Superplasticity Research Papers - Academia.edu (original) (raw)

Superplastic alloys exhibit extremely high ductility (>300%) without cracks when tensile-strained at temperatures above half of their melting point. Superplasticity, which resembles the flow behavior of honey, is caused by grain boundary sliding in metals. Although several non-ferrous and ferrous superplastic alloys are reported, their practical applications are limited due to high material cost, low strength after forming, high deformation temperature , and complicated fabrication process. Here we introduce a new compositionally lean (Fe-6.6Mn-2.3Al, wt.%) superplastic medium Mn steel that resolves these limitations. The medium Mn steel is characterized by ultrafine grains, low material costs, simple fabrication, i.e., conventional hot and cold rolling, low deformation temperature (ca. 650 °C) and superior ductility above 1300% at 850 °C. We suggest that this ultrafine-grained medium Mn steel may accelerate the commercialization of superplastic ferrous alloys.

Excellent superplasticity (elongation $720%) is observed in a novel multi-component (Mn-S-Cr-Al alloyed) ultrahigh carbon steel during tensile testing at a strain rate of 2 Â 10 À3 s À1 and a temperature of 1053 K (just above the equilibrium austenite-pearlite transformation temperature). In order to understand superplasticity in this material and its strong Al dependence, the deformationinduced microstructure evolution is characterized at various length scales down to atomic resolution, using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, energy-dispersive X-ray spectroscopy and atom probe tomography. The results reveal that 1 wt.% Al addition influences various microprocesses during deformation, e.g. it impedes Ostwald ripening of carbides, carbide dissolution, austenite nucleation and growth and void growth. As a result, the size of the austenite grains and voids remains relatively fine (<10 lm) during superplastic deformation, and fine-grained superplasticity is enabled without premature failure.

- by Dirk Ponge
- •
- Materials Engineering, Mechanical Engineering, Superplasticity, Interface Design

High-strength (1.2–1.5) C–(2–2.5) Mn–(1.5–2) Si–(0.8–1.5) Cr steels (mass%) consisting of martensite and carbides exhibit excellent superplastic properties (eg strain rate sensitivity m≈ 0.5, elongation≈ 900% at 1023K). A homogeneous martensitic starting microstructure is obtained through thermomechanical processing (austenitization plus 1.2 true strain, followed by quenching). Superplastic forming leads to a duplex structure consisting of ferrite and spherical micro-carbides.

- by Michel Bellet
- •
- Viscoplasticity, Modeling, Superplasticity, Finite element method

Titanium alloys and titanium matrix composites are useful materials in aerospace applications due to their high strength and stiffness, good corrosion resistance and low density. The gas pressure bulging of metal sheets has become an important forming method. As the bulging process progresses, significant thinning in the sheet material becomes obvious. This paper presents a simple analytical procedure for obtaining the dome height with respect to the forming time useful to the process designer for the selection of initial blank thickness as well as non-uniform thinning in the dome after forming. By thermally cycling through their transformation temperature range, coarse-grained, polymorphic materials can be deformed superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation mechanism. This mechanism was examined under biaxial stress conditions during thermal cycling of titanium alloys with and without discontinuous reinforcements. For the transformation superplasticity, the strain-rate sensitivity index is considered as unity. The radius of curvature, thickness and height of the dome with respect to the forming time are obtained. The analytical results were found to be reasonably in good agreement with the test results.

- by Chennakesava Reddy A
- •
- Composites, Superplasticity, Metal Forming

The effect of Ca element on the hot workability and microstructure evolution of AZ31 magnesium alloy was investigated. Conventional AZ31, AZ31 + 0.7 wt.%Ca, and AZ31 + 2.0 wt.%Ca alloys were tested. Electron back scattered diffraction microscopy revealed that the alloys containing Ca exhibited much finer and more homogeneous microstructure than the conventional AZ31 alloy. Hot compression and tensile tests showed that the Ca element generally increased flow strength and decreased ductility at low temperature. High temperature elongation was considerably improved by the operation of the thermally activated process. TEM work suggested that the large volume fraction of fine (Mg, Al) 2 Ca particles played an important role in preventing significant grain growth of the AZ31 + 2.0Ca alloy.

