Chapter 9: Superplasticity of Light Metal Alloys

Superplasticity is the name given to the ability of a material to sustain extremely large deformations at low flow stresses at a temperature around half the melting point expressed in Kelvin. It is only found in metals and alloys, which have, and can maintain during forming, a very fine grain structure. A parameter which indicates the degree of superplasticity is the strain rate sensitivity m, given by the high temperature flow equation: ? = K ^m, ? is the stress for plastic flow, the applied strain rate and K is a constant. Superplastic materials have m values normally between 0.4 and 0.6, while most other metals and alloys at elevated temperatures have m values of 0.2; viscous materials (e.g. glass) behave like a Newtonian fluid and have m values of 1.

A full discussion of the mechanism of Superplasticity, including methods for determining m, can be found in K. A. Padmanabhan and G. J. Davies, 'Superplasticity', Berlin, Springer-Verlag, 1980.

The tables in this chapter give alloy systems with the temperature range over which they show Superplasticity, the maximum possible percentage elongation, and the m value. The values of about 10 ^-4 quoted under remarks are preferred strain rates.