Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 10M

Year: May 2014

Plastic deformation is a process in which enough stress is placed on metal or plastic to cause the object to change its size or shape in a way that is not reversible. In other words, the changes are permanent; even when the stress is removed the material will not go back to its original shape.

Both the deformation of plastic and the deformation of metals involve changes to the makeup of the material itself. For example, metals that undergo this process of plastic deformation experience a condition known as dislocation. As stress of some type is exerted on the metal the material reaches a point known as the yield strength. When this point is achieved the patterns of the molecules that make up the metal begin to shift. The end result is that the molecules realign in a pattern that is shaped by the exterior stress placed on the object.

Plastic deformation is a change of the material dimensions remaining after removal of the load caused the deformation.

When the yield stress is achieved one plane of atoms in crystal lattice glides over another. Few parallel slip planes form a block neighboring with another block. Thus movement of the crystal planes is resulted in a series of steps forming slip bands – black lines viewed under optical microscope.

Slip occurs when the share resolved stress along the gliding planes reaches a critical value. This critical resolved shear stress is a characteristic of the material.

Certain metals (Zn and Sn) deform by a process of twinning, differing from the normal slip mechanism, where all atoms in a block move the same distance. In the deformation by twinning atoms of each slip plane in a block move different distance, causing half of the crystal lattice to become a mirror image of another half.

In polycrystalline material directions of slips are different in different crystals. If a grain is oriented unfavorably to the stress direction its deformation is impeded. In addition to this grain boundariesare obstacles for the slip movement as the slip direction should be changed when it crosses the boundary. As a result of the above strength of polycrystalline materials is higher, than that of mono-crystals.

Slip and twinning processes, occurring during plastic deformation result in formation of preferred orientation of the grains.If the stress value required for a slip is higher than cohesion strength, metal fracture occurs.