When a metal is stressed below its elastic limit, the resulting deformation or strain produced in the metal is temporary. This strain or deformation vanishes after the removal of stress and the metal goes back to the original dimensions. When it is stressed above the elastic limit, permanent deformation takes place and the metal does not returns to its original shape after the removal of stress. The ability of a metal to undergo plastic deformation is one of the important property which is utilized for shaping of metals by various fabrication process such as rolling, forging, drawing, extrusion etc.,
Mechanism Of Plastic Deformation
Plastic deformation take place by slip, twinning, some times by both
Slip is a permanent displacement of one part of crystal relative to the other part. slip involves sliding of one plane of atoms over the other. The plane on which the slip occurs are called slip planes and the direction in which this occurs are called slip direction. Slip occurs when shear stress applied exceeds a critical value.During slip each atom usually moves same integral number of atomic distances along the slip plane producing a step,but the orientation of the crystal remains the same. Slip planes are usually the most close packed planes i.e., the planes of maximum atomic density. Such planes obviously will be widely spaced i.e., the inter planar distance between such planes is more. Slip results from the motion of dislocations from one place to the other place. There are two basic types of dislocations movements called as glide and climb. In glide, the dislocation moves in a surface defined by its line and Burger’s vector (glide is conservative motion of dislocations). In climb, the dislocation moves out of the glide surface and therefore, climb becomes a non conservation motion of dislocation. Slip is the most common manifestation of glide.
Twinning is a process in which the atoms in a part of the crystal subjected to stress rearrange themselves so that the orientation of the part changes in such a way that the distorted part becomes a mirror image of the other part. The plane across which the two part are mirror images is called twinning plane or composition plane.Like slip, twinning also occurs along the certain crystallographic planes and directions. These planes and directions are called as twin planes and twin directions.The important role of twinning in plastic deformation is that it causes changes in plane orientation so that further slip can occur.