Superplasticity
Encyclopedia
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.
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.
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
, superplasticity is a state in which solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
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
Homologous temperature
Homologous temperature expresses the temperature of a material as a fraction of its melting point temperature using the Kelvin scale. For example, the homologous temperature of lead at room temperature is approximately 0.50 ....
, 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.
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
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
and the sliding of grains past each other. Also, when metals are cycled around their phase
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
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.