- by Jeoung Han Kim
- •
- Materials Engineering, Mechanical Engineering, Microstructure, Calcium

Superplastic behavior of fine and ultra fine-grained AA5083 Al alloy was examined using the shear punch test. To achieve fine-and ultra fine-grained microstructures, a relatively new severe plastic deformation (SPD) process, namely Double Equal Channel Lateral Extrusion (DECLE) was employed. The strain rate sensitivity indices (m) of samples were evaluated after 1, 2, 4, and 6 passes for shear strain rates in the range of 3 × 10 −3 to 3 × 10 −1 s −1 and temperatures in the range of 573 to 673 K. For microstructural observations, TEM images together with the corresponding SAED patterns were prepared and utilized. A considerable increase in the m-value was observed after the first pass of the operation for all testing temperatures. The best condition for achieving a good superplasticity for the alloy was found to be a single pass DECLE at 673 K in the strain rate range of 10 − 2 to 10 − 1 s − 1. This process condition resulted in an m-value of 0.43, indicative of a high strain rate superplastic deformation behavior. Further passes of the SPD process did not show any sign of superplasticity until the last pass of the operation, during which the m-value slightly increased, compared with the previous pass.

- by naemeh fakhar
- •
- Superplasticity

Spark plasma sintering (SPS) technique is being widely used to produce nanocrystalline materials by virtue of rapid sintering at relatively lower temperatures. In this investigation, fully dense Al 2 O 3-ZrO 2-MgAl 2 O 4 composite was superplastically formed into a complex shape using SPS equipment with a strain rate of approximately 10 −2 s −1 at temperatures as low as 1150 • C which is impossible to obtain by using conventional forming methods. Furthermore, a powder compact can be directly shaped into a complex shape, combining sintering and forming into one step. The product is fully dense and free of surface cracks. These results indicate that SPS can be a very competitive forming tool for ceramics as well other hard materials.

- by Joshua Kuntz
- •
- Materials Engineering, Mechanical Engineering, Materials Science, Superplasticity

Duplex and triplex microstructures consisting initially of ferrite plus carbide or of martensite, ferrite plus carbide, respectively, can undergo strain induced austenite formation during superplastic deformation at 30 K below Ae 1 (Ae 1 : equilibrium pearlite-austenite transformation temperature) and low strain rate (e.g. 2 Â 10 À 3 s À 1 ). The effect leads to excellent superplasticity of the materials (elongation $ 500%, flow stress o 50 MPa) through fine austenite grains ( $ 10 mm). Using a deformation temperature just below Ae 1 leads to a weak driving force for both, carbide dissolution and austenite formation. Thereby a sufficient volume fraction of carbides (1-2 mm, 15 vol%) is located at austenite grain boundaries suppressing austenite grain growth during superplastic deformation. Also, void nucleation and growth in the superplastic regime are slowed down within the newly transformed austenite plus carbide microstructure. In contrast, austenite grains and voids grow fast at a high deformation temperature (120 K above Ae 1 ). At a low deformation temperature (130 K below Ae 1 ), strain induced austenite formation does not occur and the nucleation of multiple voids at the ferrite-carbide interfaces becomes relevant. The fast growth of grains and voids as well as the formation of multiple voids can trigger premature failure during tensile testing in the superplastic regime. EBSD is used to analyze the microstructure evolution and void formation during superplastic deformation, revealing optimum microstructural and forming conditions for superplasticity of Mn-Si-Cr-C steels. The study reveals that excellent superplasticity can be maintained even at 120 K above Ae 1 by designing an appropriate initial duplex ferrite plus carbide microstructure.

- by Dirk Ponge
- •
- Materials Engineering, Mechanical Engineering, Superplasticity, Void

In this study, 5083 aluminum alloys modified with grain refiner, 0.25% Zr and 0.46% Mn, were processed by reciprocating extrusion to yield high-strain-rate superplasticity above 400 • C and superior room-temperature mechanical properties. Without any prior homogenization treatment, 10 extrusion passes could give the cast billets an equiaxed grain structure with a grain size of about 4.5 m and a subgrain size about 0.2 m, and a uniform distribution of fine inclusions and dispersoids in the matrix. The fine-grained structure was stable up to 525 • C, giving the alloy a highstrain-rate and low-stress superplasticity over a wide operating temperature of 400-500 • C. In the tensile test at 500 • C, a maximum elongation of 1013% and a low flow stress of 7.7 MPa at 5 × 10 −2 s −1 were achieved. The apparent and true activation energies for low temperatures (300-400 • C) without high-strain-rate superplasticity were 220.6 and 208 kJ/mol, respectively, whereas those at high temperatures (400-500 • C) were 88.4 and 98.7 kJ/mol, respectively. Further analysis confirms that grain boundary sliding is the dominant mechanism over the high-strain-rate region from 1 × 10 −2 to 5 × 10 −1 s −1 at 500 • C, and power-law breakdown mechanism occurs over the strain rate from 5 × 10 −4 to 1 × 10 −2 s −1 at 300 • C. The high-strain-rate superplasticity was more strongly enhanced by Zr addition than addition of Cr and Mn. Two enhancing mechanisms for the maximum superplastic elongation and the optimum strain rate are proposed. This study concludes that the effectiveness of Zr is caused by the fineness and the coherency of Zr-rich dispersoids in the matrix.

