189–286% elongation); this trend was attributed to a larger fraction of boundary sliding and lower friction stress for the finer material. With respect to microstructure evolution, the coarsening rate of the alpha particles during deformation was ∼12 times faster than that during static coarsening. Furthermore, both the static and dynamic coarsening rates for the Ti–6Al–2Sn–4Zr–2Mo–0.1Si were 2.7–4.7 times lower than those for the Ti–6Al–4V, a trend attributable to the lower diffusivity of Mo compared to that of V at a given test temperature. The plastic flow behavior of the coarse and fine microstructures was rationalized in terms of microstructure evolution during deformation.

Superplastic behaviour of Al–Zn–Mg–Cu–Zr alloy AA7010 containing Sc

Abstract: The influence of small addition of Sc on the superplastic potential of Al–Zn–Mg–Cu–Zr alloy AA 7010 has been examined. The alloy was subjected to a three-step thermo-mechanical process (TMP) and the resultant material (having essentially an unrecrystallized grain structure) was subjected to a strain rate–temperature combination of 1.9 × 10 −2 s −1 , 475 °C to develop recrystallized grain structure amenable to superplastic deformation. This resulted in a total elongation of 650%.

Material Models for Simulation of Superplastic Mg Alloy Sheet Forming

Abstract: Accurate prediction of strain fields and cycle times for fine-grained Mg alloy sheet forming at high temperatures (400-500 °C) is severely limited by a lack of accurate material constitutive models. This paper details an important first step toward addressing this issue by evaluating material constitutive models, developed from tensile data, for high-temperature plasticity of a fine-grained Mg AZ31 sheet material. The finite element method was used to simulate gas pressure bulge forming experiments at 450 °C using four constant gas pressures. The applicability of the material constitutive models to a balanced-biaxial stress state was evaluated through comparison of simulation results with bulge forming data. Simulations based upon a phenomenological material constitutive model developed using data from both tensile elongation and strain-rate-change experiments were found to be in favorable accord with experiments. These results provide new insights specific to the construction and use of material constitutive models for hot deformation of wrought, fine-grained Mg alloys.

Towards enhancement of properties of UFG metals and alloys by grain boundary engineering using SPD processing

Abstract: Nanostructuring of metals and alloys by severe plastic deformation techniques is an effective way of enhancing their mechanical and functional properties. The features of the nanostructured materials produced by severe plastic deformation are stipulated by forming of ultrafine-sized grains as well as by the state of grain boundaries. The concept of grain boundary engineering of ultrafine-grained metals and alloys is developed for enhancement of their properties by tailoring grain boundaries of different types (low-angle and high-angle ones, special and random, equilibrium and nonequilibrium) and formation of grain boundary segregations and precipitations by severe plastic deformation processing. In this article, using this approach and varying regimes and routes of severe plastic deformation processing, we show for several light alloys (Al and Ti) the ability to produce ultrafine-grained materials with different grain boundaries, and this can have a drastic effect on the mechanical behavior of the processed materials. This article demonstrates also several new examples of attaining superior strength and ductility as well as enhanced superplasticity at low temperatures and high strain rates in various ultrafine- grained metals and alloys.

Superplasticity and Grain Boundaries in Ultrafine-Grained Materials

Abstract: Structural superplasticity of polycrystalline materials Characteristics of the ensemble of grain boundaries Orientation distributed parameters of the polycrystalline structure Experimental investigation of the ensembles of grain boundaries in polycrystals Grain boundary sliding in metallic bi- and tricrystals Percolation mechanism of deformation processes in ultrafine-grained polycrystals Percolation processes in a network of grain boundaries in ultrafine-grained materials Microstructure and grain boundary ensembles in ultrafine-grained materials Grain boundary processes in ultrafine-grained and nanostructured nickel Duration of the stage of stable flow in superplastic deformation Determination of the main relationships in the conditions of multicomponent loading.

Effects on the Surface Texture, Superplastic Forming, and Fatigue Performance of Titanium 6AL-4V Friction Stir Welds

Abstract: The speed and feed effects of the friction stir welding (FSW) process on the surface texture along the top of a butt welded nugget were studied. The tests were conducted using fine grain (0.8-2 μm) titanium alloy 6Al-4V with a nominal thickness of 2.5 mm. It was shown that the pin tool marks along the top surface of the weld can be highly detrimental to both the superplastic forming (SPF) characteristics and the fatigue performance of welded panels. Removing the marks by machining the top surface after FSW was found to eliminate the predominant tearing of the weld during SPF and most of the fatigue life of across the weld was also restored. Through additional development of the FSW process parameters, the butt welded nugget was made to have equivalent SPF characteristics as the parent sheet material. By using a water-cooled pin tool and other cooling techniques, it is believed that the weld zone can be kept below the beta transus temperature during FSW, which enables the formation of a grain structure that is uniquely conducive to superplastic behavior, when compared to conventional fusion welding processes.

