Superplasticity
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In physics, superplasticity is a state in which solid material is deformed well beyond its usual breaking point, usually over 100% during tensile deformation. Such a state is usually achieved at high temperature, typically half the absolute melting point. Example of superplastic materials are silica glass ("molten glass"), chewing gum, and some fine-grained metals. As a material is deformed permanently (plastically), it gets thinner. Under superplasticity the stretched material gets thinner in a very uniform manner, rather than forming a 'neck' (a local narrowing) which leads to fracture. Also, the formation of internal cavities, which is another cause of early fracture, is inhibited.
In a state of superplasticity, solid matter can exhibit certain hydrodynamic qualities without becoming an actual liquid.
In metals, requirements for it being superplastic include a fine grain size (~<10 microns) and a fine dispersion of thermally stable particles which act to pin the grain boundaries and maintain the fine grain structure at the high temperatures required for superplastic deformation. Those materials which meet these parameters must still have a strain rate sensitivity (a measurement of the way the stress on a material reacts to changes in strain rate) of >0.5 to be considered superplastic.
The mechansims of superplasticity in metals are still under debate - many believe it relies on atomic diffusion and the sliding of grains past each other.
Superplasticity is used to form complex objects, by the application of gas pressure or with a tool, and often with the help of dies. Glass blowing is an early example of superplasticity. Aluminum and titanium parts are often superplastically formed for aerospace applications.
External links
FUNDAMENTAL ASPECTS OF SUPERPLASTIC DEFORMATION (http://www.imsp.da.ru/English/super_eng.html) at the Institute for Metals Superplasticity Problems (http://www.imsp.da.ru/English/main_eng.html)