- by Jien-wei Yeh
- •
- Materials Engineering, Mechanical Engineering, Inclusion, Superplasticity

Dynamic recrystallization (DRX) characteristics of a Mg Á/3Al Á/1Zn (AZ31) alloy sheet were investigated at temperatures ranging from 200Â/450 8C and constant strain rates of 1 )/10 (4 Â/2 )/10 (4 s (1 . The average grain size of the as-received alloy was 12 mm and can be refined to 6 mm via deformation at 250 8C, 1 )/10 (4 s (1 to a strain level of 60%. Grain refinement was less effective at higher temperatures due to rapid grain growth. The grain refinement was attributed to dynamic continuous recrystallization which involves progressive increase in grain boundary misorientation and conversion of low angle boundaries into high angle boundaries. During DRX, subgrains were developed through the conversion of dislocation cell walls into subgrain boundaries. The presence of precipitates was not essential for dynamic recrystallization in the magnesium alloy being investigated because of its limited slip systems, low stacking fault energy and high grain boundary diffusion rate. # : S 0 9 2 1 -5 0 9 3 ( 0 2 ) 0 0 0 9 6 -5

- by Ming-Jen Tan
- •
- Materials Engineering, Mechanical Engineering, Superplasticity, Strain Rate

Friction Stir processing is among the latest methods which are used to produce the surface composites. Surface composites are used to improve the surface property of the material. Friction Stir process is employed in the grain refinement as well as the processing of various materials by creating surface composites. Friction Stir Processing is an emerging technology which showed significant advancements in recent times. It was developed from the Friction Stir Welding. Since then, various advancements has seen in this process. Various types of elements like aluminum and magnesium can be easily processed by this method. Recently, elements like tungsten has also been reported to be processed by this method. In this study, the existing understanding and the current status of work by Friction stir processing are discussed.

— The reason for choosing this theme is to emphasize the phenomena that occur in a set (measuring device) due to external environmental factors and forces that errors involved in the measurement process. These factors influence the measurement results and their presence can not be seen with the naked eye. Thanks to the progress of last years and helping on software, advanced design, and analysis, the designer have handy ever since the concept of a device able to check the resistance to stresses and deformations that may occur in a structure, model or assembly. Following that verification, it is able to more easily decide how to strengthen the structure needed, more robust modeling benchmarks or the use of harder materials. These issues are highlighted in this paper and illustrated by finite element analysis of the structure of a measuring device using Ansys software.

High temperature plasticity of fine-grained ceramics (ZrO2, Al2O3, etc) is usually associated with a grain boundary sliding process. The aim of the present research is then to improve the high-temperature mechanical strength of polycrystalline zirconia (3Y-TZP) through the insertion of multiwalled carbon nanotubes (CNTs) or silicon carbide whiskers (SiCw), which are susceptible to pin the grain boundaries. The effect of these nano-sized particles on grain boundary sliding has been studied by mechanical spectroscopy.

Most models of internal stress superplasticity predict a linear relationship between the applied stress and the plastic strain per cycle, and are only valid at low applied stresses. In the present paper, we extend the original linear theory of phase transformation superplasticity by Greenwood and Johnson [l] and derive a non-linear closed-form solution valid over the whole range of stresses, from the low-stress regime (where a linear relationship between strain and stress is predicted in agreement with the model by Greenwood and Johnson (Proc. R. Sot. Lond., 1965, 283, 403), to the high-stress regime (where the strain increases non-linearly as the applied stress approaches the yield stress of the weaker phase). The model is found to be in agreement with literature data on transformation superplasticity of iron spanning both stress regimes. Furthermore, the model is adapted to the case where internal stresses are produced by thermal expansion mismatch: it is compared to experimental literature data for metals with anisotropic thermal expansion (Zn and U) and for metal matrix composites with inhomogeneous thermal expansion (Al-Sic).

- by David Dunand
- •
- Materials Engineering, Mechanical Engineering, Materials Science, Superplasticity

Nitrogen-rich Ca-α-sialon ceramics, prepared with CaH2 as one of the starting powders, were compressively deformed in spark plasma sintering equipment. Compared with the oxygen-rich Ca-α-sialons, increasing onset deformation temperatures (about 150 K higher) were observed for nitrogen-rich Ca-α-sialons deformed at a rate of 2 × 10−3 s−1. High hardness (HV10 = 18–20 GPa) and toughness (KIC = 4–7 MPa m1/2) were maintained after the deformation. Anisotropic grain growth was found to take place during deformation, resulting in anisotropic microstructures, containing coarse and elongated grains. The observed differences in deformation behaviour and properties between nitrogen-rich and oxygen-rich Ca-α-sialons are, as indicated by transmission electron microscopy and electron energy loss spectroscopy analysis, attributed to the presence of reduced amounts of a nitrogen-rich grain-boundary glass phase.

