Deformation is the change in dimension or forms of matter under the action of the applied forces. Deformation is caused either by the mechanical action of the external forces or by various physical and physio-chemical processes. Deformation may be permanent or temporary depending upon the type of deformation whether it is plastic or elastic. So metal deformation are of following two types:
(a) Elastic deformation
(b) Plastic deformation
Elastic Deformation – The deformation is called elastic if the strain or deformation produced in the material corresponding to a given stress completely disappears when the stress is removed. So the deformation which is fully recoverable and virtually time independent is called elastic deformation. In the figure given below, the form of the atoms before loading, after loading in tension and compression respectively is shown.
Modules of elasticity (ratio of stress to the strain) give an idea about the amount of elasticity. Modulus of rigidity (ratio of shear stress to shear strain) gives an idea about the displacement or shear stress or shear strain.
Plastic Deformation – If the stress causing deformation exceeds the elastic limit, plastic deformation is observed. Plastic deformation is typically a function of stress, temperature and the rate of straining. Plastic deformation can occur under tensile, compressive and torsional stresses. Since the material is deformed beyond elastic limit, so the stress no longer remains proportional to strain and permanent, non recoverable plastic deformation occurs. Plastic deformation of metal is a very important and desirable phenomenon which makes it suitable for various forming processes such as rolling, forging, pressing, drawing, spinning and extrusion, etc. Sometimes plastic deformation is also used to improve some mechanical properties of the material.
Plastic deformation corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors as large number of atoms or molecules move relative to one another. The mechanism of this deformation is different for crystalline and amorphous materials. For crystalline solids, deformation is accomplished by means of a process called slip, which involves the motion of dislocation.
There are two basic modes of plastic deformation:
(a) Deformation by slip
(b) Deformation by twinning