Showing papers on "Superplasticity published in 2005"

Grain refinement and superplastic flow in an aluminum alloy processed by high-pressure torsion

Abstract: Disks of an Al–3% Mg–0.2% Sc alloy were processed by high-pressure torsion (HPT) to refine the grain size to ∼0.15 μm. Inspection of the disks after processing revealed a central core region having a relatively coarse and ill-defined microstructure. The size of this core region decreased with increasing numbers of turns in HPT. Measurements showed the hardness increased with increasing applied pressure and/or increasing numbers of turns. In addition, the hardness increased with increasing distance from the center of the disk and stabilized at distances greater than ∼2–3 mm. The values of the saturation hardness in the outer regions of the disks were similar at higher applied pressures and after larger numbers of turns. This saturation hardness was ∼3× the hardness in the solution-treated condition. Within the region of hardness saturation, the microstructure was reasonably homogeneous and consisted of ultrafine grains separated by high-angle grain boundaries. Tensile testing demonstrated the occurrence of high strain rate superplasticity after HPT with elongations to failure that were similar to those obtained in samples of the same alloy processed by equal-channel angular pressing (ECAP).

A lead-free high-Curie-point ferroelectric ceramic, CaBi2Nb2O9

Abstract: Nano-powders of BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3, a mixture of the composition (BaTiO3)0.6(SrTiO3)0.4 with particle sizes in the range of 60 to 80 nm, and Bi4Ti3O12 with an average particle size of 100 nm were consolidated by spark plasma sintering (SPS). The kinetics of reaction, densification and grain growth were studied. An experimental procedure is outlined that allows the determination of a “kinetic window” within which dense nano-sized compacts can be prepared. It is shown that the sintering behaviour of the five powders varies somewhat, but is generally speaking fairly similar. However, the types of grain growth behaviour of these powders are quite different, exemplified by the observation that the kinetic window for the (BaTiO3)0.6(SrTiO3)0.4 mixture is 125 oC, ~75 oC for Bi4Ti3O12, ~25oC for BaTiO3 and SrTiO3, while it is hard to observe an apparent kinetic window for obtaining nano-sized compacts of Ba0.6Sr0.4TiO3. During the densification of the (BaTiO3)0.6(SrTiO3)0.4 mixture the reaction 0.6BaTiO3+0.4SrTiO3 → Ba0.6Sr0.4TiO3 takes place, and this reaction is suggested to have a self-pinning effect on the grain growth, which in turn explains why this powder has a large kinetic window. Notably, SPS offers a unique opportunity to more preciously investigate and monitor the sintering kinetics of nano-powders, and it allows preparation of ceramics with tailored microstructures.The dielectric properties of selected samples of (Ba, Sr)TiO3 ceramics have been studied. The results are correlated with the microstructural features of these samples, e.g. to the grain sizes present in the compacts. The ceramic with nano-sized microstructure exhibits a diffuse transition in permittivity and reduced dielectric losses in the vicinity of the Curie temperature, whereas the more coarse-grained compacts exhibit normal dielectric properties in the ferroelectric region.The morphology evolution, with increasing sintering temperature, of bismuth layer-structured ferroelectric ceramics such as Bi4Ti3O12 (BIT) and CaBi2Nb2O9 (CBNO) was investigated. The subsequent isothermal sintering experiments revealed that the nano-sized particles of the BIT precursor powder grew into elongated plate-like grains within a few minutes, via a dynamic ripening mechanism.A new processing strategy for obtaining highly textured ceramics is described. It is based on a directional dynamic ripening mechanism induced by superplastic deformation. The new strategy makes it possible to produce a textured microstructure within minutes, and it allows production of textured ferroelectric ceramics with tailored morphology and improved physical properties.The ferroelectric, dielectric, and piezoelectric properties of the textured bismuth layer-structured ferroelectric ceramics have been studied, and it was revealed that all textured samples exhibited anisotropic properties and improved performance. The highly textured Bi4Ti3O12 ceramic exhibited ferroelectric properties equal to or better than those of corresponding single crystals, and much better than those previously reported for grain-orientated Bi4Ti3O12 ceramics. Textured CaBi2Nb2O9 ceramics exhibited a very high Curie temperature, d33-values nearly three times larger than those of conventionally sintered materials, and a high thermal depoling temperature indicating that it is a very promising material for high-temperature piezoelectric applications.

