Effect of Rare Earth on Superplasticity of Zn-5Al Eutectic Alloy

The superplastic deformation curves of Zn 5Al eutectic alloy containing small amount of rare earth were measured, and the influence of rare earth on structure and superplasticity characteristics of the alloy was examined with optical microscope, XDF and TEM. The results show that the elongation of Zn 5Al eutectic alloy can be increased if less than 0.2 %(mass fraction) misch metal was added. Rare earth which exists in the form of compounds Al 2CeZn 2 and CeZn 3 can refrain the dissolution and diffusion of Zn to Al and postpone the saturation of the diffusion dissolution zone(DDZ) above 350 ℃, and in such a way boost up α/β interface sliding which benefits the superplasticity.

Investigation of Thermal-Cycling Transformation Superplasticity of As-Cast Zn-5Al-RE Alloy

Under 20～350 ℃ thermal cycling tensile condition,the as-cast Zn-5Al-RE alloy can realize transformation superplasticity. Rare-earths added into the alloy can make the strain rate and rupture elongation decrease,while the annealing treatment has an opposite effect. The longer the annealing time,the larger the strain rate and rupture elongation will be. The effect of rare earths on the thermal-cycling transformation superplasticity of Zn-5Al-RE alloy as-cast is two-faced: on the one side,rare earths can make the eutectic structure of Zn-5Al-RE alloy fine and thus benefits superplastic deformation. On the other side,rare earths make the diffusion velocity of Zn and Al decrease and postpone the diffusion coordination process,which will hinder the viscous flow of the interphase boundaries,enhance superplastic deformation stress,and make the strain rate and elongation decrease.

Effect of minor Sc and Zr on superplasticity of Al-Mg-Mn alloys

The effect of Sc and Zr on the superplastic properties of Al-Mg-Mn alloy sheets was investigated by control experiment. The superplastic properties and the mechanism of superplastic deformation of the two alloys were studied by means of optical microscope, scanning electronic microscope and transmission electron microscope. The elongation to failure of Al-Mg-Mn-Sc-Zr alloy is larger than that of Al-Mg-Mn alloy at the same temperature and initial strain rate. The variation of strain rate sensitivity index is similar to that of elongation to failure. In addition, Al-Mg-Mn-Sc-Zr alloy exhibits higher strain rate superplastic property. The activation energies of the two alloys that are calculated by constitutive equation and linear regression method approach the energy of grain boundary diffusion. The addition of Sc and Zr decreases activation energy and improves the superplastic property of Al-Mg-Mn alloy. The addition of Sc and Zr refines the grain structure greatly. The main mechanism of superplastic deformation of the two alloys is grain boundary sliding accommodated by grain boundary diffusion. The fine grain structure and high density of grain boundary, benefit grain boundary sliding, and dynamic recrystallization brings new fine grain and high angle grain boundary which benefit grain boundary sliding too. Grain boundary diffusion, dislocation motion and dynamic recrystallization harmonize the grain boundary sliding during deformation.

DYNAMIC RECRYSTALLIZATION AND SUPERPLASTICITY IN Al-Li ALLOY

The behavior of dynamic recrystallization in the superplastic deformation of 8090 and 2091 aluminum-lithium alloys have been investigated.TEM observations indicated that dynamic recrystallization occurs at thetriple junction of grain boundaries.The measurement of grain boundary angle showed that recrystallization indynamic equilibrium exists in the process of superplastic deformation of 8090 Al-Li alloy.It is also indicatedthat,besides the role of refining grains and the grain boundary sliding,dynamic recrystallization playsconcurrently a role of stablizing microstructure.Thus dynamic rccrystallization can be used to induce metalssuperplasticity,which leads to a simplification of pretreatment for superplastic deformation.

