Overview

Superplasticity is the ability of polycrystalline materials to sustain large plastic deformation in tension without rupture, and is phenomenologically delineated by tensile fracture elongations (e _f) in excess of ~300%. Originally discovered in 1912 by Bengough1, Superplasticity is now employed industrially in metal shape forming operations around the world, with primary applications in the aerospace field ( see, e.g. Ref. [2]).

Superplasticity is a viscous deformation process, and is frequently characterised by a simple flow law relating uniaxial stress ? and strain rate

(1)

where K is a temperature- and microstructure-dependent parameter, and m is referred to as the strain rate sensitivity; its reciprocal value, the stress-sensitivity, n, is also commonly reported. Whereas values of m are usually less than 0.25 for typical creep mechanisms in polycrystals, superplasticity is distinguished by values of m ? 0.3. Higher values of m directly promote tensile ductility, by increasing the stability of necks, and by suppressing the tendency for grain-boundary cavitation. In the Newtonian limit of m = 1, there is no theoretical limitation to the achievable ductility, although inhomogeneities in temperature or microstructure can substantially impact superplastic extension.

There are two main classes of superplasticity, broadly referred to as microstructural superplasticity and internal-stress superplasticity, distinguished by deformation mechanisms at the microstructural level.

Microstructural superplasticity is by far the more studied mode of superplastic deformation, and relies on the increase in strain-rate sensitivity, m, observed in fine-grained materials...