Plasticity

Plastic deformation of materials in the general sense can be defined as irreversible deformation, which means that in a tensile or compression test after loading and unloading a deformation remains. Different mechanisms can be responsible for such an irreversible or plastic deformation: dislocation motion, vacancy motion, twinning, phase transformation, and viscous flow of amorphous materials. In a polycrystalline or multiphase material the deformation can take place within the grains or predominantly along the grain or phase boundaries. In metals at room temperature dislocation motion is the most important deformation mechanism. In ceramics dislocation motion requires high shear stresses due to covalent bonds. Therefore under most loading conditions ceramics fail by the extension of flaws, whereas the competing failure mechanism by dislocation motion would require higher stresses. Nevertheless the plastic deformation and the formation of dislocations have been observed under specific loading conditions.