Processing of Materials for Superplasticity
Introduction
It is well established that a fine grain size is an essential pre-requisite for superplasticity. An understanding of the basic metallurgical principles underlying grain refinement and grain growth is therefore important to the development of superplasticity in materials which would not normally be superplastic. Several methods are available for grain refinement including phase separation, phase transformation, and mechanical working with recrystallization. It should be possible in principle to develop fine grain microstructures using thermal treatments alone. However, the imposition of mechanical working during any stage of the heat treatment may give a thermomechanical process which produces the required grain size in a fewer number of processing steps.Unfortunately, merely achieving a fine grain size is not, itself, sufficient to guarantee that a material will exhibit superplasticity, since the grain size needs to remain stable throughout the deformation process. Grain growth during superplastic flow has been reported for a number of materials with the extent of grain growth being greater in the superplastically deformed part of the samples studied than in the undeformed areas. From the available data it is clear that strain enhanced grain growth is a widespread problem of superplastic deformation in both pseudo-single phase and microduplex materials.
In this section the metallurgical principles underlying the thermomechanical and thermal treatments that are capable of refining the grain size of a material are described.
For a more detailed account of the procedures that have been applied to develop superplasticity in particular alloys see
- Aluminium Alloys,
- Titanium Alloys,
- Iron alloys,
- Copper alloys and
- Other alloys, including Ceramics.
Grain Refinement by Mechanical Working
Duplex alloys
Grain refinement in equilibrium duplex alloys is accomplished by hot working the material close to the temperature range where superplastic deformation is to be carried out. In most cases the temperature is as high as possible concomitant with a microstructure consisting of approximately equal volume fractions of the two phases. If the phases have different deformation characteristics, such as one being harder and more brittle than the other, then working would fragment the harder phase. The softer phase would then be forced to infiltrate and separate the harder phase. The phases can recrystallize or spheroidize during the hot working process. Alternatively, if the two phases have very similar mechanical responses then working first elongates the grain structure, then fragments it by the development of intense shear bands. Again the original structure can be reformed, but on a much finer scale, by recrystallization. In some cases, the fine equi-axed structure is developed from a cold rolled material by recrystallization during heating immediately prior to superplastic deformation.Grain growth during both heat treatment and superplastic flow is restricted as the individual phases have different chemical compositions. In order that grain growth can occur solutes must diffuse from the smaller grains of each phase through or around the other phase to the larger grains. As alloy additions partition to the phase in which they are most soluble, the rate of mass transfer of the alloy element through the other phase is restricted by its limited solubility. The grain size is then said to be "segregation stabilised". A duplex superplastic material of very high structural stability would be one in which the component metals (or ceramics) show little or no solid solubility.
Examples of duplex materials which can be hot worked to develop fine grain microstructures include alpha/beta titanium alloys alpha/beta copper alloys, ultra high carbon steels, ferrite/austenite duplex stainless steels and eutectics such as Al-Ca , Al-Ca-Zn , Pb-Sn and Bi-Sn.
Al-Ca-Zn Eutectic (as cast and processed)
Pseudo-single phase alloys
Grain refinement as a result of warm working and recrystallization treatments has been used extensively in the development of superplastic aluminium-based alloys. The alloys are termed pseudo-single phase since they consist almost entirely of a solid solution strengthened matrix with <10% by volume of a precipitate phase which is present to stabilise the microstructure against grain growth. The alloys are constituted such that two types of particles are present during warm working but that usually only one, the finer of the two, remains at the superplastic forming temperature.The high density of dislocations which form during warm working would normally tend to re-arrange themselves by climb to form dislocation walls and possibly sub-grain boundaries. In the absence of any fine particles the dislocation arrays would migrate by climb allowing the microstructure to undergo continuous recovery and ultimately, recrystallization. However, the presence of fine particles, which are usually less than 0.2µm in diameter, prevents recovery by exerting a drag on the migrating dislocations, dislocation walls, and sub-grain boundaries. Mechanical working in the presence of the fine particles therefore generates and maintains a large amount of stored energy and introduces into the microstructure a large number of potential nucleation sites for subsequent recrystallization.
