The Dislocation-Based Mechanism to Plastic Deformation

Slip relies on chemical bond breaking and bond reformation as two planes of atoms pull apart. It is observed that the critical resolved shear stress required to cause plastic deformation in real materials is much lower (by several orders of magnitude) than the shear stress required in deforming perfect defect-free crystals, the so-called ideal shear stress. The latter is equivalent to the stress required for the simultaneous ghding motion (bond breaking and reformation) of aU the atoms in one plane, over another plane. 

Dislocations are present in the natural states of crystalline materials but they drastically increase in number (expressed as the dislocation density, or dislocation length per unit volume) with plastic deformation as existing dislocations spawn new ones. This dislocation multiplication with plastic flow causes an increase in the number of mutual interactions, which hinders their motion. As a consequence, a shear-stress increase must be 

It may be energetically favorable for a dislocation, b, to spht into two dislocations if the product dislocation Burgers vectors and satisfy the condition b b -hb. Dislocation reactions can even produce stable imperfect dislocations, if they result 

The parent dislocations are in the same (I I I) plane. Thus, the product dislocation is also in this plane. Thus the dislocation reaction is  

The sum of the energies of the parent dislocations is proportional to a. The energy of the product dislocation is proportional to a /2. Since there is a net reduction in energy, the reaction is favorable. 

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