Showing papers on "Superplasticity published in 1987"

Superplasticity in a high strength powder aluminum

Abstract: The superplastic properties of a rapidly solidified, high strength P/M Al alloy and the same alloy reinforced with SiC particulates (SiCp) have been studied. To prepare superplastic test materials, a matrix alloy powder of composition 7.2Zn-2.4Mg-2Cu-0.2Zr-0.12Cr-0.2Co (Kaiser PM-64) and the powder mixed with 10 to 20 vol pct SiCp (~5 μm diameter) were thermomechanically processed to very fine equiaxed grain structures of ~6 μm and ~8 μm, respectively. Superplasticity in these materials was evaluated by characterizing (1) high temperature stability, (2) dynamic grain growth, (3) strain rate sensitivity, (4) flow stress behavior, (5) cavitation and cavitation control, and (6) total superplastic strain. It was observed that the PM-64 alloy could achieve a total elongation of over 800 pct, while the SiCp reinforced alloy could attain an elongation greater than 500 pct before failure. Also, it was shown that with the use of hydrostatic pressure during superplastic flow, cavitation could be controlled. Observations were made of the effect SiCp reinforcement particles had on the superplastic flow stress behavior. Interpretations are proposed to explain the role of particulates during superplastic straining.

Superplastic aluminum products and alloys

Abstract: Superplastic forming of aluminum work stock is improved by including therein about 0.05% to about 10% or 15% scandium together with up to 0.2 or 0.25% zirconium. In preferred practices, soluble elements such as magnesium are also included in the aluminum alloy. One or more of the elements from the group of scandium, yttrium, gadolinium, holminum, dysprosium, erbium, ytterbium, lutetium, and terbium, may be included in addition to or in lieu of scandium. Heat treatable aluminum alloys such as 7XXX alloys and 2XXX alloys can be made superplastic by including scandium and zirconium to provide very high strength in superplastically formed products.

Solid state bonding of superplastic AA 7475

Abstract: Solid state (diffusion) bonds have been developed in AA 7475E sheet material using different regimes of bonding temperature, pressure, and time. The bonds produced have exhibited shear strengths in the range 30–150 MN m−2 and have been found to be capable of withstanding peel during a slow high-temperature superplastic forming operation. However, the bonds formed under any one set of bonding conditions showed an extremely wide variation in both shear strength and shear fracture mode. The source of this wide variation has not yet been identified.MST/601

Internal stress superplasticity in anisotropic polycrystalline zinc and uranium

Abstract: Internal stress superplasticity is assessed in powder metallurgy and wrought polycrystalline Zn and in polycrystalline α-U. These materials are anisotropic in their thermal expansion coefficients, and, as a result, internal stresses are generated during thermal cycling. A creep model is developed based on the contribution of internal stress to the enhancement and inhibition of normal plastic flow. This creep model, which has no disposable parameters, is shown to describe quantitatively the flow behavior of anisotropic materials under thermal cycling conditions, and correctly predicts the attainment of Newtonian flow characteristics at low stresses. It is predicted that certain polycrystalline ceramics can be made superplastic in tension when thermally cycled under small applied stress.

Some developments in the numerical simulation of metal forming processes

Abstract: The paper describes developments in the numerical analysis of metal forming processes mainly motivated by industrial applications. It deals with a complete consideration of the unsteady contact developing between the material and the die, the regeneration of the finite element mesh during the course of the calculation, and with the simulation of superplastic forming processes. In particular, an approach relating both the contact pressure and the friction force to the motion of the material relative to the die surface leads to a convenient computational procedure and to a smooth numerical behaviour under friction. The topological part of the contact algorithm appears well‐suited also for the redefinition of the discretization mesh. As a selected application, superplastic forming is considered in conclusion. Industrial practice requires the adjustment of the forming pressure to a prescribed value of the maximum rate of deformation in the material.

