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 is of following two types:
- Elastic deformation
- Plastic 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.
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 neighbours and then reforming bonds with new neighbours 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:
- Deformation by slip
- Deformation by twinning
Deformation by Slip
Slip is defined as sliding of blocks of crystal over one another along definite crystallographic planes called slip planes. So, it is the relative displacement along a definite direction. When slip takes place, one part of the lattice moves with respect to the other. Generally, slip plane is the plane of highest atomic density and slip direction is the closest packed direction within the slip plane. This is because the bond between these planes is weakest, so when force is applied in proper direction, relative movement takes place very easily.
The shear stress required to produce slip on a crystal plane is called the critical resolved shear stress. A slip plane and a slip direction together make a slip system. There are twelve slip systems in each of FCC and BCC crystals, while there are only three in HCP crystal.
Slip planes are generally fixed for every type of lattice. In FCC Lattice (111) plane, in BCC lattice (110) plane and in HCP lattice (001) planes are the most common slip planes. As the magnitude of the applied stress increases, the number of active slips planes and the distance of slip along these planes increases. The extent of slip is limited to an interatomic distance or an integral multiple of that distance. Slip in all metals of similar crystal structure will occur along the same crystallographic planes and directions. The slip on each plane may be of the order of several microns.
Deformation by Twinning
Twinning is the process by which a portion of the crystal takes up an orientation which makes that portion a mirror image of the parent crystal. In order to form a mirror image across the twin plane, the amount of movement of each plane of atom in the twinned region should be proportional to its distance from the twinning plane.
Twinning planes are mostly fixed like that of slip planes. In FCC (111) plane is the twinning plane, in BCC lattice (112) plane is the twinning plane and in HCP lattice (112) plane is the twinning plane.
If a shear stress is applied, the crystal will turn about the twinning plane. The region to the left of the twinning plane is undeformed. To the right of this plane, the planes of atoms have sheared in such a way so that the lattice makes a mirror image across the twin plane.
Twins may be of the following two types:
- Mechanical Twins. These are produced by mechanical These are produced in BCC or HCP metals under conditions of rapid rate of cooling and decreased temperature.
- Annealing Twins. Twins which forms during the process of recrystallisation are called annealing twins. These are produced as a result of That is why these are called annealing twins.