Q. Explain the mechanism for dislocation in plastic deformation.

Ans.  Stresses are acting on dislocation in a crystal that is known as stress field, due to these stress force is produce on another dislocations. Hence there is interaction between dislocations and salute atoms. In screw dislocation only shear stress is there but in edge dislocation all type i.e. tensile, compressive and shear stress field are present.

         Dislocation Climb: The process of dislocation climb occurs by jump of atoms from the bottom row of extra half plane into adjacent vacancies. The dislocation climb is controlled by diffusion and activate by temperature. At elevated temperature dislocation climb increase and atoms moves from extra half plane i.e. known as positive climb. In negative climb a row of atoms are added in the extra half plane.

          Dislocation Loop: Dislocation in crystals may be in straight lines, plane or loop, but the probabilities of line, plane dislocation are very less hence this dislocation takes the form of a curve or loop i.e. known as dislocation loops.     

          Jogs in Dislocation: Jogs is defined as the increase in length of dislocation line created by the intersection of dislocation. Jog or step is created when dislocation is move from one slip plane to another slip plane.

          Cross Slip: The process where, by a screw dislocation glides into another slip plane having a slip direction in common with the original slip plane is known as cross slip. Cross slip occurs at higher stress than normal slip and occurs mostly in crystals with large number of slip systems, BCC and FCC structures have six and four plane of easy slip and there is more change for cross slip.

           Frank Read Source: Slip dislocation present in a few numbers of crystals initially. But slip dislocation spread out throughout in the crystal immediately. Hence there is some mechani9sm which initiate these dislocations on a slip plane. The starting point of this process is known as Frank Read Source.