Showing papers on "Superplasticity published in 1985"

Creep of crystals : high-temperature deformation processes in metals, ceramics, and minerals

Abstract: Preface 1 Mechanical background 2 The agents of deformation: lattice defects 3 Phenomenological and thermodynamical analysis of quasi-steady-state creep 4 Dislocation creep models 5 The effect of hydrostatic pressure on deformation 6 Creep polygonization and dynamic recrystallization 7 Diffusion creep, grain-boundary sliding and superplasticity 8 Transformation plasticity 9 Scaling and classification References Indexes

Microstructural aspects of superplasticity

Abstract: The microstructural aspects of the superplastic phenomenon are reviewed. The experimental results of a very large number of investigations are critically analysed in the context of: grain shape and size; grain growth; grain boundary sliding and migration, grain rotation and rearrangement; diffusion and dislocation activity. It is shown, that in spite of often conflicting evidence in the literature, a common pattern of microstructural behaviour emerges for all the materials and conditions investigated to date.

Superplastic Deformation in Fine-Grained MgO 2Al2O3 Spinel

Abstract: Fine-grained polycrystals of MgO 2A12O3 spinel were deformed to large strains at strain rates ranging from 1O-5 to 1O-3 s-1 and at temperatures from 1723 to 1885 K. These polycrystals were ductile at low strain rates and high temperatures; the ductility was especially remarkable at temperatures near the solvus. The mechanism of deformation, which was determined by measuring the change of flow stress with grain size, was dislocation creep, with the stress exponent in the power-law creep equation having a value of 2.1 ± 0.4. Despite deformation of several hundred percent, the grain size remained equiaxed, and deformation led to the evolution of a grain size which depended only on the strain rate and temperature. The initial microstructure had an influence on whether the poly-crystal would fracture or flow; a small initial grain size and a supersaturated solid solution were conducive to ductile flow. The ductility is attributed to dynamic recrystallization. It is proposed that the onset of fracture and the onset of dynamic recrystallization are competitive processes. Conditions which promote dynamic recrystallization also promote ductile flow.

High temperature deformation of ultra-fine-grained oxide dispersion strengthened alloys

Abstract: The high temperature deformation properties of two oxide dispersion strengthened (ODS) alloys, MA 754 and MA 6000, with initial grain sizes of 0.67 µm and 0.26 µm, respectively, have been studied. Tensile tests have been conducted at 1173, 1273, and 1373 K at strain rates ranging from 2 X 10−4 to 5 s−1. Tension creep tests were conducted on MA 6000 at 1273 K to extend the strain rate regime to 3 X 10−8 s−1. Microstructures of both undeformed and deformed samples have been characterized by transmission electron microscopy. MA 754 exhibits a maximum elongation of 200 pct and a maximum strain rate sensitivity of 0.30 at 1273 K. MA 6000 is superplastic, exhibiting a maximum elongation of over 300 pct and a maximum strain rate sensitivity of 0.47 at 1273 K. The microstructure of MA 754 is unstable during deformation, showing recrystallized grains and grains which have grown to 1 µm in diameter. No evidence for ordinary recrystallization is found for MA 6000, and grain growth is slight. For both alloys, strain rates less than about 1 s−1 alter the initial microstructure and prevent grain coarsening on subsequent annealing at higher temperature. Deformation of the fine-grained MA 6000 can be described as a combination of power law creep and diffusional (Coble) creep, with a threshold stress caused by the presence of γt’ particles existing only for the diffusional creep process. Structural instabilities do not permit a simple description of deformation of MA 754.

Tooling system for superplastic forming of metals

Abstract: An oblate spheroid shaped shell has upper and lower component parts which are held together by means of spring loaded clamps. The upper and lower component parts each contain liquid hardenable ceramic material forming die member halves which have surfaces defining a die cavity for superplastic forming of a metal workpiece therein. The ceramic material has electrical heating elements embedded therein and positioned close to the die cavity in order to heat the metal workpiece in the cavity. Tubes in the upper and lower halves of the shell supply pressurized gas and quenching fluid to the die cavity for superplastic forming and quenching of the metal workpiece.

