Superplasticity

Superplastic materials are polycrystalline solids which have the ability to undergo large uniform strains prior to failure. For deformation in uni-axial tension, elongations to failure in excess of 200% are usually indicative of superplasticity, although several materials can attain extensions greater than 1000%. The highest elongations reported are 4850% and 7750% in a Pb-Sn eutectic alloy; 5500% and greater than 8000% for an aluminium bronze.

Historical Background

Observations of what appeared to be superplastic behaviour were initially made in the late 1920's with a maximum of 361% for the Cd-Zn eutectic at 20°C and strain rates of ~10-8/s and 405% at 120°C and a strain rate of ~10-6/s. Jenkins also reported a maximum of 410% for the Pb-Sn eutectic at room temperature but at strain rates of ~ 10-6/s. However, the most spectacular of the earlier observations was that by Pearson in 1934 (see commentary by Ridley). While working on eutectics, he reported a tensile elongation of 1950% without failure for a Bi-Sn alloy. Pearson also examined the bulging characteristics of his materials using internally pressurised tubular specimens.

Following these observations there was little further interest in the Western world in what was clearly regarded as a laboratory curiosity. Later, and unrelated, studies carried out in the USSR (see commentary by Novikov, ICSAM 1994) discovered the same phenomenon and the term superplasticity was coined by Bochvar and Sviderskaya in 1945 to describe the extended ductility observed in Zn-Al alloys. The word superplasticity is derived from the Latin prefix super meaning excess and the Greek word plastikos which means to give form to; sverhplastichnost was used in the original russian language paper of Bochvar and was subsequently translated as superplasticity in Chemical Abstracts (1947). In 1962 Underwood reviewed previous work on superplasticity and it was this paper, along with the subsequent work of Backofen and his colleagues reported in 1964, which was the forerunner of the present expanding scientific and technological interest in superplasticity.

There are two main types of superplastic behaviour: micrograin or microstructural superplasticity and transformation or environmental superplasticity. Both types of behaviour have been reviewed by Padmanabhan and Davies.

Micrograin Superplasticity

Micrograin superplasticity is shown by materials with a fine grain size, usually less than 10µm, when they are deformed within the strain rate range 10-5 to 10-1/s at temperatures greater than 0.5Tm, where Tm is the melting point in Kelvin. Superplastic deformation is characterised by low flow stresses and this combined with the high uniformity of plastic flow has led to considerable commercial interest in the superplastic forming of components using techniques similar to those developed for the bulge forming of thermoplastics. Superplastically formed parts find many uses particularly in aerospace. Superplastic forging of nickel-base alloys has been used to form turbine discs with integral blades, while diffusion bonding and superplastic forming (DB-SPF) of titanium alloys is used to produce fan and compressor blades for aeroengines. Aluminium alloys can be used in the fabrication of airframe control surfaces and small scale structural elements where low weight and high stiffness are required. Non-aerospace applications of Al alloys include containers with complex surface profiles and decorative panels for internal and external cladding of buildings.

Mechanical Aspects of Superplasticity

The most important mechanical characteristic of a superplastic material is its high strain rate sensitivity of flow stress, referred to as m and defined by:-

For superplastic behaviour, m would be greater than or equal to 0.5 and for the majority of superplastic materials m lies in the range 0.4 to 0.8. The presence of a neck in a material subject to tensile straining leads to a locally high strain rate and, for a high value of m, to a sharp increase in the flow stress within the necked region. Hence the neck undergoes strain rate hardening which inhibits its further development. Thus a high strain rate sensitivity confers a high resistance to neck development and results in the high tensile elongations characteristic of superplastic materials.

If the relationship between Stress and Strain Rate is measured and then plotted logarithmically the slope of the plot is equal to the strain rate sensitivity of the flow stress, m. In practice most superplastic materials show a sigmoidal variation of the flow stress with strain rate and that the strain rate sensitivity passes through a maximum. A value of m>0.4 delineates the superplastic regime, Region II. Both the high and low strain rate ranges exhibit values of m in the range 0.1 to 0.4. The region at high strain rates, Region III, is generally believed to correspond to conventional recovery controlled dislocation creep (power law creep). Deformation within this region leads to the observation of slip lines and to the development of high dislocation densities within the grains. Crystallographic texture within the material is increased and significant grain elongation occurs during deformation.