Superplasticity and ductile fracture of synthetic feldspar deformed to large strain

Abstract: [1] We performed high-strain torsion experiments on fine-grained (≈4 μm) anorthite aggregates in a Paterson-type gas deformation apparatus. The dense hydrous (≈0.1 wt% H2O) samples contain < 3 vol% Si-enriched residual glass located at triple junctions. Specimens were twisted at constant rate to a maximum shear strain of about 5 at experimental conditions of 100–400 MPa confining pressure, temperatures of 950°C–1200°C, and shear strain rates of ≈2 × 10−5, 5 × 10−5, and 2 × 10−4 s−1. Resulting maximum shear stresses at the sample periphery were in the range of ≈2–80 MPa. The samples showed strain hardening at slow deformation rate and strain weakening at fast strain rate, respectively. Fitting the stress strain rate data to a power law yields linear viscous behavior. Microstructural analysis shows locally enhanced dislocation density, suggesting diffusion-assisted and/or dislocation-assisted grain boundary sliding as the dominant deformation mechanism. Deformed samples exhibit abundant cavities, nucleated mostly at grain triple junctions and at grain boundaries in response to cooperative grain boundary sliding. At shear strains ≥ 2, growth and coalescence of the cavities form an anastomozing network of regularly spaced strings oriented at about 30° to the direction of maximum compressive stress and ∼15° to the shear plane. Strain is localized along these shear bands associated with a shape-preferred orientation of high-aspect ratio feldspar grains and with segregation of initial pore fluids (residual glass) into the bands. More than one third of the samples were deformed to terminal failure that occurred suddenly at shear strains of ≈3–5. The experiments indicate that cavitation damage may facilitate fluid flow and deep seismicity in highly strained shear zones.

Plastic Flow and Microstructure Evolution during Low-Temperature Superplasticity of Ultrafine Ti-6Al-4V Sheet Material

Abstract: The low-temperature superplastic (SP) flow behavior of two lots of Ti-6Al-4V sheet, each with an ultrafine microstructure, was established by performing tension tests at temperatures of 775 °C and 815 °C and true strain rates of 10−4 and 10−3 s−1. The as-received microstructures of the two materials comprised either equiaxed or slightly elongated alpha particles in a beta matrix. The material with equiaxed alpha particles exhibited flow hardening, which was correlated with concurrent (dynamic) coarsening. The rate of dynamic coarsening was rationalized in terms of static coarsening measurements and the enhancement of kinetics due to pipe diffusion. By contrast, the material with initially elongated alpha particles exhibited comparable flow hardening at the lower strain rate but a complex, near-steady-state behavior at the higher strain rate. These latter observations were explained on the basis of the evolution of the alpha particle shape and size during straining; dynamic coarsening or dynamic spheroidization was concluded to be most important at the lower and higher strain rates, respectively. The plastic flow behavior was interpreted in the context of a long-wavelength flow localization analysis.

Deformation and Strain Storage Mechanisms during High-Temperature Compression of a Powder Metallurgy Nickel-Base Superalloy

Abstract: High-temperature compression testing combined with high-resolution electron backscatter diffraction (EBSD) analysis has been used to study microstructural-scale straining processes that occur during high-temperature deformation of a powder-consolidated nickel-base superalloy, Rene 88DT (GE Aviation, Evendale, OH). Orientation imaging has been employed to study grain-level straining and strain storage at temperatures between 1323 K (1050 °C) and 1241 K (968 °C) for strain rates between 0.1/s and 0.00032/s at nominal strain levels between 0.1 and 0.7. Two distinct deformation mechanisms were observed. At strain rates below 0.01/s, superplastic deformation dominates, while power-law creep occurs during high rate compression. Stored strain and evolution of the grain structure during deformation are dependent on strain rate during compression. At low strain rates in the superplastic regime, low levels of stored strain and some grain growth are observed. At high strain rates, dynamic recrystallization occurs along with higher levels of stored strain within selected grains, particularly those at the high end of the grain size distribution. A constitutive model for superplastic deformation was employed to predict the temperature and strain rate dependence of the transition from superplastic to power law deformation. The transition in rate sensitivity was consistent with the transition in stored strain measured by EBSD. Superplasticity-enhanced grain growth is observed and the implications for the transition in deformation mechanisms are discussed.

Blow forming of AZ31 magnesium alloy at elevated temperatures

Abstract: In this work, the forming behaviour of a commercial sheet of AZ31B magnesium alloy at elevated temperatures is investigated and reported. The experimental activity is performed in two phases. The first phase consists in free bulging test and the second one in analysing the ability of the sheet in filling a closed die. Different pressure and temperature levels are applied. In free bulging tests, the specimen dome height is used as characterizing parameter; in the same test, the strain rate sensitivity index is calculated using an analytical approach. Thus, appropriate forming parameters, such as temperature and pressure, are individuated and used for subsequent forming tests. In the second phase, forming tests in closed die with a prismatic shape cavity are performed. The influence of relevant process parameters concerning forming results in terms of cavity filling, fillet radii on the final specimen profile are analysed. Closed die forming tests put in evidence how the examined commercial magnesium sheet can successfully be formed in complicated geometries if process parameters are adequately chosen.