Effect of coalescence on cavity growth during superplastic deformation

Abstract: The accumulation of cavitation damage with increasing strain during the biaxial deformation of two superplastic aluminium-base alloys has been studied using densitometry and quantitative metallography. A model based on constitutive relationships for the diffusive and plastic growth of voids has been extended to account for cavity coalescence, a phenomenon commonly observed during superplastic deformation. Good agreement between the measured and calculated cavity growth rates was obtained for one alloy (AI 7475), while discrepancies observed for the second alloy (Supral 220) are explained in terms of the effects of continuous cavity nucleation.MST/189

Deformation mechanisms and the theory of structural superplasticity of metals

Abstract: A new model of structural superplasticity is proposed on the basis of recent experimental data on the operative deformation mechanisms and the results of investigations of grain boundary properties. The concepts of hardening and recovery are used to analyse the superplastic nature, the development of these processes being associated with grain boundaries. According to the proposed model during superplastic flow the hardening due to easy generation of lattice dislocations (LD's) in the boundaries at the formation of grain boundary dislocation pile-ups is insignificant while the rate of the recovery whose kinetics is determined by LD absorption by boundaries is high. On the basis of these conceptions a system of equations of the dislocation kinetics is obtained which describes the experimental curve of superplastic deformation and the principal causes of a number of structural effects which have not been satisfactorily explained before are given. [Russian Text Ignored].

Superplasticity and superplastic forming processes

Abstract: Superplasticity, first observed some seventy years ago, remained a scientific curiosity until about twenty years ago. It is now recognized as a property which can be utilized in forming processes. There are two types of superplastic behaviour, known as fine–grained (or fine–structure) and internal–stress superplasticity. Fine–grained superplastic materials have a strain–rate sensitivity exponent of 0·5, and deform principally by a grain–boundary sliding mechanism. In this paper the microstructural features important in the development of fine structure super plasticity are discussed, and phenomenological equations for describing superplastic flow are presented. The superplastic properties of fine–grained materials can be optimized by promoting grain–boundary sliding and inhibiting slip. A number of fine–grained superplastic materials have been developed for commercial use, and their number is increasing. Internal–stress superplastic materials can have a strain–rate sensitivity exponent as high as ...

Superplastic Al-Cu-Li-Mg-Zr alloys

Abstract: Superplastic properties have been developed in Al-Cu-Li-Mg-Zr alloys. The alloys have low Zr levels (≤0.2 wt pct) and are experimental compositions that were originally designed as low-density, highmodulus, and high strength alloys for room temperature, aerospace structural applications. The alloys have been manufactured both by an ingot metallurgy (IM) route and a powder metallurgy (PM) route involving rapid solidification processing. After conventional manufacturing, the alloys are not superplastic but require further thermomechanical processing. The microstructural changes that occur during this processing are described. Superplastic properties have been evaluated as a function of temperature, strain rate, and processing history. Prior to thermomechanical processing, the alloys have elevated-temperature ductilities of 100 to 200 pct, strain rate sensitivities of about 0.25, and activation energies corresponding to lattice diffusion. After thermomechanical processing, the alloys have ductilities of 500 to 1000 pct, strain rate sensitivities of about 0.4, and activation energies corresponding to grain boundary diffusion. In addition, room temperature properties have been measured in the solution-treated and peak aged (T6) condition for all the alloys and comparison is made with other commercial, non-Li containing, superplastic alloys. Particular emphasis is placed on properties of interest in aerospace applications such as specific modulus, specific strength, and a buckling failure criterion.

Superplastic forming process

Abstract: Heated superplastic material is deformed using gas pressure which forces the material into a die cavity. Improved three dimensional models for deforming superplastic materials are based upon spherical shapes penetrating the die cavity which is approximated by one of two different rectangular box models. The three dimensional and box models produced radius and thickness equations from which an accelerated gas pressure versus time profile and a minimum thickness value are calculated. The gas pressure deforms the superplastic material at the maximum possible strain rate without rupturing thereby reducing the speed at which parts are formed. Die frictional effects and variable flow stress phenomena are included into the pressure versus time profile computation and thickness equations so as to improve the speed and accuracy of the superplastic forming process.

