The phenomenon of superplasticity is explored in the range of high strain rate for both nanocrystalline and microcrystalline materials. True tensile superplasticity has been demonstrated in nanocrystalline grain size range. The difference in the details of such superplasticity between nanocrystalline and microcrystalline state is emphasized.

The international joint project “Ceramics Superplasticity” has been conducted from 1995 till 1999 by a cooperation between the Japan Science and Technology Corporation (JST) and Max-Planck-lnstitute for Metals Research (MPI-MF). The new type of covalent ceramics was developed through atomic level processing, and novel properties such as superplasticity were investigated. The development of local analysis technique of disordered structure combined the two research fields, grain boundary of crystalline solids and structure of amorphous.

It is necessary for grain boundary dislocations to slide and climb during the grain boundary sliding process that dominates fine-grained superplastic deformation. The process of climb requires either an influx of vacancies to the grain boundary plane or a local generation of vacancies. Transmission electron microscopy (TEM) observations of grain boundaries in superplastically deformed Al-Mg-Mn alloys quenched under load from the deformation temperature have revealed the presence of nano-scale cavities resulting from a localized supersaturation of vacancies at the grain boundary. Compositional measurements along interfaces have also shown an effect of solute atoms on the local structure. This is shown to result from a coupling of vacancy and solute atom flows during deformation and quenching. Calculations of the localized vacancy concentration indicate that the supersaturation along the grain boundary can be as much as a factor often. The effects of the local supersaturation and solute atom movement on deformation rates and cavity nucleation and growth will be discussed.

Superplastic behavior associated with grain growth in ceramics at high temperature is predicted by using a modified constitutive equation for superplasticity that incorporates the rate of grain growth. The equation reveals that there is an optimum deformation conditions – i.e., the strain rate and deformation temperature - so that ceramics have low flow stress when the grain growth is significant. The equation provides an explanation of increased flow stress for liquid-phase-sintered SiC in the slow-strain-rate region or higher-temperature region in which grain growth plays an important role.

The linear relation between dynamic grain growth rate and strain rate appears to be constant across all superplastic materials—regardless of whether the system examined is metallic or ceramic, and regardless of the stress exponent exhibited during deformation. The simplicity and universality of the dynamic grain growth law suggest it might be useful as a foundation for a theory for superplasticity. One attempt at such a theory is presented. It is argued that stress leads to the development of anisotropic grain shapes that then require recovery through directionally biased grain growth events. Once this mathematical relationship between stress and grain growth rate is developed, it is inserted into the existing dynamic grain growth - strain rate law to arrive at a phenomenological law for superplasticity.

Superplastic tensile deformation is simulated in 2 dimensions by incorporating grain boundary diffusion and concurrent grain growth derived from static and dynamic growth mechanisms. The following relationship is found between microstructural changes and deformation behavior for constant stress conditions. Grain boundary diffusion produces an increase in the aspect ratio of the matrix grains during deformation and the increased aspect ratio causes a change in creep rate parameters: the stress exponent is decreased from the initial value of 1.0 for equiaxed grains and the grain size exponent is increased from the initial value of 3.0. Accelerated grain growth is also found by the present simulation.

This paper examines grain boundary sliding under the conditions of plastic strain incompatibility that is the most frequent case in polycrystalline materials. Two components of grain boundary sliding: dependent and independent on intragranular slip are distinguished. Theoretical estimate of a ratio between slip induced sliding and intragranular slip is obtained. It is concluded that slip and sliding are rather independent than interrelated processes.

Polycrystalline materials having crystallographic anisotropy show internal stress superplasticity under thermal cycling conditions. The deformation mechanism is analyzed using a theoretical model based on continuum micromechanics including the effects of crystallographic texture. The model is experimentally verified through the thermal cycling creep tests using polycrystalline zinc having two kinds of fiber-texture, and agrees well with the experimental results.

Over the past thirty years superplastic aluminum forming has advanced from a laboratory curiosity to a well established manufacturing route. Applications are now found in many and diverse markets, including: medical systems, specialist automobiles, rail cars, architectural panels and all types of aircraft.