Effect of microstructures on superplasticity of Al-11%Si alloy

Three kinds of Al-11%Si (mass fraction) alloy samples with different processes were produced to investigate the effect of microstructures on its superplasticity. Among them, the as-ECAP sample pressed 16 passes has ultrafine grains (300 nm) and the finest secondary particles. The ECAP-T6 sample, with ECAP 16 passes followed by T6 treatment, has fine secondary particles (3 μm) but the largest grains (8 μm). Contrarily, the T6-ECAP sample, with T6 treatment followed by ECAP 16 passes, has ultrafine grains and the large secondary particles (7 μm). The tensile testing results show that the as-ECAP sample exhibits superplasticity at high strain rate of 5.75×10-1 s-1 due to its fine secondary particles and ultrafine grains. The ECAP-T6 sample, however, does not exhibit superplasticity at the same high strain rate of 5.75×10-1 s-1 because it has relatively large secondary particles and large grains. Remarkably, the T6-ECAP sample does not have superplasticity even at the lower strain rate of 1.15×10-1 s-1, attributing to its comparatively large secondary particles. When most secondary particles are larger than 7 μm, the high strain rate superplasticity could not be obtained even if this sample has ultrafine grains.

Effect of scandium on superplasticity of Al-Mg alloys

The superplastic behavior of adding 0.22%Sc into the Al 6Mg alloy was studied by simple superplastic pretreatment process—warm rolling and cold rolling. The optimum superplastic temperature and strain rate of the alloys were defined and satisfactory results were obtained during the superplastic deformation at 811 K and initial strain rate 0=1.67×10 -3 s -1 . The average elongation of Al 6Mg 0.22Sc alloys reaches to 1 125%, and the maximum elongation is 1 200%, maximum m value (strain rate sensitive index) is 0.879. But under the same condition the elongation and maximum m value for Al 6Mg alloy were only 377% and 0.595, respectively. The superplastic deforming mechanism for Al 6Mg 0.22Sc alloys was also discussed. [

Effect of two-stage aging on superplasticity of Al-Li alloy

The effect of two-stage aging on the microstructures and superplasticity of 01420 Al-Li alloy was investigated by means of OM, TEM analysis and stretching experiment. The results demonstrate that the second phase particles distributed more uniformly with a larger volume fraction can be observed after the two-stage aging (120℃, 12 h+300℃, 36 h) compared with the single-aging(300℃, 48 h). After rolling and recrystallization annealing, fine grains with size of 8-10 urn are obtained, and the superplastic elongation of the specimens reaches 560% at strain rate of 8×10-4s-1 and 480℃. Uniformly distributed fine particles precipitate both on grain boundaries and in grains at lower temperature. When the sheet is aged at high temperature, the particles become coarser with a large volume fraction.

SUPERPLASTICITY OF Al-Zn-Mg-Zr-RE AND Al-Zn-Mg-Ti ALLOYS AND THEIR CAVITATION BEHAVIOUR DURING DEFORMATION

The superplasticity of the Al-Zn-Mg-Zr-RE(alloy A)and Al-Zn-Mg-Ti(alloy B),their change of volume fraction of metastable micro-cavities under various reduction of area and strain rates,as well as the growth of stable cavities in them have been studied by means of small angle X-ray scattering analyses and quantitative metalloraphy respectively.Experimen- tal results show that the volume fraction of metastable micro-cavities of the alloy A is more than that of the alloy B,but the growth rate of stable cavities in alloy A is less than that in al- loy B.Thus,it is believed that the superplasticity of alloy A is superior to that of alloy B.

Effect of compression ratio on microstructure evolution of Mg−10%Al−1%Zn−1%Si alloy prepared by SIMA process

A new Mg−10%Al−1%Zn−1%Si alloy with non-dendritic microstructure was prepared by strain induced melt activation(SIMA)process.The effect of compression ratio on the evolution of semisolid microstructure of the experimental alloy was investigated.The results indicate that the average size ofα-Mg grains decreases and spheroidizing tendency becomes more obvious with the compression ratios increasing from 0 to 40%.In addition,the eutectic Mg2Si phase in the Mg−10%Al−1%Zn−1%Si alloy transforms completely from the initial fishbone shape to globular shape by SIMA process.With the increasing of compression ratio,the morphology and average size of Mg2Si phases do not change obviously.The morphology modification mechanism of Mg2Si phase in Mg−10%Al−1%Zn−1%Si alloy by SIMA process was also studied.

