Superplasticity

In materials science, superplasticity is a state in which solid crystalline material is deformed well beyond its usual breaking point, usually over about 200% during tensile deformation. Such a state is usually achieved at high homologous temperature, typically half the absolute melting point. Examples of superplastic materials are some fine-grained metals and ceramics. Other non-crystalline materials (amorphous) such as silica glass ("molten glass") and polymers also deform similarly, but are not called superplastic, because they are not crystalline; rather, their deformation is often described as Newtonian flow. Superplastically deformed 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.

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In metals and ceramics, requirements for it being superplastic include a fine grain size (~<20 micrometres) 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 >0.3 to be considered superplastic.

The mechanisms of superplasticity in metals are still under debate - many believe it relies on atomic diffusion and the sliding of grains past each other. Also, when metals are cycled around their phase transformation, internal stresses are produced and superplastic-like behavior develops.

Superplasticity is used to form complex objects, by the application of gas pressure or with a tool, and often with the help of dies. Aluminum and titanium parts are often superplastically formed for aerospace applications.

More recently, superplasticity has been used to form parts for automotive applications. For application in the automotive industry, aluminum alloys are formed at a faster rate (compared to aerospace applications) to support high volume production. For references, see:

"Evaluation and Prediction of Material Response During Superplastic Forming at Various Strain Rates", Sumit Agarwal, PhD Dissertation, 2006.