The superplastic forming of bulk metallic glasses

Abstract: Superplastic forming (SPF) is introduced in this article as a net-shape processing method for bulk metallic glasses (BMGs), commercially known as Liquidmetal® alloys. This method decouples fast cooling and forming of the BMG. Forming takes place in the supercooled liquid region, where the BMG exists as a highly viscous liquid and increases its fluidity with increasing temperature. The SPF method is very similar to techniques used for processing thermoplastics. In this work, a simple flow law is used to quantify the forming ability and to estimate both the potential and the limitations of the SPF method. This process is especially well suited to replicate small features and thin sections with high aspect ratios, which makes this process appropriate for microelectromechanical systems, nano- and microtechnology, jewelry, medical and optical applications, and data storage.

Relationship between texture and low temperature superplasticity in an extruded AZ31 Mg alloy processed by ECAP

Abstract: The development of low temperature superplasticity and texture is examined in an AZ31 Mg alloy after extrusion and processing by equal-channel angular pressing (ECAP). It is demonstrated that an elongation of ∼460% may be attained at a temperature of 150 °C, equivalent to 0.46 T m where T m is the absolute melting temperature. This result demonstrates the potential for achieving low temperature superplasticity. The experimental results show that the mechanical properties of the alloy are influenced by the different textures present after extrusion and after extrusion and subsequent processing by ECAP.

Superplastic Flow of Fine‐Grained Yttria‐Stabilized Zirconia Polycrystals: Constitutive Equation and Deformation Mechanisms

Abstract: Available deformation data for superplastic yttria-stabilized zirconia polycrystals with grain size 0.10 wt%) over the entire stress range. The strain-rate enhancement with respect to high-purity materials is related to the grain-boundary amorphous phase present in such materials.

Deformation mechanisms in superplastic AA5083 materials

Abstract: The plastic deformation of seven 5083 commercial aluminum materials, produced from five different alloy heats, are evaluated under conditions of interest for superplastic and quick-plastic forming. Two mechanisms are shown to govern plastic deformation in AA5083 over the strain rates, strains, and temperatures of interest for these forming technologies: grain-boundary-sliding (GBS) creep and solutedrag (SD) creep. Quantitative analysis of stress transients following rate changes clearly differentiates between GBS and SD creep and offers conclusive proof that SD creep dominates deformation at fast strain rates and low temperature. Furthermore, stress transients following strain-rate changes under SD creep are observed to decay exponentially with strain. A new graphical construction is proposed for the analysis and prediction of creep transients. This construction predicts the relative size of creep transients under SD creep from the relative size of changes in an applied strain rate or stress. This construction reveals the relative size of creep transients under SD creep to be independent of temperature; temperature dependence resides in the “steady-state” creep behavior to which transients are related.

Improving the superplastic properties of a two-phase Mg–8% Li alloy through processing by ECAP

Abstract: Significant grain refinement was achieved in a cast Mg–8 mass% Li alloy through processing by equal-channel angular pressing (ECAP) using a die having an internal channel angle of 110° and a pressing temperature of 473 K. Following extrusion and subsequent ECAP through two passes, the alloy exhibited excellent superplastic properties including a maximum elongation of ∼970% at 473 K when using an initial strain rate of 1.0 × 10−4 s−1. The strain rate sensitivities under the optimum superplastic conditions were measured as m ≈ 0.4–0.6. The maximum elongation achieved in this investigation is very high by comparison with other Mg alloys tested in tension at similar temperatures and strain rates.

