Superplasticity—Recent advances and future directions

Superplastic materials exhibit high values of the strain-rate-sensitivity exponent, m, in the equation sigma = K epsilon exp m , where sigma is the true flow stress, epsilon is the true strain rate, and K is a constant. Ideal or Newtonian-viscous behaviour is found in materials where M = 1. Most normal metals alloys exhibit m < 0.2 whereas superplastic alloys have values of m > 0.4. The two well-established types of superplastic behaviour in polycrystalline solids, fine-structure superplasticity (FSS) and internal stress superplasticity (ISS) are described. In FSS materials, a strain-rate-sensitivity exponent equal to approx 0.5 is usually found and the materials deform principally by a grain boundary sliding mechanism. (The possibility to achieve values of m = 1 in FSS through the development of fine-grained materials that incorporate glide-controlled slip as the accommodation process during grain boundary sliding is also described). In the case of ISS materials, however, the strain-rate-sensitivity exponent is usually unity, i.e. they exhibit Newtonian-viscous flow. These ISS materials need not be fine grained, and generally deform by a slip deformation mechanism. The concepts and principles described in FSS and ISS superplasticity have been applied to enhance powder consolidation through superplastic flow and the development of superplasticity in laminated composites containing at least one superplastic component. These developments are described. Other observations of large tensile strains in metals that do not fit into the above classifications are described including: (a) the observations of superplastic-like behaviour (up to several hundred percent) in coarse-grained Class I solid solutions, (b) the possibility of achieving high elongations in relatively coarse-grained materials exhibiting values of m = 1 at low strain rates through Coble creep (grain boundary diffusion controlled), Nabarro--Herring creep (lattice diffusion controlled), and Harper--Dorn creep (slip controlled), and (c) the observations of large plastic strains in Cu and Al under the extremely high strain rates found in anti-armor shape charge liners. 160 ref.--AA