This paper examines the reasons for success in these markets and explores future prospects including the potential for fast forming alloys currently under development.

This paper examines the possibilities of manufacturing large-scale aluminium sandwich structures using superplastic forming. The materials tested were Mg-alloyed production quality aluminium (Al 5083-0, Al 5083-H321) and Aluminium 1561. Tensile tests at elevated temperatures were performed in order to establish the suitability of the test materials for superplastic forming. The microstructural changes in the test material specimens were examined. Thereafter numerical simulation of the Al 5083-0 forming process was conducted based on the tensile test results. The numerical simulation results were subsequently used to estimate forming parameters and the feasibility of manufacturing large-scale structures by superplastic forming.

The results indicate that higher strain can be reached at higher temperatures for the test materials. Aluminium alloy 1561 exhibited the largest elongation to fracture and Al 5083-H321 the smallest. Strain appeared to be temperature dependent but not much affected by the strain rate. Metallographic examination clarified that Al-5083-0 and AI-5083-H321 showed susceptibility for cavity forming whereas Aluminium 1561 formed relatively few voids. The numerical simulation indicated that Al 5083-0 can be superplastically formed using relatively low forming pressure (0.9 – 1.4 bar).

The superplastic forming (SPF) of titanium alloys is an established technology. A reduction in grain size from that of the typical sheet materials would lead to enhanced SPF properties and hence a reduction in production cycle times. This study describes the microstructural development and superplastic behaviour of fine-grained Ti-6%Al-4%V alloys. Ball-milling Ti-6%Al-4%V powder produces a nanocrystalline material; however on consolidation by hot isostatic pressing rapid grain growth occurs. Addition of boron powder during milling leads to boride precipitates in the matrix of the consolidated alloy. The precipitates are dispersed inhomogeneously, resulting in localized grain refinement. Superplastic testing revealed cavitation formation but in comparison to conventional sheet material, large elongations were achieved at relatively high strain rates.

In this paper we address the controversial issue of nucleation of cavities in Al 5083 alloys. We focus on the origin of cavities during the manufacture of these alloys into SPF (superplastic forming) sheet form. Experimental observations on the pre-existing cavities in this alloy are made using optical and electron microscopy. The effects of rolling direction and state of stress during superplastic deformations on the formation of cavities are also discussed. Numerical simulations of the sheet manufacturing process are carried out to understand the effect of hard phase/matrix, mechanical properties and interfacial strength on the origin of cavities. Based on the numerical results, a simplified model relating the process, material parameters and the cavity nucleation is presented.

This paper has two distinct goals. First, we argue in an extended introduction that high velocity forming, as can be implemented through electromagnetic forming, is a technology that should be developed. As a process used in conjunction with traditional stamping, it may offer dramatically improved formability, reduced wrinkling and active control of springback among other advantages. In the body of the paper we describe the important factors that lead to improved formability at high velocity. In particular, high sample velocity can inhibit neck growth. There is a sample size dependence where larger samples have better ductility than those of smaller dimensions. These aspects are at least partially described by the recent model of Freund and Shenoy. In addition to this, boundary conditions imposed by sample launch and die impact can have important effects on formability.

An experimental work on the superplastic bulge forming of machined sheets is presented in this study. Unlike the previously employed incremental-iterative method, a reverse deformation model was used to estimate the initial thickness distribution of the machined sheets from which a constant final thickness can be obtained when the shape of the bulged sheet is hemisphere. The reverse deformation modci was obtained by modifying previously-known models, which were based on the axisymmetric membrane and the incremental strain theory.

Bulge forming experiments were conducted on machined sheets of Al alloy, A5083, at about 530°C under constant pressure control mode. The result of this simulation to estimate the final constant thickness distribution agreed well with the experiment, and confirmed that the reverse deformation model can be successfully applied in optimizing the thickness distribution of the starting sheets in order to obtain the desired final thickness distribution of the free bulged hemispherical product.

The optimum materials design in microstructural control could be developed for the highstrain-rate superplastic materials in the industrial scale. In the present work, it is reported that the high-performance-engine pistons with near-net-shape can be fabricated by the superplastic forging technology in the high-strain-rate superplastic PM Al-Si based alloy, which is produced by using this optimum materials design.

