The history of superplasticity is reviewed. The first scientific observation of superplasticity dates back to 1912. Sporadic reports of subsequent observations appear in the literature up to the review by Underwood in 1962. Since that time there have been a number of milestone achievements that have propelled the technology of superplasticity forward. These include commercial developments based on nickel, titanium, iron, and aluminum alloys. In more recent times the advent of superplasticity in ceramics and intermetallics has stimulated yet further interest in the field. At the present time there are major research efforts underway in the USA, USSR, Japan, China, and Europe to apply superplastic forming technology to the manufacture of complex components.

A two-dimensional model for diffusional creep (Coble creep) and diffusionaccommodated grain boundary sliding in polycrystals has been developed. The results obtained for small symmetrical clusters of grains reproduce Spingam and Nix's work [Acta Metall. 2M, 1389 (1978)]. For clusters of irregularly shaped grains the material deposition and removal rates, the fluxes along the grain boundaries, the grain boundary tractions as well as the grain boundary sliding rates are irregular and can all be calculated.

A model is presented to explain the grain growth that is often observed during superplastic deformation. The atomic structure of grain boundaries leads to a coupling between boundary sliding and boundary migration. There is a similar coupling between the absorption or emission of vacancies from a boundary and boundary migration. Because of these couplings, the grain boundary sliding and diffusional flow of superplastic deformation produce extensive boundary migration. We propose that this forced migration leads to random changes in the sizes of grains, and that this evolution of the grain size distribution leads to grain growth.

Grain growth depending on strain is generally observed during structural superplasticity. A new deformation model is proposed in order to explain this grain growth.

This model is based on grain switching by Ashby and Verrall. The modification is to introduce grain size irregularity. The grain structure with minimum irregularity consists of one 5–7 pair in hexagonal array, which corresponds to edge dislocation. While the tensile stress oblique to the pair results the grain switching described by dislocation glide, the stress parallel to it results the grain switching which transforms the pentagon into the quadrilateral and then removes it. The latter can be described by dislocation climb and results the enhancement of grain growth. The expansion into three dimensions is discussed.

The characteristics of superplasticity are discussed from the viewpoint of irreversible thermodynamics, in particular, system evolution and the formation of dissipative structures. A mathematical model combining determinism with indeterminism is used to analyze plastic deformation and superplasticity in metals. Since the plastic deformation of a metal is in essence a dissipative process occurring in a system far away from thermodynamic equilibrium, the instability of the system resulting from structure fluctuations (defects) and thermodynamic bifurcations (possible thermodynamic states) will result in many deformation mechanisms being available to provide the deformation observed in the system. Thus it is virtually impossible to describe all the possible models in conventional terms i.e. by the application of the method of determinism. A new model is proposed, and is shown to account for much of the experimental data reported.

It is well known that cavities are nucleated and grow during the superplastic deformation of many materials. The various theories for cavity growth are examined with special emphasis on the role of growth by diffusion. It is demonstrated that the normal model for the diffusion growth of cavities is inadequate for superplastic materials when the grain boundary lengths are very small. By developing a new model for the growth of an isolated cavity to sizes exceeding the grain size, it is shown that the diffusion process may play a major role in cavity growth under a range of experimental conditions.

An investigation was conducted on the superplastic Zn-22% Al alloy to examine the effect of impurity level on mechanical behavior and cavitation in the alloy. In conducting the investigation, three grades of Zn-22% Al containing different levels of impurities (6 ppm, 100 ppm, and 180 ppm) were used. The results show that the deformation behavior of Zn- 22% Al at low stresses is influenced by the impurity level of the alloy, and that the extent of cavitation increases with increasing impurity content. These two findings lend support to the concept of attributing low-stress superplastic deformation to strong impurity segregation at boundaries.

Cavity nucleation during superplastic deformation of a high strength aluminium alloy has been studied using a high voltage electron microscope and an optical microscope. The results show that cavities nucleation is due only to superplastic deformation and not to pre-existing microvoids which may be introduced during thermomechanical processing. The main reason for cavity nucleation is the high stress concentration at discontinuties in the plane of the grain boundary due to grain boundary sliding.