In the superplastic regime, Region II, where high uniform strains are observed, experimental studies have so far failed to identify a unique rate controlling mechanism of deformation. It is clear, however, that grain boundary sliding and grain rotation make a substantial contribution to the total strain. In contrast to Region III the grains remain equi-axed throughout deformation and materials which initially show microstructural banding develop a more uniform equi-axed microstructure. Crystallographic texture may be reduced during deformation in this region. Transmission electron microscopy studies have shown only limited evidence for dislocation activity within the grains of superplastically deformed materials. The flow stress decreases and the strain rate sensitivity, m, increases with increasing temperature and decreasing grain size. The elongation to failure in this region tends to increase with increasing m.

The origin of the low strain rate regime, Region I, is at present unknown. The experimental evidence available at these low strain rates is both limited and often contradictory. It has been suggested that the decrease in the strain rate sensitivity with strain rate is only apparent and results from a threshold stress for deformation, or from the effects of microstructural instability (grain growth hardening). Alternatively, the similarity in stress exponent (n=1/m) between Regions I and III has been used to imply that Region I also involves recovery controlled dislocation creep. Other experimental investigations, however, have shown that at low strain rates, the strain rate sensitivity can increase, taking values close to unity and thereby implying diffusion creep.

The mechanical behaviour of superplastic materials is very sensitive to both temperature and grain size. In general, increasing the temperature or decreasing the grain size of the material has a similar effect on the variation of flow stress with strain rate. Increasing the temperature decreases the flow stress, particularly at the lower strain rates corresponding to the transition from Region II to Region I. The maximum strain rate sensitivity has been found to increase with increasing temperature and the strain rate of maximum 'm' moves to higher strain rates. The increases in m-values are much greater in Region II than in Region III. The strain rates at which superplasticity is normally observed lie in the range 10-5 to 10-1/s, although this is more usually between 2x10-4 and 2x10-3/s. These strain rates are less than those used in conventional hot deformation processes.

More recently, superplasticity has also been reported at strain rates >1 with elongations as high as 1000% at strains rates approaching 102/s in mechanically alloyed aluminium alloys and around 500% at strain rates of 10 to 50 /s in aluminium based metal matrix composites. In each of the above instances, and others, deformation was carried out at temperatures just below the solidus and there is evidence to suggest that the high strain rates employed result in heating of the sample such that the temperature rises just above the solidus and partial melting occurs in the grain boundary, allowing the accomodation of superplastic flow to occur in a liquid layer.

Superplastic Materials

For superplastic behaviour a material must be capable of being processed into a fine equi-axed grain structure which will remain stable during deformation. The grain size of superplastic materials should be as small as possible, but is normally in the range 2 to 10µm although a limited amount of superplasticity is still observed for grain sizes up to 20µm or even higher. In the presence of a suitable microstructure, superplasticity occurs over a narrow range of temperatures which generally lies above 0.5Tm.

There are two main types of superplastic alloys: pseudo-single phase and microduplex. In the former class of material, a combination of hot and cold working and heat treatment is employed to develop a fine scale distribution of dispersoids so that on recrystallization the alloy will have a grain size of the order of 5µm or less. Ideally the dispersion of particles will also prevent any further grain growth during superplastic deformation, however, most of the second phase particles have usually passed back into solution at the superplastic forming temperatures. The precipitation strengthened aluminium alloys based on the Al-Cu, Al-Mg, Al-Zn-Mg and the Al-Li & Al-Cu-Li alloys can be included in this group. Other materials include dispersion strengthened copper alloys where silicide or aluminide particles are used to limit grain growth. Some steels and ultra fine grain ceramics such as ZrO2 and UO2, can also be classed as essentially single phase. The aluminium alloys, which from a commercial viewpoint are the most important of the pseudo-single phase materials, can be further sub-divided into those which are recrystallized prior to superplastic forming and those which acquire their fine grain structure only after a limited amount of deformation at the forming temperature. Superplasticity has also been observed in a large number of aluminium-base metal matrix composites.

The microduplex materials are thermomechanically processed to give a fine grain or phase size. Grain growth is limited by having a microstructure that consists of roughly equal proportions of two or more chemically and structurally different phases. This latter group of materials includes a/b titanium alloys, a/g stainless steels, a/b & a/b/k copper alloys, eutectics and some ceramics. In the case of the ceramic materials, tensile elongations greater than 400% have been reported. Although these elongations are small by comparison with those commonly attained in metallic materials, they are much larger than the 1 to 2% normally observed in structural ceramics.

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