The superplastic properties and microstructures of three contrasting fine grained aluminium alloys were investigated. These included (i) a powder metallurgy MMC, (ii) a severely deformed spray cast alloy, and (iii) the Zn-22%Al eutectoid alloy. The results showed some differences in the details of behaviour between the alloys. One of these was the presence of true work hardening, associated with dislocation activity, in the MMC, and its absence in the microduplex Zn-Al eutectoid alloy. In addition, the powder route MMC had the high threshold stress (up to 10MPa) commonly encountered in such materials, whereas this was not the case with the cast and severely deformed alloy, indicating that the threshold stress was associated with the presence of ceramic particles in the MMC (oxide + reinforcement). In all the alloys, however, the m value tended to increase with temperature, and this led to a corresponding increase in elongation with temperature until the microstructure became destabilised. In the two predominantly single phase alloys studied this destabilisation corresponded to the solvus temperature. The constantly changing m value indicates that there is no unique value of this parameter. The same was found to be true of the grain size exponent, p. The direct interpretation of apparent activation energies in terms of simple physical processes should be made with care in the light of the present results, as (i) the microstructures of two of the alloys were found to change continuously and significantly with small changes in temperature, and (ii) the fact that m is a function of temperature necessarily implies that the apparent activation energy is a function of stress.

Ultra-fine microstructure consisting of equiaxed Al-grains and aluminide particulate was produced by powder metallurgy process using gas-atomized powders of Al-10wt%Ti-2wt%Fe alloy. High strain rate superplasticity (HSRS) has been investigated at 873-923K and strain rates higher than 10−3s−1 in tension, and total elongation up to 500% was observed at the strain-rate of 10−1s−1. The strain rate vs. flow stress behavior exhibits the typical aspect of HSRS such as the increase of strain-rate sensitivity exponent with increase in strain-rate and an apparent activation energy higher than that for lattice diffusion in aluminum. The concept of threshold stress has been incorporated to illustrate the HSRS behavior, where the stress exponent of 3 describes the experimental data. The determined threshold stress showed strong temperature dependency as in the case of a similarly processed Al-10wt%Ti alloy, which exhibited the stress exponent of 2 in the same testing conditions. Solute drag mechanism has been postulated for the Al-Ti-Fe alloy.

Superplastic properties of magnesium alloys and their composites were reviewed with a special emphasis on the achievement of high strain rate superplastic forming. The role of grain size on superplastic deformation mechanisms was particularly addressed. Commercial Mg-Al-Zn alloys and a ZK60-based composite are used as model materials to illustrate the underlining principles leading to the observation of high strain rate superplasticity. In this paper, experimental results from several processing routes, including thermomechanical processing, severe plastic deformation, and extrusion of machined chips and rapidly solidified powders, are presented. High strain rate superplasticity (HSRS) is demonstrated in ZK60-based composites.

We have investigated the flow stress and elongation of superplastic deformation in a La55Al25Ni20 (at%) metallic glass that has a wide supercooled liquid region of 72 K before crystallization. The superplasticity that appeared in the supercooled liquid region was generated by the Newtonian viscous flow that exhibits the m value of unity. The elongation to failure was restricted by the transition of the Newtonian flow to non-Newtonian one and the crystallization during deformation. We succeeded in establishing the constitutive formulation of the flow stress in the supercooled liquid region. Its formulation was expressed very well by a stretched exponential function σflow=Dε exp(H*/RT) [1-exp(E/{ε exp(H**/RT)}0.82)]. Formulations describing the elongation to failure in constant-strain-rate and constant-crosshead velocity tests were, moreover, established. It was found from the simulation that the maximum elongation in the constant-strain-rate test reached more than 106% which was two orders of magnitude larger than that in the constant-crosshead-velocity test.

Using the technique of equal-channel angular (ECA) pressing, it is possible to reduce the grain size of polycrystalline materials to the submicrometer level. Thus, this processing technique has the potential for producing materials which may exhibit superplasticity. This paper describes various factors affecting the development and evolution of the microstructure produced by ECA pressing. Optimization of such factors is then presented for the advent of superplasticity.