The ultrafine grain sizes and high diffusivities in nanophase materials assembled from atomic clusters suggest that these materials may have a strong tendency toward superplastic mechanical behavior. Both small grain size and enhanced diffusivity can be expected to lead to increased diffusional creep rates as well as to a significantly greater propensity for grain boundary sliding. Recent mechanical properties measurements at room temperature on nanophase Cu, Pd, and TiO2, however, give no indications of superplasticity. Nonetheless, significant ductility has been clearly demonstrated in these studies of both nanophase ceramics and metals. The synthesis of cluster-assembled nanophase materials is described and the salient features of what is known of their structure and mechanical properties is reviewed. Finally, the answer to the question posed in the title is addressed.

The sintering and deformation kinetics of nanocrystalline (n-) TiO2 were studied. It was found that pressureless sintering yields densities above 95 % with modest increase in grain size whereas pressure assisted sintering (1GPa) results in high density samples without grain growth. In addition compression tests were performed. Large true strains up to 0.6 and strain rates as high as 8 × 10−5 s−1 were observed at 810 °C without fracturing. The sintering and creep results are discussed in terms of the microstructure and diffusion coefficients in n-TiO2.

Methods of preparing non-agglomerated powders for three systems -- yttria, titania, and yttria-stabilized zirconia -- are reviewed. The non-agglomerated nature of these powders should make it possible to sinter them into dense ceramic bodies with nanocrystalline grain sizes. Experiments with yttria-stabilized zirconia have shown that this is indeed the case, with mean linear intercept grain sizes of 60 nm resulting from original powder particle diameters of 13 nm. This ultrafine-grained zirconia is shown, in turn, to have superplastic forming rates 34 times faster than a 0.3 μm-grained commercial zirconia of the same composition.

A non-isothermal analysis of grain growth in tetragonal zirconia ceramics doped with various amounts of yttria and/or ceria in the temperature range 700–1150°C is presented. In these nanocrystalline ceramics, prepared by the gel precipitation technique, two grain growth regimes are recognized. At temperatures up to 900–1000°C grain growth proceeds slowly at high values of porosity (55%→35%); probably by means of a surface diffusion mechanism. At higher temperatures grain growth is much faster and occurs in locally dense regions by a normal grain growth or an impurity drag mechanism. Apparent activation energies are given for five compositions in both regimes.

The isothermal sintering behavior of 3 mol% Y2O3-containing TZP at 1050°C is analysed. A dense ceramic with a crystallite size in the nanometer regime (< 100nm) is obtained.

Transmission electron microscopy (TEM) has been employed to examine the role of dislocations in the superplastic deformation of a duplex stainless steel. In particular, matrix, grain boundary and interphase interface dislocation distributions have been documented after testing in region II. It is concluded that dislocation activity is of importance during superplastic flow and both loop sources and dislocation pile-ups have been observed. The presence of highly distorted dislocation arrays also suggests that grain and interphase boundary sliding is intimately linked to the motion of grain and interphase boundary dislocations. Finally, it is argued that dislocation multiplication in interphase interfaces can involve the operation of loop sources.

High-carbon steels with carbon content of more than 0.8wt% have been known to be superplastic by refining their microstructure. Many methods such as warm rolling have been proposed to refine the microstructure. But most of them are considered to be difficult to use as an actual production method. To make high-carbon steels superplastic and economical, application of a cold rolling and continuous annealing process has been examined.

Changes in microstructure of high-carbon hot rolled steels during the cold rolling and continuous annealing was investigated. By cold rolling with a reduction of more than 50%, a lamellar pearlite structure is deformed and heavily sheared, and a large strain energy is stored. Rapid annealing at temperatures just below the A1 temperature(1000K) is shown to spheroidize the sheared pearlite and to recrystallize the ferrite grains. After processing, the final structure consists of very fine cementite particles with 0.05–0.2μm diameter in a very fine equiaxed ferrite matrix with 0.1–0.5μm diameter. The processed steel shows total elongations of more than 700% and a strain rate sensitivity index of about 0.5 when deformed at 973K.