- by Saeid Esmaeilzadeh
- •
- Materials Engineering, Mechanical Engineering, Microstructure, Superplasticity

Gas pressure bulging of metal sheets has become an important forming method. As the bulging process progresses, significant thinning in the sheet material becomes inevitable. A prior knowledge about non-uniform thinning in the product after forming helps the designer in the selection of initial blank thickness. This paper presents a simple analytical procedure on the thinning of superplastically formed Pb-Sn eutectic alloy spherical domes. It also addresses the issue of instability of deformation.

- by Chennakesava Reddy A
- •
- Superplasticity

Environmental and economical issues have been increasingly demanding reduced fuel-consumption and exhaustemission vehicles. Among the different means to satisfy these demands, reduction of mass remains the most influential and least costly one, provided that large cuts of 20-40 % are achieved. Leading automotive manufacturers have shown that more than 50 % of fuel consumption is mass dependent, a fact that keeps increasing the interest in lightweight materials over conventional ones. Being the lightest constructional metal on earth, it is rather natural and quite expected for magnesium to be receiving such great attention over the last decade. Magnesium's low density makes it 35 % lighter than aluminum and 78 % lighter than steel. And with proper design considerations, magnesium could replace these two metals in many areas, promising significant weight reductions. Many examples of magnesium auto parts that have evolved recently proof the initial signs of such promises. However, those examples fall mainly into the cast-components category. Unless magnesium usage is expanded to cover other areas, mainly sheet metal forming, feasible weight reductions will be limited. The metal's inferior ductility at room temperature is a key factor in limiting such an expansion.

Ti-6Al-4V ELI (extra low interstitials) grade alloy provides improved ductility and fracture toughness comparing to grade 5 Ti-6Al-4V alloy. In order to find the optimum superplastic forming and diffusion bonding (SPF/DB) condition, a series of tensile tests was carried out at the strain rate range of 10 −4 to 10 −2 s −1 and temperature range of 1073-1223 K. The maximum elongation of 1898% was obtained at the strain rate of 10 −3 s −1 at 850 • C. It was shown that the ELI grade alloy performs better than the grade 5 alloy in terms of the optimum superplastic condition for Ti-6Al-4V. Based on this result, diffusion bonding process of superplastic Ti-6Al-4V ELI sheet metals was developed. Bonding was completed by means of inert gas pressure applied in a bonding tool at high temperature. The microstructure of the bonding area was investigated and the bonding interface was microscopically undetectable. The evidence of nucleation of new grains and migration of grain boundaries at the interface proves the diffusion bonding process is successful. It is shown that the superplastic forming and diffusion bonding of Ti-6Al-4V ELI grade is possible at the temperature lower than those of conventional Ti-6Al-4V.

- by Ho-Sung Lee
- •
- Materials Engineering, Mechanical Engineering, Materials Science, Superplasticity

Abstract Superplastic behavior of fine and ultra fine-grained AA5083 Al alloy was examined using the shear punch test. To achieve fine- and ultra fine-grained microstructures, a relatively new severe plastic deformation (SPD) process, namely Double Equal Channel Lateral Extrusion (DECLE) was employed. The strain rate sensitivity indices (m) of samples were evaluated after 1, 2, 4, and 6 passes for shear strain rates in the range of 3 × 10− 3 to 3 × 10− 1 s− 1 and temperatures in the range of 573 to 673 K. For microstructural observations, TEM images together with the corresponding SAED patterns were prepared and utilized. A considerable increase in the m-value was observed after the first pass of the operation for all testing temperatures. The best condition for achieving a good superplasticity for the alloy was found to be a single pass DECLE at 673 K in the strain rate range of 10− 2 to 10− 1 s− 1. This process condition resulted in an m-value of 0.43, indicative of a high strain rate superplastic deformation behavior. Further passes of the SPD process did not show any sign of superplasticity until the last pass of the operation, during which the m-value slightly increased, compared with the previous pass.

