Super plasticity
Superplasticity is the property of a polycrystalline material to be superplastically deformed under certain conditions. When superplastic forming (Engl. Super plastic forming , SPF ) extends the material by several 100% without breaking, and its Poisson distributed it fairly evenly over the length, rather than at one point ( reduction of area to focus). Some alloys achieve elongations at break of more than 1000%.
Typical superplastic alloys have grain sizes below approx. 10 μm, require a forming temperature of 0.5 T m ( melting temperature in Kelvin ) and low forming speeds (~ 5% / min = 0.0008 / s).
Explanation
In a forming by train the material is stretched. Most materials constrict across the train, as their volume remains almost the same. In general, the necked area is weaker than the uncut area. The further forming is therefore concentrated more and more on this area until it breaks. In the case of a superplastic deformation, on the other hand, the constricted area solidifies, so that further deformation is shifted to the less constricted areas. In this way, the constriction is much more even and the material can be stretched much more until it breaks.
A similar behavior of plastics is not called superplasticity, since plastics are not crystalline and the property is not based on grain boundary effects.
Examples
Examples of superplastic alloys are:
- Ti6Al5V (reached at 850 ... 525 ° C and 5% / min deformation, elongations of> 700%)
- AlZnMg , AlCu Zr-, AlLi Zr (reach an elongation of up to 1200% at 490 ... 540 ° C)
literature
- G. Gottstein: Physical basics of materials science . Scientific Springer Verlag, Berlin 2007, ISBN 978-3-540-71105-6 , Chapter 6.8: Time-dependent deformation, p. 278-281 .
- W. Weissbach: Materials science: structures, properties, testing . vieweg, Wiesbaden 2007, ISBN 978-3-8348-0295-8 , Chapter 2.4: Processes in the metal grid at higher temperatures, p. 63 .