The microstructural changes associated with phase transformation and the straining behavior in polycrystalline structures during transformational superplastic deformation have betransformatonal superplasticen investigated. In-situ observations have been carried out with specially designed hot-stage microscopes. The strain distribution has been examined by microscopic strain analyses using a micro-grid pattern with 12.7 and 6 μm intervals in pure iron, 0.1%C steel and eutectoid steel. The results are as follows: (1) In the initial stage of Ac3 transformation in pure iron, superplastic strain is induced by the sliding at o/y interface along the prior ferrite grain boundaries and by the grain rotation. In the intermediate stage, the sliding deformation is generated at the migrating tranformation interface associated with the growth of austenite grains. (2) Accumulated strain by sliding has been observed within the previously transformed region during Ac3 transformation in 0.1%C steel. (3) The characteristic straining behavior in the eutectoid steel is due to the behavior at the transformation interface in the microstructure including secondary phase Fe3C particles. These observations suggest that a sliding mechanism at the migrating interface is a principal mechanism of transformation superplasticity.

The phenomenon of internal stress superplasticity is reviewed and the theoretical mechanical foundations of current mechanism models of the process examined. These models are shown to be based on two different principles: i.e. a biased internal plastic flow, or an enhanced creep generated by a superposition of internal and external stresses. Mechanical test data is shown to be more consistent with enhanced plasticity models in zinc and metal matrix composites for deformation during thermal cycling.

The transformation of the phenomenon Superplasticity into a manufacturing technique has been pushed ahead over the last two decades mainly by the aerospace industries. The possibility of producing complicated structures in one step offers well known economic advantages. The combination of superplastic forming and diffusion bonding further enhances design flexibility thanks to the ability of fabricating multisheet structures.

This paper provides an overview of the practical application of Superplasticity in forming components. It focusses on the major aerospace materials i.e. Ti alloys and Al alloys and discusses specific characteristics, the material specifications, and supply situation.

Superplastic forming processing is presented including the techniques used for diffusion bonding; design concepts and exemplary SPF and SPF/DB parts are presented.

Finally a critical discussion concerning current development trends, and the factors limiting broad application of this manufacturing technique are given. Future prospects are outlined and further research needs emphasized.

Generally, the elimination of the voids in bond interface is considered to be the rate-controlling mechanism for diffusion bonding. In an earlier paper, the author proposed a new theoretical model for solid-state diffusion bonding of similar matals, in which the grain growth across the bond interface was considered to be the governing mechanism for diffusion bonding. The aim of the model is to understand how the joint strength is affected both by process variables (temperature, pressure, time) and by the microstructures of the materials being joined. In this paper, the mechanism of grain growth and recrystallization is discussed, and the calculation of an appropriate kinetic equation is made based on a diffusion bonding experiment on a hot-rolled 7075 aluminum alloy. The calculated results basically agree with those of the experiment.

Solid state and transient liquid phase diffusion bonded joints have been made between Al-Li 8090 alloy and 8090 alloy/SiC particulate metal matrix composite. In this paper some shear and peel strengths are reported and the factors that affect the strength and fracture behaviour are discussed. Comparisons are made with other published data for bonded joints in Al-Li alloys.

Because of the presence of surface oxide films, aluminum alloys are very difficult to join by diffusion bonding processes when there are no interlayers between mating surfaces. In the present investigation, the diffusion bonding of superplastic 7075 aluminum alloy has been carried out using a special method for surface treatment in the temperature range 500∼520°C, at pressures of 2.07∼3.OMPa, times of 90∼150min and a vaccum of 1×10−5torr. The joint strength obtained is the same as the base metal strength of the alloy, and the joint microstructure is indistinguishable from that of the base metal. In this paper, the effect of the superplastic treatment of the alloy on the bonding process is discussed, and it is proposed that the bonding mechanism is the migration of the initial bond interface caused both by the diffusion of atoms and by the growth of grains during the diffusion bonding process.