- by naemeh fakhar
- •
- Materials Engineering, Mechanical Engineering, Materials Science, Superplasticity

In the present work, the influence of c eutectic phase on the cavitation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate hot tensile tests. The tensile tests were executed in two temperature range of 300-425°C and 450-500°C; i.e. somewhat below and higher eutectic melting temperature of c precipitates ($437°C). The hot compression characteristics of the experimental alloy were also considered to assist explaining the related deformation mechanisms. The results indicated that a large amount of cavities originates from the c second phases, specially located on grain boundaries. A sharp transition was realized from higher (>3%) to lower cavity area fraction (<0.02%) between 450°C and 500°C. The latter was attributed to the effect of c liquid phase on stress relaxation through accommodating the grain boundary sliding phenomena. In addition, the current work explores the room temperature mechanical properties of tensile deformed specimens using shear punch testing method.

- by Hamid Reza Abedi
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- Superplasticity, Thermomechanical Processing, Cavitation, Magnesium Alloy AZ31

ÐPlastic deformation, microstructure and deformation substructure in an Al±14 wt% Ni±14 wt% Mm (misch metal) alloy produced from amorphous powders were examined focusing on the eects of the initial average grain size d 0 , the grain boundary sliding and the motion of dislocations on the superplasticity. The as-received and annealed specimens with d 0 =0.8, 1.6 and 2.5 mm deformed at 873 K at an initial strain rate of 1/s. The specimen with d 0 =0.8 mm showed high-strain-rate superplasticity accompanied by low¯ow stress and 350% elongation, while the¯ow stress increased and elongation decreased with greater grain size. Grain re®nement occurred and the frequency of small angle boundaries showed a complicated change during superplastic deformation. On the other hand, in the specimen with d 0 =1.6 mm, the grains coarsened rapidly. A large number of dislocations were observed in grains even in the superplastically deformed specimen. The high-strain-rate superplasticity of Al±14 wt% Ni±14 wt% Mm alloy is controlled by the grain boundary sliding but the motion of dislocations also plays an important role in accommodating the highly strained region near triple junctions of the grain boundaries.

- by Takayoshi Nakano
- •
- Materials Engineering, Mechanical Engineering, Materials Science, Microstructure

This paper presents the numerical results obtained from the finite element analyses of the superplastic forming (SPF) of AlTi alloys. The models are used to optimise the process and predict forming times in terms of deformed shapes,... more

- by Clayton Adam
- •
- Materials Engineering, Mechanical Engineering, Superplasticity, Finite Element

The concept of efficiency can be used for comparing the relative performance of various pozzolans when incorporated into concrete. In this paper, an alternative approach for the evaluation of efficiency factor k of a pozzolanic material has been introduced. The method, developed following Abram's strength–W/C ratio rule, calculates efficiency in terms of relative strength and cementitious materials content. The advantage of this method is that only two mixtures are required to determine the k factor of a specific mixture. A laboratory investigation on silica fume (SF) and metakaolin (MK) concrete found that the computed efficiency factors varied with pozzolan type, replacement level and age. At 28 days, the k values ranged from 1.6 to 2.3 for MK and 2.1 to 3.1 for SF mixtures, while at 180 days, the k values varied from 1.8 to 4.0 for MK and 2.4 to 3.3 for SF mixtures. Generally, the k factors increased with age but declined with higher pozzolanic content. It was also observed that change in W/CM ratio from 0.33 to 0.27 did not significantly affect the resultant efficiency factors.

- by Hashim Razak
- •
- Civil Engineering, Mineralogy, Modeling, Superplasticity

The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (11SO"C), the grain boundaries are covered by an ultrathin (1 nm) yttrium-rich amorphous film. After deformation at 1200"-1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress.

- by Louis Winnubst
- •
- Materials Engineering, Mechanical Engineering, Materials Science, Microstructure

In recent years, there have been a lot of research efforts in studying high-strain-rate superplasticity of aluminum matrix composites. However, the superplastic deformation behavior of the composites has not been fully understood. Superplastic deformation mechanism in aluminum matrix composites was considered to be grain boundary sliding and interfacial sliding. In this paper, the model proposed recently by the authors to explain the superplastic deformation of a particulate reinforced aluminium matrix composite at temperatures above and below its solidus temperatures is further developed. The theoretical predictions are in agreement with the published experimental findings.