Superplasticity of δ-ferrite/Austenite Duplex Stainless Steels

Abstract: Superplastic behavior of δ-ferrite/austenite duplex stainless steels has been studied by means of isothermal hot tensile test at temperatures (T)from 700 to 1100°C at initial strain rates (e) from 10-4 to 10-1 s-1 in relation to microstructural aspects prior to and during deformation. Superpiastic elongations were observed in wide ranges of T and e. The maximum elongation greater than 2500% was obtained under the condition where c phase precipitation occurred during deformation. The elongation depended on the precipitation rate of c phase especially in the lower temperature deformation as well as on the prior microstructure. The most suitable microstructure obtained for superplasticity was fine grained δ-ferrite matrix with fine dispersion of γ particles. In the deformation of the specimens with such optimun microstructure, elongations greater than 200% were obtained at T≅1000°C even at e_??_10-1s-1. During superplastic flow, the large microstructural changes were observed. At above 1000°C γ phase was separated and was refined into spherical particles within the δ-ferrite matrix and at temperature below 1000°C, a γ/σ mixed structure was formed by the eutectoid decomposition of δ-ferrite, resulting finally in the stable equiaxed micro-duplex structures with δ/γ and γ/σ, respectively. Such microstructural changes can play an important role for the superplastic behavior in addition to the effect of m-value.

Ultrahigh carbon steels containing aluminum

Abstract: An ultrahigh carbon steel having a composition of carbon in an amount of from about 0.8 weight percent up to the maximum solubility limit of carbon in austenite, aluminum in an amount of from about 0.5 to about 10 weight percent, an effective amount of a stabilizing element acting to stabilize iron carbide against graphitization, and the balance iron. Preferably, the aluminum is present in an amount of from about 0.5 to about 6.4 weight percent and the stabilizing element is chromium. The steel has excellent ductility and is readily hot, warm and cold worked without cracking. It is particularly useful in superplastic forming operations, and may be processed to a suitable microstructure by any technique which reduces its grain size to about 10 microns or less, and preferably to about 1 micron. Such a very fine grain size is readily acheived with the steel, and the aluminum and stabilizing additions act to retain the fine grain size during superplastic processing.

Superplastic deformation of strongly textured Ti-6Al-4V

Abstract: Changes in microstructure and texture during superplastic deformation of strongly textured Ti-6Al-4V bar have been determined in order to establish the cause of stress and strain anisotropy. The effect of strain on the microstructure of the alloy was to cause a progressive break-up, due to grain-boundary sliding, of an initially directional microstructure containing contiguousα-phase. The texture of theα-phase, however, changed very little with superplastic strain but that of theβ-phase was randomized. Shape changes predicted by permitted deformation modes in theα-phase did not correlate with the observed shape changes, whereas the observed anisotropy could be explained by the break-up of the contiguousα-phase. A model to account for this anisotropy is described briefly, together with a typical microstructure which should exhibit isotropic superplastic deformation.

Forming of stiffened panels

Abstract: A method of forming a stiffened panel from first and second metal sheets, at least the first sheet being capable of both superplastic deformation and diffusion bonding, and also provided with at least one control region of different thickness compared with other regions of the sheet, includes the steps of: attaching the sheets together at a series of attachment lines across their faces, the attachment lines and the control region or regions being in predetermined relationship with one another, placing the attached sheets in a mould and heating to within that temperature range within which superplastic deformation and diffusion bonding takes place, urging those areas of the first sheet between the attachment lines away from the second sheet by a common differential pressure at a rate within that range of strain rates at which superplastic deformation occurs to form a series of cavities between the two sheets such that peripheral parts of those areas urged away from the second sheet form side walls of neighboring cavities and become diffusion bonded together to provide internal stiffeners of the finished panel, the control region or regions effecting local modification of the rate of superplastic deformation such that the internal stiffeners adopt a desired configuration and location.

Method for forming structural parts

Abstract: Method for producing structures from metal workpieces which may or may not be capable of superplastic forming, such as titanium alloy and steel, using a forming die having a predetermined contour or shape and a cooperating pressure diaphragm formed of a metal capable of superplastic forming. Fluid pressure is applied to one side of the pressure diaphragm to cause stretching or expansion thereof by superplastic forming, and forcing the diaphragm against the workpiece, which in turn is caused to move or expand into contact with the forming die, to produce a structural component of predetermined shape. With the proper application of pressure, temperature and time, metal parts, e.g. 8 to 12 inches thick, can be formed in addition to parts formed from comparatively thin sheet metal.