Mechanical properties of Friction Stir Processed 2618/Al2O3/20p metal matrix composite

Abstract: The effect of Friction Stir Processing (FSP) on the mechanical properties of 2618 aluminium alloy reinforced with 20% of alumina particles aluminium alloy has been studied in the present paper. The material was processed into the form of sheets of 7 mm thickness after T6 treatment and was tested in tension and fatigue at room temperature. Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone, in order to analyse the superplastic properties of the recrystallized material and to observe the differences with the parent materials as a function of the strong grain refinement due to the Friction Stir Process. The high temperature behaviour of the material was studied, in longitudinal direction, by means of tensile tests in the temperature and strain rate ranges of 400–500 °C and 10 −3 –10 −1 s −1 , respectively. Fracture surfaces of the deformed fatigue test specimens were comprehensively examined in a scanning electron microscope equipped with field emission gun to determine the macroscopic fracture mode and characterize the fine-scale topography and microscopic mechanisms governing fatigue fracture. The mechanisms governing fatigue life, cyclic deformation and fracture characteristics are analysed in function of magnitude of applied stress, intrinsic micro structural evolution and material deformation behaviour.

Deformation and Diffusion Modes in Nanocrystalline Materials

Abstract: An overview of theoretical models of plastic deformation and diffusion processes in nanocrystalline (nano–grained) materials is presented. The key experimentally detected facts – abnormal Hall–Petch relationship, anomalously fast diffusion, grain rotations, plastic flow localisation, high-strain–rate superplasticity, etc. – in nanocrystalline materials are discussed. Special attention is paid to theoretical models which describe such plastic deformation mechanisms (modes) as lattice dislocation slip, grain boundary (GB) sliding, GB diffusion creep, triple junction diffusion creep and rotational deformation in nanocrystalline materials with emphasis on competition and interaction between these deformation mechanisms and explanation of the key experimentally detected facts. Theoretical models of diffusion enhancement associated with transformations of GB defects in nanocrystalline materials are also considered.

Processing routes leading to superplastic behaviour of magnesium alloy ZK60

Abstract: There has recently been discussion on processing of magnesium alloys by equal channel angular extrusion (ECAE) to achieve a high elongation-to-failure under superplastic tensile deformation. A two-step processing route was suggested to improve superplastic capabilities of such alloys. In this work, superplastic behaviour of ZK60 processed by ECAE alone was compared with that for the material that was rolled prior to the ECAE processing. It was shown that deformation preceding the ECAE step does not bring any benefit with regard to superplastic behaviour in the low temperature range around 200 °C. An important finding is that ECAE processing by itself leads to a record elongation of 1400 and 2040% at the strain rates of 3 × 10 −3 and 3 × 10 −4 s −1 , respectively.

Superplasticity and superplastic forming ability of a Zr-Ti-Ni-Cu-Be bulk metallic glass in the supercooled liquid region

Abstract: The superplastic deformation behavior and superplastic forming ability of the Zr41.25Ti13.75Ni10Cu12.5Be22.5 (at.%) bulk metallic glass (BMG) in the supercooled liquid region were investigated. The isothermal tensile results indicate (hat the BMG exhibits a Newtonian behavior at low strain rates but a non-Newtonian behavior at hiqh-strain rates in the initial deformation stage. The maximum elongation reaches as high as 1624% at 656 K. and nanocrystallization was found to occur during the deformation process. Based cm the analysis on tensile deformation. a gear-like micropart is successfully die-forged via a superplastic forgings process. demonstrating that the BMG has excellent workability in the supercooled liquid region. (C) 2004 Elsevier B.V. All rights reserved.

Effective Grain Alignment in Bi4Ti3O12 Ceramics by Superplastic-Deformation-Induced Directional Dynamic Ripening†

Abstract: Effective grain alignment in Bi4Ti3O12 ceramics by superplastic-deformation-induced directional dynamic ripening

Microstructural prototyping of ceramics by kinetic engineering: applications of spark plasma sintering.