- by Bing Han
- •
- Mechanical Engineering, Civil Engineering, Superplasticity, Metal matrix composite

In this investigation, the ductility and cavitation behavior of five grades of the superplastic Zn-22 pct A1 were studied under identical conditions of grain size, temperature, and initial strain rate. These five grades were prepared from high-purity A1 and Zn using the same procedure but different Fe impurity levels; grades A, B, C, D, and E contain 1, 40, 120, 400, and 1460 ppm of Fe, respectively. A comparison between the present results and those reported earlier for the creep behavior of the five grades of Zn 22 pct A1 demonstrates that while the presence of Fe has no noticeable effect on the steady-state creep rates during deformation in the superplastic region (where maximum ductility occurs), it has a pronounced effect on the ductility and fracture behavior of the alloy. The experimental data show that there is a significant drop in the average elongation to fracture when Fe concentration exceeds 125 ppm, and that the ductility of grade E is approximately 50 pct of grade A for initial strain rates less than 10-2 s-1. In addition, the data reveal two important findings regarding cavitation behavior. First, neither the initial grain size nor the occurrence of grain growth has an effect on cavitation that is as significant as that arising from the presence of excessive impurities. Second, the presence of other impurities in addition to Fe in Zn-22 pct AI has the effect of enhancing cavitation. The effect of Fe, alone or in the presence of other impurities, on ductility and cavitation in Zn 22 pct A1 is examined in terms of phenomena associated with impurity segregation at boundaries.

- by James Earthman
- •
- Materials Engineering, Mechanical Engineering, Materials Science, Superplasticity

It is shown that Garofalo's hyperbolic sine creep relation can predict superplasticity at moderate strain rates in materials from their mechanical behavior in a higher strain rate region. Fine grained Ti-6 Al-4 V alloy has been used as an... more

- by M. Carsí
- •
- Engineering, Materials Science, Superplasticity, Creep

Ultrafine grains were developed in Hadfield manganese steels through appropriate thermomechanical processing. The steels contained from 1.2 to 1.7 wt.% C and 12.3-16.3 wt.% Mn. The austenite grains. 2-8 ktm in size, were stabilized against grain growth by a dispersioia of fine carbides, typically less than i pm. The processed materials were evaluated for superplastic properties at elevated temperatures (750-900 °C). Values for the strain rate sensitivity exponent m in the expression o = kg m ranged from 0.37 to 0.65. The value of m in the superplastic regime was found to depend on composition, grain size and temperature. At 23 °C, the fine-grain steels showed higher yield strengths and hardness values, but lower ductility, relative to values reported for commercially processed materials.

- by O. Ruano
- •
- Materials Engineering, Mechanical Engineering, Microstructure, Superplasticity
- by O. Ruano
- •
- Superplasticity, Iron, Composite Material, Tensile Stress
- by Nihat Akkus
- •
- Materials Engineering, Mechanical Engineering, Modeling, Prediction

Recent measurements of cavitation in superplastically strained 3Y-TZP ceramics by scanning electron microscopy and ultra small-angle neutron scattering (USANS) lead to systematic differences in volume fractions of cavities determined by both methods. We showed that the scattering curves measured by double crystal USANS diffractometers can be calculated directly from scanning electron micrographs taken from a single section of the material, without any assumptions of cavity shape or distribution. This technique was used to compare the USANS measurements with electron micrographs and showed that the calculated and measured curves agreed well for medium scattering vectors. It confirmed that the electron micrographs represented well the microstructure of small cavities up to about 0.6 µm in radius. On the contrary, the calculations resulted in systematically larger intensities for small scattering vectors. This indicates that large cavities are responsible for the observed differences in volume fraction for the samples with nominal strain above 100%. An explanation by inhomogeneous distribution of large cavities generated at high strains in the bulk of the tensile specimen and/or by surface artifacts is suggested.

- by D. Ponge and +1
- •
- Materials Engineering, Mechanical Engineering, Superplasticity, Interface Design

ABSTRACT An Al 7075 alloy (5.63mass%Zn-2.56mass%Mg-1.68mass%Cu-0.21mass%Fe- 0.19mass%Cr-0.14mass%Si-0.02mass%Ti with balance of Al) was processed by high-pressure torsion (HPT) under an applied pressure of 6 GPa for 1, 3 and 5 revolutions with a rotation speed of 1 rpm at room temperature. Vickers microhardness saturated to a level of 220 Hv after the HPT processing and the grain size was refined to 120 nm at the state of the hardness saturation. Tensile tests were conducted with initial strain rates from 2.0 × 10-4 to 2.0 × 10-2 s-1 at temperatures as 200 °C and 250 °C (equivalent to 0.52Tm and 0.57Tm, respectively, where Tm is the melting point of the alloy). The HPT-processed samples for 3 revolutions exhibited superplastic elongations of 640% and 510% at 250 °C with initial strain rates of 2.0 × 10-3 s-1 and 2.0 × 10-2 s-1, respectively.