Alloys which contain predominantly fine particles develop a fine grain equi-axed microstructure during the initial stages of superplastic deformation by in-situ recrystallization. The particles and solutes which prevent recrystallization during warm working at lower temperatures cease to be effective in restraining dislocation migration at the higher deformation temperatures. The majority of the grains which form on heating to the superplastic deformation temperature have very similar crystallographic orientations because of the texture introduced into the material during warm working. As high temperature deformation and hence grain boundary sliding proceeds, the misorientation between the grains increases and leads to the formation of high angle grain boundaries and a true superplastic microstructure which can undergo grain boundary sliding.
A second group of pseudo-single phase alloys contains both fine and coarse particles during processing. The presence of larger particles, which are usually greater than 1µm in diameter, provide nucleation sites for recrystallization and cause localised distortions in the orientation of plastic flow during warm working. The resulting differences in crystallographic orientation of the material, on a very fine scale, lead to the nucleation of recrystallised grains with widely varying orientations and the formation of high angle boundaries as the embryonic grains impinge. Materials which contain predominantly the larger particles are statically recrystallised to produce the fine equi-axed microstructure by annealing prior to superplastic deformation.
Alloys which are refined by recrystallization include Al-Cu-Zr (2xxx series alloys including Supral), Al-Zn-Mg-Cr (7xxx series) and Cu-Al-Si-Co (Coronze CDA-638).
Grain Refinement by Phase Transformation
Several studies have shown that repeated thermal cycling of a material through a phase transformation can result in a very fine grain size. The mechanism of grain refinement is the nucleation of the reaction product at several sites on the grain boundaries of the parent phase. The product phase then grows as the transformation proceeds, replacing the single parent grains by a multitude of smaller grains. Repeated cycling through the phase transformation further refines the structure until a saturation grain size is reached. A similar effect can be achieved by controlled rolling above the transformation temperature such that a heavily deformed but unrecrystallized parent phase is produced. The high dislocation density within the parent phase results in a large number of nucleation sites for the product phase. The parent phase transforms directly to a fine grain product on cooling. Ideally superplastic deformation would take place in the temperature range where the two phases are present in order that the grain size is stabilised. However, if superplastic deformation is to be carried out at a temperature where the material is single phase then additional steps need to be taken to prevent grain growth - e.g. the introduction of fine dispersoids to pin the grain boundaries.Controlled rolling coupled to phase transformation has been successfully exploited in high strength low alloy steels (HSLA), ultra high carbon steels and could equally be applied in the Cu-Al based alloys where the eutectoid transformation (b = a+g2 or b = a+k3) occurs on cooling.
Grain Refinement by Phase Separation
It is often possible to anneal duplex materials at a temperature where only one phase is stable. Quenching the resulting single phase structure will then either effect a martensitic transformation or produce a supersaturated solid solution. A subsequent annealing treatment will result in the separation of the two equilibrium phases from the metastable microstructure. If sufficient nucleation sites are available then a fine grain microstructure is produced. A martensitic structure, such as that formed on quenching alpha/beta titanium alloys from the beta-phase field, provides such a density of potential nucleation sites for the alpha-grains and hence the reversion of the b'- Ti to the equilibrium alpha+beta phases. Another example of grain refinement by phase separation is the spinodal decomposition of Zn-22%Al. Slow cooling the Zn-Al alloy results in a lamellar eutectoidal decomposition product with very poor superplastic properties. If however, the alloy is quenched to room temperature a supersaturated solid solution of Al in Zn is formed which decomposes in a spinodal fashion producing a very fine grain, highly superplastic, microstructure.Despite the wide variety of methods available for developing fine grain microstructures, only a very small number of upwards of 100 distinct alloys which show extensive superplasticity are, or have the potential to be, exploited on a commercial scale. These include medium to high strength aluminium alloys, some duplex titanium alloys and ultrahigh carbon and stainless steels.
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