Superplasticity of Metals

Superplasticity of Metals Robert Cahn, Paris Laboratoire de Métallurgie Physique, Université Paris-Sud Forming metals that are in a superplastic state offers the promise of obtaining large and regular deformations at reasonable levels of stress. It is a familiar experience that a rod of the stress there rises. In a material which is soda glass, heated in the flame of a Bunsen inherently plastically stable, while deforma burner and pulled by hand, will stretch out tion would tend to become concentrated in into a long, uniform fibre ; the strain can be the local constriction, the local strain rate thousands per cent, without fracture. A would then become much greater than the bar of mild steel, however, when pulled In a strain rate along the rest of the rod. If, tensile machine, will neck down locally and however, m is fairly high, the stress re break after a mean strain of at most some quired to sustain deformation in the con tens per cent. The glass stretches easily striction would rise to a high level and, long and uniformly, the steel with difficulty and before this could happen, the strain would non-uniformly. The glass, but not the steel, become distributed along the unnecked behaves superplastically. Nevertheless, it is (and major) part of the rod. Moreover, if now possible to produce a polycrystalline the strain-hardening exponent, n, is alloy that will mimic hot glass in its substantial, then a rapidly deforming neck mechanical behaviour, which has led to im requires an even higher stress to sustain portant industrial applications in the past continuing deformation than with a non few years. hardening material like glass, for which n = 0. A proper analysis of the triaxial Fig. 1 — The dependence on strain rate, ε, of the flow stress, a, and the exponent, m, for the Mechanics of Plastic Instability stresses in a developing neck is very com eutectic Al-Cu alloy deformed at 520° C, for 3 The key to all superplastic behaviour, in plex and theories abound. One theory con grain sizes. (After Holt D.L. and Backofen glasses and crystalline materials alike, lies cludes that the critical strain, ε', for a neck W.A., Trans. Quart. ASM 59 (1966) 755. in the relationship between plastic strain to develop in a sustained fashion is given rate, ε, and the stress, σ, required to sus by: ε' = 2n/(1-2m) which implies that if m a very low stress, whereas at 20°C (~ 0.36 tain it. The relationships is : σ = Kεm, where reaches 0.5, the material is plastically stable Tm) it is strong and its ductility is small. K and m (≤ 1) are functions of the material (i.e. ε' is infinite) so long as n>0. For the We can now summarise the essential and of the temperature. The exponent m many superplastic alloys now known, m is characteristics of a superplastic alloy: It (which for hot glass is unity) is not normally in the range 0.4—0.7 and n>0. must be (and remain during its deforma constant, but varies with ε, as seen in Fig. tion) very fine-grained ; the index of strain- 1 which shows both the stress and m as Conditions for Superplasticity rate sensitivity, m, must exceed ~0.4, functions of strain rate for an Al-Cu alloy. When a sustained neck develops, frac which in turn is found to Imply strain rates Fig. 1 is, in practice, characteristic of the ture quickly and necessarily follows: for not usually exceeding 10 -2/s ; high values of majority of superplastic alloys. Such plots any ductile material, therefore, the m must be restricted to temperatures well are, by implication, standardised for a fixed avoidance of neck development is a neces above ambient, and at such temperatures strain, ε, as work-hardening will result in sary condition for very large strains to be the flow stress must be very low, whereas the stress required to produce a given attained, although not sufficient in itself. It at ambient temperature, m must be low strain rate, increasing with strain according is also necessary to avoid, in substantial and the flow stress high. to a relationship often adequately describ proportions, the presence of a phase, ed by the equation : σ∞ εn (n < 1). which is much harder than the matrix; if Mechanisms of Superplasticity In most metal forming operations, defor that happens, cavities develop at grain We next turn to the mechanisms which mation is performed under a constant im boundaries and the material breaks at small make superplastic behaviour possible. A posed strain rate. The Implications can be strains, especially at low strains rates, even very large amount of microstructural understood by considering a long rod stret if no neck develops. The close correlation ched at fixed strain rate, ε*. If a small in between m and the strain to fracture in ten stability (incipient neck) develops, the sion, valid for many alloys, is shown in Fig. cross-sectional area is locally reduced and 