Study on the hot deformation behavior of Al-Zn-Mg-Cu-Cr aluminum alloy during multi-stage hot compression

The mechanical behavior and microstructures of an Al-Zn-Mg-Cu-Cr aluminum alloy during multi-stage hot deformation were investigated by thermal stimulation test, optical microscopy, and transmission electron microscopy. The true stress vs true strain curves and the microstructure evolution of two hot deformation procedures were gained. The flow stress of the alloy studied decreases with increasing the deforming passes and declining the temperature, and the larger the temperature decline between adjacent stages, the larger the peak stress fall is. The stress-strain behavior mainly result from the dynamic recovery during deformation, the static recovery and recrystallization in the delay time, and the second phases precipitated from the matrix at high temperature.

SUPERPLASTICITY OF A RAPIDLY SOLIDIFIED AI-Li ALLOY

A rapidly solidified microcrystalline Al-Li-Cu-Mg-Zr alloy and its superplasicity have been investigated.An optimum tensile elongation of 585% was obtained at 540℃ and strain rate 1.67×10^(-2)s^(-1).The superplastic Al-Li alloy is manufaetured using thermomechanical pro- cessing:solution,overaging,warm rolling and recrystallization.Microstructural changes in thermomechanical processing and cavitation occurred during superplastic deformation have been observed.The superplastic failure of alloy may be caused mainly by nucleation and growth of cavities as well as the linkage around grains.

ELECTRO-SUPERPLASTICITY IN 2091 Al-Li ALLOY

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Superplasticity and Fracture Behavior of Fine Grained 5083 Al Alloy

The microstructure with uniform equiaxed fine gain was obtained by the thermo-mechanical processing （TMP） for the 5083 Al alloy （Al-4.56%Mg-0.61%Mn） plate. Uniaxial tensile test was carried out at a temperature range of 500-570 ℃ and a strain rate range of 4.17×10^-4 s-1^- 1×10^- 2 s^- 1. Maximum tensile elongation 530% was obtained at 550 ℃ and strain rate ε＆ =4 .17×10^-4 s^-1. Dislocations were observed in grain interiors and at grain boundaries during uniaxial tensile deformation by transmission electronic microscopy （TEM）, respectively. Results show that the grain boundary sliding （GBS） accommodated by dislocation motion is the principal reason for superplastic deformation. The cavities and fracture were observed during uniaxial tensile deformation of the alloy by scanning electronic microscopy （SEM）, indicating that linkage of cavities in large region would induce failure of the material. Moreover, presence of liquid phase at grain boundary also affects the superplastic deformation and behavior of cavities.

Superplasticity in an Aluminum Alloy 6061/Al_2O_3p Composite

The superplasticity of an Al203p/6061Al composite, fabricated by powder metallurgy techniques, has been investigated. Instead of any special thermomechanical processing or hot rolling, simple hot extrusion has been employed to obtain a fine grained structure before superplastic testing. Superplastic tensile tests were performed at strain rates ranging from 10-2 to 10-4 s-1 and at temperatures from 833 to 893 K. A maximum elongation of 200% was achieved at a temperature of 853 K and an initial strain rate of 1.67×103 s-1. The highest value obtained for the strain rate sensitivity index (in) was 0.32. Differential scanning calorimeter was used to ascertain the possibility of any partial melting in the vicinity of optimum superplastic temperature. These results suggested that no liquid phase existed where maximum elongation was achieved and deformation took place entirely in the solid state.

Stored energy analysis of Zn-5Al eutectic alloy in superplastic deformation

The stored energy and the energy release during SPD (superplastic deformation) of a Zn-5 Al alloy were studied. The alloy after rolling process gains more stored energy, and the as-rolled specimen can obtain maximum elongation and minimum flow stress without hot holding treatment before SPD. Experimental results show that stored energy release process is along with SPD process and is also an impetus to SPD. The as-rolled Zn-5Al alloy has 48 J/mol stored energy which was measured with DSC (differential scanning calorimeter) and conforms well to the calculated value. The as-rolled Zn-5Al alloy after SPD with an elongation of 2 500% releases 112 J/mol stored energy. Analysis shows that the strain rate is in direct ratio to the rate of stored energy release.