- by Seungwon Lee
- •
- Superplasticity

In this study, the twin roll casting process has been utilized to prepare initial as-cast strip of AZ91 alloy that has been further tested in uniaxial tension at 325, 350 and 375 ºC, and at strain rates from 10 -2 to 10 -4 s -1 . The ability of AZ91 strip to undergo superplastic or superplastic-like deformation in longitudinal direction, in transverse direction, and in 45 degrees to longitudinal direction was investigated with no further thermal or thermomechanical processing applied prior to the testing. At temperature 350 ºC and medium strain rate the maximum elongation reached almost 200% in direction parallel to the strip casting, however in the transverse and 45 degrees direction, different temperature, and at higher strain rates the maximum elongation was lower. Based on microstructural investigation by means of OM and SEM it could be seen that continuous dynamic recrystallization of initial coarse dendritic as-cast microstructure during elevated temperature deformation is active and responsible for high elongations. Additional analysis is provided by means of a strain rate sensitivity characterization and its evolution with increased level of strain. Strain rate sensitivity of AZ91alloy increased with increasing level of strain from 0.20 to 0.33.

- by Gaurav Bali
- •
- Superplasticity, Magnesium, Magnesium Alloys, Superplastic Forming

The microstructural mechanism of grain growth during superplastic deformation is studied in this paper. A new model that considers grain growth is proposed and applied to conventional superplastic materials (Zn-Al alloy, LY12CZ alloy, and 7475 Al alloy). The relationships between the strain, strain rate, test temperature, initial grain size and grain growth in superplastic materials are discussed. The theoretical predictions

- by Ming-Jen Tan
- •
- Engineering, Materials Science, Microstructure, Modeling

The grain refinement after thermo-mechanical treatment (TMT) was investigated in AZ91, AE42, und QE22 magnesium alloys. The optimal over-aging temperature was determined to be 300 °C in the case of AZ91 and AE42 alloys and 350 °C for QE22 alloy. After optimized TMT, the average grain sizes were 13.5 µm (AE42), 11.1 µm (AZ91) and 1.9 µm (QE22). The QE22 alloy exhibited the superior superplastic properties, with maximum elongation to failure 750 % and strain rate sensitivity parameter m=0.73. The Friction Stir Welding showed that the original base material grain structure of the alloys AZ31 and AZ91 replaced by ultrafine grains in the stir zone. The purpose of the present paper is to present the results of the grain refinement in magnesium alloys by thermo mechanical treatment and stir welding.

- by Robert Král
- •
- Materials Science, Metallurgy, Superplasticity, Friction Stir Welding

Using a model that gives a realistic description of the plastic flow of polycrystals with refined grain size resulted in discrepancies being found in the postulates of the mathematical theory of plasticity. The new proposed time-dependent... more

- by Victor Conte Pozo
- •
- Materials Engineering, Mechanical Engineering, Superplasticity, Creep

In this study, a novel method of friction stir forming (FSF) was conducted for mechanical interlocking of Zn-22Al superplastic alloy and thin copper wire insulated by polyimide. The potential development of a composite material capable of transmitting electrical energy or electric signals was studied experimentally, and it was concluded that FSF can successfully interlock insulated copper wire with Zn-22Al superplastic alloy. The authors suggest the possibility that FSF could join sheets of Zn-22Al alloy by pressure welding and superplastic forming and diffusion bonding (SPF/DB). Trials of FSF were carried out on a modified vertical milling machine. The results are discussed in terms of microstructure observations, hardness distributions, and temperature measurements.

- by Hamed Mofidi Tabatabaei
- •
- Engineering, Materials Science, Metallurgy, Superplasticity

Fine grained, three-phase ceramic composites that exhibit favorable toughness, hardness, and high room-temperature strength were evaluated for high-temperature mechanical stability. A 50vol%Al2O3–25vol%NiAl2O4–25vol%3 mol%yttria-stabilized tetragonal zirconia polycrystal (3Y–TZP) and a 33vol%Al2O3–33vol%NiAl2O4–33vol%3Y-TZP composite were compression creep tested at temperatures between 1350 and 1450 °C under constant stresses of 20–45 MPa. The three-phase microstructure effectively limited grain growth (average d0 = 1.3 μm, average df = 1.6 μm after 65% true strain). True strain rates were 10−4 to 10−6 s−1 with stress exponents n = 1.7 to 1.8 and a grain-size exponent p = 1.3. A method for compensating for grain growth is presented using stress jump tests. The apparent activation energy for high-temperature deformation for 50vol%Al2O3–25vol%NiAl2O4–25vol%3Y–TZP was found to be 373 kJ/mol-K.