Microstructural change during superplastic deformation of δ-ferrite/austenite duplex stainless steel

Abstract: Superplasticity of a 25 pct Cr-6.5 pct Ni-3 pct Mo-0.14 pct N δ/γ duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation. Two large superplastic elongations are obtained at temperatures around 1323 K in δ/γ duplex phase region and 1173 K where σ phase particles precipitate dynamically at a strain rate of ~10−3 s−1. During deformation in the higher temperature region, fine Widmanstatten γ particles coarsen and coarse γ grains formed during the prior treatments are broken into spherical particles, resulting in a homogeneous dispersion of γ particles within the σ-ferrite matrix. The dynamic recrystallization of soft σ-ferrite matrix occurs locally in the region where the strain reaches some critical value, and the final microstructure consists of equiaxed σ and γ grains. In the case of lower temperature deformation, a eutectoid decomposition of δ-ferrite into γ and σ phases occurs. The relatively soft γ grains which are severely deformed by hard σ particles recrystallize dynamically, and these processes lead to the γ/σ equiaxed duplex structure. The extremely large superplasticity of this alloy can mainly be explained in terms of the above microstructural change during deformation.

Method for fabricating monolithic aluminum structures

Abstract: A method for forming monolithic structures from a plurality of aluminum or aluminum alloy sheets is provided which integrates a method for producing a selective fine grain structure in each of the sheets, with roll bonding and superplastic forming. The sheets are subjected to isothermal aging or controlled slow cooling to obtain a desired grain size in each of the respective sheets. Different heating and cooling treatments can be individually applied to each of the sheets so that the sheets can be tailored to have a fine grain or a coarser grain. Stop-off material is applied at selected areas of the sheets, and the resulting stack of sheets is subjected to cold roll bonding to bond adjacent surfaces of the sheets where stop-off material has not been applied. The roll bonding step also serves to plastically deform the sheets. The roll-bonded stack of sheets is then subjected to rapid heating for recrystallization of one or more of the sheets into a fine grain structure. After recrystallization, the sheets are subjected to superplastic forming into the desired monolithic structure.

Superplastic Behavior of Al-Mg-Cu Alloys

Abstract: A new superplastic Al-Mg-Cu alloy is described. The effect of copper addition on superplastic properties of Al-Mg base alloy (AA5083) has been studied. By addition of 0.6%Cu, a high elongation of about 700% is obtained in deformation at a strain rate of 2.8×10-3s-1 and 550°C. Typical mechanical properties of formed parts made from the new alloy are 340MPa in tensile strength and 25% in elongation. The present alloy sheet has a potential for general applications in building and construction because of its properties very similar to the conventional 5000 system alloys and high productivity in the conventional casting and rolling process.

On the contribution of concurrent grain growth to strain sensitive flow of a superplastic Al-Cu eutectic alloy

Abstract: Flow behavior and microstructural evolution in an Al-Cu eutectic alloy of equiaxed grains were investigated over e ≃ 2× 10−6 to 2 × 10−2 s−1 andT = 400° to 540 °C Depending on the test conditions, there appeared either strain hardening or strain softening predominantly in the early part of the σ-e curves The microstructural observations showed evidence for grain growth, development of zig-zag boundaries, dislocation interactions, and cavitation The grain growth adequately accounts for the observed strain hardening at higher temperatures and lower strain rates However, at lower temperatures the strain hardening can be only partly accounted for by the observed grain growth; under this condition, some dislocation interactions also contribute to the strain hardening The presence of cavitation causes strain softening predominantly at higher strain rates Therefore, to develop a proper understanding of the superplastic behavior of the Al-Cu eutectic alloy, it is necessary to take into account the influence of dislocation interactions and cavitation along with that of grain growth

Method of making heat resistant heavy-duty components of a turbine by superplasticity forging wherein different alloys are junctioned

Abstract: The method concerns fabricating heat-resistant heavy-duty components of a rbine, e.g. disk, wheel, etc. wherein two or more different kinds of alloys are junctioned, which comprises solidifying the one alloy powder by hot isostatic pressing or extrusion, junctioning and solidifying the solidified alloy with the other alloy powder by hot isostatic pressing or extrusion, and subjecting the alloys thus obtained to superplasticity forging thereby to secure the junction boundary. Ni-base superalloys having different complete solid solution temperatures of gamma prime phase are selected for use in the hub and rim of a turbine disk or the like, the one alloy which is higher in the temperature being disposed for the hub and the other alloy for the rim. The difference in the temperature between both alloys is at least 8° C. Further solution heat treatment is performed at a temperature between the complete solid solution temperatures to adjust crystal grain size. Heat-resistant heavy-duty components of a turbine, e.g. disk, etc. are thus fabricated by superplasticity forging from Ni-base superalloys.

