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.^{[citation needed]}

In metals and ceramics, requirements for it being superplastic include a fine grain size (less than approximately 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. Recently high-temperature superplastic behaviour has also been observed in iron aluminides with coarse grain structures. It is claimed that this is due to recovery and dynamic recrystallization.

See also

• Superplastic forming
• High explosive anti-tank warhead

References

Bibliography

• Agarwal, Sumit (2006). Evaluation and Prediction of Material Response During Superplastic Forming at Various Strain Rates (Ph.D. thesis). Brown University. OCLC 549697889.