2. We have not yet considered the in fluence of temperature and grain size. No practically useful superplastic alloy is superplastic at ambient temperature: superplasticity is normally restricted to the temperature range 0.4 Tm-0.7 Tm(Tm = ab solute melting temperature). Fig. 3 shows, for a Zn78 Al22 alloy, the variation of flow stress with temperature for constant ε, for two quite different grain sizes illustrating a fact which has long been familiar to metal lurgists, that fine-grained metals are stronger than coarse when cold, while the Fig. 2 — Strain rate sensitivity, m, vs. elongation reverse is true when hot. When the grain- Fig. 3 — The influence of grain size on the to failure for various metals and two superplastic size of Zn78 Al22 is kept below about 5 µm strength of Zn78Al22 alloy as a function of tem alloys. (After Langdon T.G. Met. Trans. 13A and it is strained at ~ 0.6 T (200°C), it can perature. (After Ball A. and Hutchison M.M. (1982) 698.) be stretched to strains exceeding 500%, at Met. Sci. J. 3 (1969) 1.) 4 stituent processes are diffusional flow (by sheet ; the starting sheet is pressed by gas combined lattice and grain- boundary diffu or fluid pressure, or by isothermal forging, sion), grain-boundary shear and grain into a die, which can be made from inex boundary migration; a measure of grain pensive materials since the pressures invol rotation is also involved. The version of dif ved are modest. This feature is one of the fusional flow in Fig. 4 differs slightly from principal economic advantages of super the older models and gives faster flow. At plastic forming vis-à-vis such processes as high stresses (near the upper limit for high-pressure pressing of non-superplastic superplastic behaviour) dislocation motion steel sheets. Also the accessible strains are intervenes and grains begin to elongate ; at much higher in superplastic forming than in Fig. 4 — Diffusional flow in a grain under ten low temperatures only diffusional flow ope ordinary pressing or deep drawing, so that sion. (After Ashby M.F. and Verrall R. A., 1973.) rates, grains remain equiaxed and also a complex shape can be made in a single rotate bodily (as evidenced by the gradual forming operation with just one die, in research has been done in the past decade disappearance of any initial preferred orien stead of a multiple process with several to address this question, and even though tation of the grains). The balance of pro dies. The technique has proved particularly the relative emphasis to be given to the dif cesses, and thus also the controlling pro useful for complex consumer goods and ferent constituent processes is still subject cess, varies as stress is increased (and as (using titanium alloys) for air frame com to disagreement, a clear picture is begin the temperature changes). ponents. ning to emerge. Practical superplastic alloys have been The following structural observations are Practical Aspects developed only in the past decade ; serious generally valid : For the metallurgist, the principal task is research has been in progress for about (1) Superplastic behaviour requires grains to provide alloys in which the grains not two decades, whereas the first observation which are not only fine, but "equiaxed" (of only are fine to begin with, but resist of the phenomenon dates back to 1920. Of roughly equal diameter in all directions); coarsening at the temperature of optimum the many alloys which have been develo these equiaxed grains remain equiaxed superplasticity. This is done in practice throughout a deformation which may ex ped, only four are in current industrial use ; ceed 1000% (whereas in normal plastic either by designing a duplex microstructure this is no doubt due to the demanding spe deformation, initially equiaxed grains be in which two phases, both equiaxed in cifications required. The greatest possible come progressively elongated, as a single form, are present in roughly equal propor superplastic strain rate is necessary if pro crystal which is plastically stretched). tions, or else by dispersing a small fraction duction times are not to be unreasonably of second phase in very finely distributed long : 10-2/s is probably the lower accep (2) Neighbouring grains are mutually dis form. In the "duplex" case, a majority of table limit (and is still much lower than placed by shear at grain boundaries. the boundaries are interphase ones, which strain rates current in other forming pro (3) Grain boundaries migrate continuously can behave similarly to ordinary mono cesses). during superplastic straining. phase grain boundaries. In the "disperse" A reasonably wide temperature range of (4) Except at the highest stresses (right- case, the fine particles (too small to cause superplastic working is needed, since it is hand side of Fig.

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