On the pressure forming of two superplastic alloys

Abstract: Superplastic forming of the Ti-6Al-4V and Sn-Pb eutectic alloys was attempted using the pressure forming (sheet thermoforming) process. It has been demonstrated that true hemispheres could be formed out of sheets of both the alloys. The thickness strains in both the alloys were less than those predicted theoretically and this could be traced to material flow from the flange and gripped regions. This flow, however, was greater in case of the titanium alloy than the Sn-Pb alloy, on account of the greater strain-rate sensitivity of the former material. Due to the same effect, the “thinning factor” actually increased with deformation in the titanium alloy, but it decreased on increasing deformation in the Sn-Pb alloy. Within the experimental range, the hold-down pressure (titanium alloy) and initial sheet thickness (Sn-Pb alloy) had very small effects, although the deformation became slightly more uniform on decreasing the hold-down pressure or increasing the initial sheet thickness. The thickness and circumferential strains increased with deformation and in particular when the bulge height (h 0) to base diameter (D 0) ratio was greater than 0.35, non-uniformity in deformation along the bulge profile became noticeable. These strains were largest at the pole and its vicinity. On account of its lower strain-rate sensitivity, these effects were more pronounced in the Sn-Pb alloy than in the titanium alloy. Although initially the bulging rate was rapid, later the (h 0/D 0) ratio increased linearly with the forming time and at any instant the bulge profile corresponded to an arc of a circle.

Volume control superplastic forming

Abstract: An apparatus and method for controlling the superplastic forming process by measuring and controlling the volume displaced by the blank being formed so as to measure total strain or surface area increase of the blank.

Influence of prior straining on superplastic behaviour of an Fe–Cr–Ni alloy

Abstract: The superplastic behaviour of a microduplex Fe–Cr–Ni (25·7Cr–6·6Ni) alloy was investigated in the as-worked, annealed, and prestrained conditions. In the early stages of deformation, flow stress depends significantly on strain, and also on the instantaneous microstructural state in the case of as-worked and annealed specimens. Under these conditions, the empirical parameters of the constitutive equation for superplastic deformation were found to depend systematically on strain. At 1000°C, strain hardening predominates, and this could be accounted for by grain growth and by the hardening produced by the noticeable dislocation activity. After suitable prestraining, steady-state deformation conditions may be attained; this may facilitate the collection of σ–e data, which could then be used to assess the relative importance of the appropriate deformation mechanisms.MST/125

Prediction of the optimum pressure cycle for the superplastic forming of hemispheres from Ti-6Al-4V sheet

Abstract: Application du durcissement par deformation, de la sensibilite a la vitesse de deformation et du coefficient R au formage de demi-spheres, en utilisant les concepts de deformation equivalente et de contrainte equivalente

Deformation of Cu–P alloys at high temperatures

Abstract: The deformation behaviour of Cu–P alloys has been investigated by torsion and tensile testing over a range of strain rates and temperatures. The torsion flow curves are interpreted in terms of dynamic softening processes, and the curves obtained during interrupted testing are used to examine static-restoration behaviour. Constitutive equations relating flow strength to strain rate and temperature are deduced, with allowance made for the effect of deformation heating, and implications of the equation constants are discussed. It is shown from tensile results that a state of superplasticity can be achieved in alloys containing 3·8 and 6·8 wt-%P. Superplasticity can occur only if the small α grain size is stable and if the temperature and strain rate fall within certain limits. The activation energy associated with superplastic flow has been determined.MST/52

Hot working method for superplastic duplex phase stainless steel

Abstract: A duplex α (ferrite) and γ (austenite) phase stainless steel exhibiting superplasticity at high strain rate is disclosed. The steel comprises Fe, Cr, and Ni as primary elements, and N in an amount of 0.05-0.25%, preferably 0.1-0.2% by weight. The amount of Cr+Mo+1.5xSi is preferred to be substantially three times as much as that of Ni+0.5×Mn+30×C+25×N. The disclosed steel shows good superplasticity in the temperature range of from 700° C. to the point 100° C. lower than the temperature at which the steel transforms into a single ferrite phase and at a strain rate of at least 1×10 -6 s -1 and less than 1×10 0 s -1 , and can be elongated by more than 1000% at 900° C. and at a strain rate of 1.5×10 -2 s -1 .