SUPERPLASTICITY IN LARGE-GRAINED Fe_3Al ALLOYS

The superplasticity behavior of Fe-28Al, Fe-28Al-2Ti, Fe-28Al-4Ti (all composition reported in this paper are in atomic percent) alloys has been investigated by tensile testing.optical microscopy and transmission electron mocroscopy.Tensile test were performed at 700 to 900℃ under a strain rate range of about 10-5/s to 10-2/s. Maximum strain rate sensitivity index m was found to be 0.5 and the largest elongation reached 620%. The flow activation energy was measured to be 263kJ/mol for Fe-28Al and 191kJ/ mol for Fe-28Al-2Ti, which are much lower than the creep activation energy generally observed in Fe3Al alloys. After deformation grain size became much finer from about 100 μm to 20-30μm.As combined with TEM observations, we suggested that a continuous recrystallization process took place and superplasticity may arise from this process.

Superplasticity of a Ti-24Al-14Nb-3V-0.5Mo Intermetallic Alloy

Superplastic properties and microstructural evolution of a Ti-24Al-14Nb-3V-0.5Mo (at. pct) intermetallic alloy were studied. Optimum superplastic properties were obtained for temperatures in the interval 960℃≤5 T≤5980℃. The apparent activation energy in the superplastic regime was determined and the deformation mechanism was also discussed. Based on the studies, a curve panel with three sheets sandwich structure was fabricated successfully. The microstructures corresponding to different strain in the part were also studied.

Superplasticity of Spray Deposited 5083 Al-Mg Alloy

The superplasticity of spray deposited and thermomechanical processed 5083Al-Mg alloy is investigated in this paper. The results show that spray deposited 5083 Al exhibits anequiaxed grain morphology with an average size of 15 m and porosity in the range of 0.1 vol. % to 5vol. % . Two distinct TMP procedures are employed to close porosity and refine grain size:extrusion plus rolling and direct rolling. The material processed using the former method exhibits arelatively high superplasticity with a maximum superplastic elongation of 465 % , whereas thatprocessed using the latter method exhibits a maximum superplastic elongation of 295 % . Materialsprocessed using extrusion plus rolling and direct rolling both exhibit similar stress-strainbehavior and strain rate sensitivity factors. The strain rate factors are in the 0.3 to 0.5 range.The difference in their superplastic elongation is possibly the result of differences in grain sizeand available cavity nucleation sites provided by closed gas pores.

Superplasticity-like creep behavior of coarse grained ternary Al alloys

Enhanced tensile ductilities in coarse grained Al-Mg-Zn and Al-Mg-Fe materials were studied.The materials were Al-2Mg-5Zn,Al-3Mg-5Zn,Al-4Mg-5Zn,Al-3Mg-0.11Fe,Al-3Mg-0.27Fe,and Al-3Mg-0.40Fe.Tensile elongation-to-failure tests were conducted at constant cross-head speed and constant temperatures from 300 to 450℃.Strain rate change tests were conducted at a constant temperature from 300 to 450℃and in strain-rate range from 4.31×10-5to 1.97×10-2s-1.Experimental results show that over 100%ductilities are consistently achieved in these materials.This superplasticity-like behavior is rate-controlled by solute-drag creep.Although ternary Zn and Fe additions do not have an adverse effect on solute-drag creep and ductility,they increase stress exponent and its sensitivity to Mg content during solute-drag creep.

Superplasticity and superplastic forming ability of Al-Mg-Sc alloy

The superplasticity and formability of a cold-rolled Al-6Mg-0.2Sc alloy with coarse grains were investigated. High temperature tensile tests were performed in the temperature range of 400-500℃with strain rates ranging from 2.0×10-4 to 2.0×10-2s-1, and the maximum elongation of 421% was obtained at 450℃and strain rate of 2.0×10-4s-1. Superplastic bulging tests of a square conical shaped part were carried out under an optimum superplastic condition, with the wall thickness reduction less than 8%, which illustrates that the alloy has good superplastic formability. The cavities in the sections of the tensile specimens and the bulging parts were analyzed, which can be reduced greatly by imposing proper backpressure in the bulging process.