Superplastic Deformation Mechanism of δ/γ Duplex Stainless Steels

Abstract: Superplasticity of δ/γ duplex stainless steels has been studied in relation to the microstructural change during deformation in order to elucidate the role of the second phase particles. In the case of deformation in the δ/γ duplex phase region, local strain concentration within the δ-ferrite matrix due to dispersion of relatively hard γ particles and the subsequent recrystallization occur repeatedly. When the alloys containing high Cr and Mo are deformed at temperatures around 900°C, σ phase particles precipitate via eutectoid decomposition of δ-ferrite into new γ and σ phases, and the γ/σ duplex phase structure forms. The dynamic recrystallization of γ phase matrix occurs locally and intermittently, and extremely large superplasticity is also obtained. Prior cold work largely accelerates these processes, and leads to equiaxed δ/γ or γ/σ duplex structure in the early stage of deformation. The superplasticity in both cases can be obtained by subtle balance between the local strain hardening and the subsequent recrystallization, and thus the flow stress exhibits large strain rate dependence. This model can also explain granular appearance of the fracture surface as a coalescence of microvoids induced by the individual hard phase particles. In order to obtain large superplasticity, the microstructure should consist of hard particles embedded within a soft matrix and the amount of hard particles should be at least greater than 10% in the case of γ/δ duplex structure. The structure with hard phase matrix within which the soft second phase particles disperse do not exhibits superplasticity.

High-temperature deformation of a fine-grained zirconia

Abstract: Etude par MET de l'evolution de la microstructure d'une zircone lors de sa deformation sous contrainte a 1220 o -1300 o C

An experimental study on the alignment of cavities in a superplastic commercial copper alloy

Abstract: A detailed study of cavitation was conducted in a commercial copper alloy deformed superplastically at a strain rate of 1.3 × 10-3 s-1. Cavities are observed to form in stringers parallel to the tensile axis due to cavity nucleation around aligned stringers of large cobalt-rich particles present in the as-received alloy. The increase in the number density of cavities and the range of cavity sizes observed indicates that cavities nucleate continuously during superplastic deformation. At large elongations, the interlinkage of cavities in a direction perpendicular to the tensile axis tends to mask the alignment of cavity stringers. It is demonstrated that the present results can rationalize satisfactorily the previous observations of cavity alignment in the superplastic copper alloy.

Cavitation and cracking in as-cast and superplastic Pb-Sn eutectic during high-temperature fatigue

Abstract: On montre que la cavitation et la fissuration ont lieu aux joints separant les colonies lamellaires adjacentes dans l'alliage eutectique moule et aux joints d'interphase dans les materiaux superplastiques a structure a grains fins

Grain refinement and superplastic forming of an aluminum base alloy

Abstract: A rapidly-solidified aluminum alloy powder having a nominal composition of 7% Zn, 2.5% Mg, 2% Cu, 0.3% Zr, and 0.3% Cr is used to make a high forming-rate, superplastic, high-strength aluminum alloy. The powder is outgassed, consolidated, and extruded, thereby developing a wide range of particle size distribution of dispersoids in the process, containing respectively zirconium and chromium dispersoids, as well as age hardening precipitates. The consolidated powder is then rolled to 85% reduction to provide a sheet material which is superplastically formed at a temperature in the range of 450° C. to 490° C. and at a rate between 5×10 -3 to 5×10 -2 per second.

Method of making a metal bone implant

Abstract: The metal bone implant is constructed with thin sheets of superplastic material which are deformed into irregular contours. The sheets are welded to the solid base element of the implant and thereafter deformed outwardly under fluid presure to an irregular shape.

In-situ microstructural observations and micro-grid analyses of transformation superplasticity in pure iron.