Abstract: The significance of kinetics on the development of microstructures in connection with sintering of ceramics is well recognized. In practice, however, it still remains a challenge to prepare designed microstructures via engineering the sintering kinetics because of an insufficient understanding of the different operative mechanisms that are in many cases overlapping. In this article the kinetic issues involved in sintering are described and discussed with respect to their potential for prototyping microstructures that yield desired properties. By exploiting and mastering the differences present in the kinetics of grain sliding, densification, chemical reactions, and grain growth, respectively, we have established processing principles for producing bulk ceramics with microstructures consisting of nano-sized grains, aligned grains, and/or non-equilibrium-phase constitutions, and for achieving radically improved superplasticity in brittle ceramics. Although the studies quoted in this article were mainly carried out by spark plasma sintering, more general implications of them are expected, including efficient particle sliding, deformation-induced dynamic ripening, superplastic deformation-induced dynamic ripening, and non-equilibrium integration.

Severe plastic deformation of metals

Abstract: This paper provides an introduction in the field of severe plastic deformation (SPD). First of all the main methods to produce SPD materials are discussed. In the following section, the mechanisms leading to the formation of fine grains are reviewed and the influence of changes in strain path is highlighted. During post-SPD thermal annealing, some typical microstructural changes take place. The influence of SPD and subsequent annealing on strength, ductility and superplastic properties are reviewed. Finally the paper provides a short overview of fatigue resistance and corrosion properties of those materials.

Grain refinement and superplasticity in a magnesium alloy processed by equal-channel angular pressing

Abstract: The extrusion/equal channel angular pressing (EX-ECAP) processing procedure, in which magnesium-based alloys are subjected to extrusion followed by ECAP, was applied to a Mg-7.5 pct Al-0.2 pct Zr alloy prepared by casting. Microstructural inspection showed the EX-ECAP process was effective in reducing the grain size from ∼21 µm after extrusion to an as-pressed grain size of ∼0.8 µm. It is shown through static annealing that these ultrafine grains are reasonably stable up to 473 K, but grain growth occurs at higher temperatures. Tensile specimens were cut from the billets prepared by EX-ECAP and testing showed these specimens exhibited superplasticity at relatively low temperatures with maximum elongations up to >700 pct. By processing through EX-ECAP to a higher imposed strain and thereby increasing the area fraction of high-angle boundaries, it is demonstrated that there is a potential for achieving high-strain-rate superplasticity.

Extraordinary superplastic ductility of magnesium alloy ZK60

Abstract: Exceptionally high tensile ductility of commercial Mg alloy ZK60 is reported. It was achieved by equal channel angular pressing without any extra processing steps. The tensile ductility at 220 °C was 2040% and 1400% for the strain rates of 3 × 10−4 s−1 and 3 × 10−3 s−1, respectively. The strain rate sensitivity of the flow stress exhibited a value slightly above 0.5, which is characteristic of superplastic deformation. The grain structure associated with this behavior was shown to be bi-modal with further separation in two fractions with different grain sizes within the small grain size population.

Cavity nucleation and growth in superplasticity

Abstract: Most superplastic materials fail by the nucleation, growth and interlinkage of cavities. Professor Langdon was amongst the first to recognize the significance of concurrent cavitation in superplasticity. This report examines the relaxation of stress concentrations caused by grain boundary sliding, and the nucleation of cavities at triple points, which is relevant for many superplastic ceramics as well as some Mg alloys. It is shown that cavity growth is dominated by plastic deformation. Data for superplastic zirconia are utilized for purposes of illustration. Experimental conditions to limit concurrent cavitation are also identified.

An Analysis of Flow Mechanisms in High Temperature Creep and Superplasticity

Abstract: The flow of crystalline solids at elevated temperatures is dependent upon creep mechanisms associated with the movement of dislocations, the relative displacements of adjacent grains and the stress-directed flow of vacancies. It is convenient to express the steady-state creep rate in terms of the dependences of these various creep mechanisms on the applied stress, the testing temperature and the grain size of the material. In practice, however, there are similarities in some of the predicted dependences for different creep processes and this may lead to experimental difficulties in unambiguously identifying the rate-controlling creep mechanism. The difficulties in identifying the creep mechanism become especially significant at low stresses. This paper reviews the characteristics of the various flow processes occurring in simple metallic systems such as pure metals and metallic alloys and describes procedures that may be adopted to provide an unambiguous identification of the rate-controlling flow mechanism.

