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Plasticity

Plasticity
Definition:Property of a material to be able to endure a permanent deformation caused by external stress.
Explanation:Plasticity is one of the most important properties of metallic materials because the final shape of a part is normally achieved by forming processes like rolling or forging. To depict the formability, the yield point Re, which represents the upper limit between elastic and plastic deformation, and the tensile strength Rm, which means the maximum tolerable load, are taken into account. Steels with a relatively low yield point and high tensile strength are easily deformed and show good plasticity.

From a materials science point of view, good plasticity requires a microstructure with a low density of lattice defects and precipitations along with a high number of possible slip systems. A slip system consists of a gliding plane (the close-packed plane of the crystal system) and a slip direction (close-packed direction of the crystal system). For the deformation of polycrystals, five independent slip systems are necessary, which leads to differing plasticity properties in different Materials. For instance, magnesium has very weak formability due to its small number of activating slip systems

In addition to crystallographic glide, which represents the most dominant deformation process in metallic materials, other mechanisms can be observed. A recently intensive investigated phenomenon is the plastic deformation by the formation of mechanical twins (→TWIP), or in the future, also microband induced plasticity (→MBIP).
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Diagram:

Stress-strain curves of different steel grades showing different plasticity
SFB-Link:Which deformation mode dominates the plastic deformation is also close correlated to the stacking fault energy (→SFE) which plays an important role in the materials design concept of the SFB. By supporting different deformation mechanisms like TWIP, different combinations of plasticity and strength can be achieved.
References:Gottstein, G: Physikalische Grundlagen der Materialkunde 3. Auflage, Springer-Verlag Heidelberg New York 2007