Abstract: In the present paper, the microstructural changes associated with phase transformation and the straining behavior in polycrystalline structures dueing transformation superplastic deformation are investigated. In-situ observations of microstructural changes during Ac3transformation on rapid heating in pure iron have been carried out with a high temperature optical microscope and a dark field reflection high temperature microscope. Both of hot-stage microscopes were specially designed for this study. The distribution of superplastic strain has been examined by microscopic strain analyses by using a micro-grid pattern with 12.7μm intervals. The main results are as follows:(1) Ac3 transformation process at a heating rate under 50K/s is predominantly nucleation of austenite grains at the prior ferrite grain boundaries and triple points. A subsequent growth of grains into the prior ferrite matrices is observed. The growth of austenite grains is not always isotropic under a tensile stress.(2) In the initial stage of transformation, superplastic strain is induced by the sliding at γ/α transformation interface along the prior ferrite grain boundaries. The observed superplastic strain is also associated with the grain rotation, corresponding to the growth of austenite grains which surround ferrite grains.(3) In the intermediate stage of transformation, the sliding deformation is generated at the migrating transformation interface associated with the growth of austenite grains. Accumulated strain by the sliding is left within the previously transformed region.(4) These observations suggest that the sliding mechanism at the migrating transformation interface is a principal mechanism of transformation superplasticity.

Post-forming tensile properties of superplastic Ti–6Al–4V alloy

Abstract: A study has been made of the influence of uniaxial superplastic deformation on the ambient temperature tensile properties of Ti–6Al–4V sheet. Material was deformed to various strains up to 200% at temperatures from 850 to 970°C at strain rates in the range 1·1−18 × 10;amp;#x2212;4s−1 (0·7−11% min−1). Tests were also performed on statically annealed material to separate the effects of high temperature exposure and superplastic deformation. Mechanical property changes were complex and depended on the relative contributions from the strengthening and softening mechanisms occurring during either superplastic deformation or heat cycling. Structural features influencing mechanical properties were phase size and morphology, dislocation density, and crystallographic texture. The strength after superplastic deformation was always less than that of as-received material but a significant reduction in strength was attributable to heat cycling. In some cases, the strength of the superplastically deformed mate...

Microstructures and superplastic behavior of eutectic Fe-C and Ni-Cr white cast irons produced by rapid solidification

Abstract: Superplastic behavior of two commercial grade white cast irons, eutectic Fe-C and Ni-Cr white cast irons, was investigated at intermediate temperatures (650 to 750 °C). For this purpose, rapidly solidified powders of the cast irons were fully consolidated by compaction and rolling at about 650 °C. The volume fractions of cementite in the eutectic cast iron and in the Ni-Cr cast iron were 64 pct and 51 pct, respectively, and both cast irons consisted of fine equiaxed grains of cementite (1 to 2 μm) and ferrite (0.5 to 2 μm). The cast iron compacts exhibited high strain-rate sensitivity (strain-rate-sensitivity exponent of 0.35 to 0.46) and high tensile ductility (total elongation of 150 pct to 210 pct) at strain rates of 10-4 to 10-3 s-1 and at 650 °C to 750 °C. Microstructure evaluations were made by TEM, SEM, and optical microscopy methods. The equiaxed grains in the as-compacted samples remained unchanged even after large tensile deformation. It is concluded that grain boundary sliding (e.g., along cementite grain boundaries in the case of the eutectic cast iron) is the principal mode of plastic deformation in both cast irons during superplastic testing conditions.

composite magnetic compacts and their forming methods

Abstract: Composite magnetic compacts having good conductivity and excellent mechanical and magnetic properties and their forming methods. The composite magnetic compacts are basically made by forming mixtures consisting essentially of 1 to 50 percent by weight of a magnetic powder and the remaining percentage of a powder of superplastic Zn-22A1 alloy. A drop in the strength of the compacts that occurs when the mixing percentage of the magnetic powder increases is made up for by the impregration of plastic in the compacts or the simpler addition of a plastic power to the mixture of the powders of magnetic material and superplastic Zn-22A1 alloy. The forming methods of the composite magnetic compacts are carried out at different temperatures and under different conditions depending on the composition of the powder mixtures and so on.

Superplasticity of aluminium alloys grain-refined by zirconium

Abstract: The superplasticity of aluminium alloys containing magnesium, oint, iron and manganese as well as zirconium as a grain-refining addition element was investigated by high-temperature tensile tests. The tool elongation and the strain rate sensitivity as a function of strain rate and temperature were determined. The activation enthalpy and activation volume were also determined in the superlastic region of the deformation. !n addition to the tensile tests metallo-graphic investigations were also made. The results obtained show that the superplasticity of the alloys investigated is increasing by the addition of iron but it is decreasing by the simultaneous addition of iron and manganese.