Superplasticity in a 7055 aluminum alloy processed by ECAE and subsequent isothermal rolling

Abstract: The superplastic properties and microstructural evolution of a 7055 aluminum alloy was examined in tension at temperatures ranging from 300 to 450 °C and strain rates ranging from 2.7 × 10 −5 to 5.6 × 10 −2 s −1 . A refined microstructure with an average grain size of ∼1.4 μm and a portion of high-angle grain boundaries (HAGBs) of ∼67% was produced in sheets by equal channel angular extrusion (ECAE) followed by isothermal rolling (IR) at 250 °C. The alloy exhibited a maximum elongation-to-failure of ∼820% at a temperature of 450 °C and an initial strain rate of 5.6 × 10 −3 s −1 . Superplastic elongation of ∼670% was recorded at strain rates higher than 10 −2 s −1 , where the strain rate sensitivity coefficient was around 0.44. The relationship between superplastic ductilities and microstructure stability was analyzed. It was shown that increasing the portion of HAGBs by IR results in enhancement of superplastic properties.

Superplasticity of zirconia-alumina-spinel nanoceramic composite by spark plasma sintering of plasma sprayed powders

Abstract: Zirconia 3 mol% yttria–alumina–alumina magnesia spinel nanoceramic composite was synthesized by spark plasma sintering of plasma sprayed particles. For compacts sintered from high energy ball milled powders, superplasticity was observed at temperatures between 1300 and 1450 °C and at strain rates between 10−4 and 10−2 s−1, while for those without high energy ball milling, deformation at the same temperature and strain rate range did not show superplastic behavior. Also, the apparent activation energy (945 kJ/mol) of the high energy ball milled batch was much higher than that of the same composite processed from nanopowder mixtures (621 kJ/mol). The flow stresses were also higher at the same temperatures and strain rates. The difference may be related to the unique low angle grain boundaries in the grains that nucleated and grew from the metastable phase inside the plasma sprayed agglomerate at elevated temperatures. Such boundaries were not intrinsically easy to slide.

Effect of Friction Stir Processing on the Kinetics of Superplastic Deformation in an Al-Mg-Zr Alloy

Abstract: The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 °C to 600 °C and at initial strain rates of 1×10−3 to 1 s−1. A combination of a fine grain size of 1.5 μm and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility, much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy exhibited the highest superplastic ductility of 1015 pct at 580 °C and an initial strain rate of 1×10−2s−1, whereas a maximum elongation of 1280 pct was obtained at 525 °C and an initial strain rate of 1×10−1s−1 for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation kinetics in the FSP condition is proposed.

Strengthening and Toughening of Silicon Nitride by Superplastic Deformation

Abstract: Mechanical properties of silicon nitride which was superplastically deformed in plane-strain compression were investigated. Superplastically deformed silicon nitride exhibited a highly anisotropic microstructure, where rod-shaped grains tended to be aligned along the extruding direction. The bending strength and fracture toughness were increased substantially by the deformation process when a stress was applied in the extruding direction. It appears that these improvements were mainly due to effective operation of grain bridging and pull-out by the grain alignment.

Effect of acute tool-angles on equal channel angular extrusion/pressing

Abstract: Acute tool-angles in equal channel angular extrusion (ECAE) can increase the strain induced in the material within minimum number of passes. Such increased strains can yield ultrafine grains and high fraction of high angle grain boundaries, which enhance mechanical and superplastic properties. However, the deformation and punch pressure requirements will be more stringent demanding detailed investigation. In this study, finite element simulations of ECAE were carried with Abaqus/Explicit for die/channel angles of 60° and 75° in comparison to 90°. Outer corner angle of 10° was considered for all simulations. Factual phenomena like strain hardening of material and friction were considered for the simulations. Deformation was found to take place in three steps for acute channel angles phi = 60° and 75°, two steps for that of 90°. Corner gap, which is often the major drawback in the case of 90° channel angle, was not a factor for the acute channel angles. Punch pressure requirements were also found to be high with acute tool-angles.