It was reported that superplastic boronizing process (SPB) provides a much faster boronizing rate than the conventional boronizing process (CB). This process was conducted on duplex stainless steel (DSS) which exhibit superplasticity. The study concentrated on the effect of strain rate and compression strain on SPB. The process was conducted under four different strain rates and three diferent strains condition. Boronizing was successfully conducted with the best result obtained under the high strain rate range of 5 x 10-5 s-1 to 1 x 10-3 s-1 which is associated with the superplastic region. Through SPB, movement of atoms into the specimen was highly accelerated by the grain boundary sliding process leading to a formation of thick and hard boronized layer in extraordinarily short period of time.

- by Hanis Ayuni Mohd Yusof
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- Superplasticity, Strain Rate, Surface Properties, Duplex Stainless Steel

A continuum theory for non-Newtonian flow of a two-phase composite containing rigid inclusions is presented. It predicts flow suppression by a factor of (1-V)p, where V is the volume fraction of the rigid inclusion and q depends on the stress exponent and the inclusion shape. Stress concentrations in the rigid inclusion have also been evaluated. As the stress exponent increases, flow suppression is more pronounced even though stress concentration is less severe. To test this theory, superplastic flow of zirconia/mullite composites, in which zirconia is a soft, non-Newtonian superplastic matrix and mullite is a rigid phase of various size, shape, and amount, is studied. The continuum theory is found to describe the two-phase superplastic flow reasonably well.

- by I-wei Chen
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- Materials Engineering, Mechanical Engineering, Materials Science, Microstructure

Multi-level modeling adaptive to theoretical treatise of superplastic deformation is proposed not only to make process simulation for superplastic forming but also to describe the microstructural change taking place in materials processing. Use of the unit cell modeling enables us to make theoretical linkage between the macroscopic and the mesoscopic models. The fundamental theory is introduced with some comments on the unit cell model for macro-micro mechanics linkage and the advanced granular modeling for mesoscopic representation of superplasticicty. The sinter-forging process is employed for practical demonstration of the present multi-level modeling.

- by Tatsuhiko Aizawa
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- Superplasticity, Finite element method, Homogenization, Forging

Intrinsic difficulty of metal forming in magnesium alloys must be overcome by new processing. Superplatic injection forming is one of the most promising methodologies. Stroke velocity controlled forming system was developed to evaluate the formability of AZ91 at 573 K both in upsetting and backward-extrusion modes. Uniaxial constitutive equation was used together with consideration of grain growth to predict the stress-strain rate and the stress-stroke relations in upsetting experiments. Double flange, thin-walled cup can be superplastically formed from a cylindrical billet.

- by Tatsuhiko Aizawa
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- Materials Science, Superplasticity, Strain Rate, Grain Growth

The instability and the localization process of unstable plastic flow during constant pressure bulging of superplastic Ti6Al4V sheets at 900°C are analyzed. A generalized stability criterion is developed by generalizing Hart's... more

- by Li Chung
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- Materials Engineering, Mechanical Engineering, Materials Science, Superplasticity

Wires of 99.5% pure iron with 406 and 508 m diameter were subjected to a uniaxial tensile stress while being thermally cycled about the ␣-␥ allotropic transformation temperature. The strain increments per cycle are proportional to the applied stress in the range 1-22 MPa, indicating that transformation-mismatch plasticity is the dominant deformation mechanism. The strain increments for the wires have the same magnitude as those reported for bulk iron samples, thus, indicating that the internal mismatch strains responsible for this deformation mechanism are undiminished in the wires, despite their high surface-to-volume ratio. Very high average strain rates (up to 3 × 10 −3 s −1) were achieved through resistive heating and convective/radiative cooling of the thinnest wires.

- by David Dunand
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- Materials Engineering, Mechanical Engineering, Materials Science, Plasticity

Ti-6Al-4V, with a network of elongated, open pores aligned along two perpendicular directions, is produced by a two-step replication process: (i) Ti-6Al-4V powder or foil preforms containing low-carbon steel wire meshes are densified by hot pressing under transformation superplasticity conditions; (ii) porosity is created by electrochemical dissolution of the low-carbon steel wires and the adjacent Fecontaining Ti-6Al-4V matrix. If high-carbon steel wires are used, Fe diffusion into Ti-6Al-4V is inhibited by a carbide layer forming at the wire/matrix interface, and pores exactly replicate the shape of the wires. Ti-6Al-4V with $19% and 34% porosity, without and with Fe-Ti interdiffusion respectively, shows low oxygen contamination and good compressive ductility. Strength and stiffness, as measured by compression testing and ultrasonic measurements, are compared with simple analytical models and numerical finite-element models.

- by David Dunand
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- Materials Engineering, Mechanical Engineering, Materials Science, Superplasticity