Explain the mechanism of plastic deformation – What is slip and twinning ?

In this article, we are going to learn about the mechanism of plastic deformation and what is mean by slip and twinning?

What is plastic deformation?

  • Plastic deformation is a process in which the size and shape of the object changed due to external load or forces. Plastic deformation is a permanent deformation which does not change after removal of external load or forces. 
  • Plastic deformation is an irreversible process of deformation.
  • In a plastic deformation, we can observe a strain hardening effect.
  • It depends on a loading path.
  • It takes place due to the slip and twinning mechanism.

    Use of plastic deformation:

    Plastic deformation is used for shaping different types of metals by the process such as rolling, forging, drawing and extrusion.

    Mechanism of plastic deformation:

    As we know, plastic deformation takes place due to slip, twinning, and the combination of slip and twinning.

    Slip:

    Slip Mechanism Diagram
    Slip Mechanism Diagram

    1. In this mechanism, one place slides over another plane. This sliding of one plane over another plane does not affect the crystal structure of the metal. Also, no change observed in the arrangement of atoms at this time.

    2. Slip takes place in one particular direction and in one particular plane. This particular directions and planes are known as slip directions and slip planes respectively.
    Slip planes and slip directions for basic crystal structures are given below in table form.

    Crystal Structure Slip Planes Slip Directions Metal Example
    B.C.C.   (1 1 0) < 1 1 1 > Mo, Fe
    F.C.C.   (1 1 1) < 1 0 >    Al, Cu
    H.C.P.   (0 0 0 1)    <1 1 2 0 >    Mg, Co

    3. A slip occurs due to the motion of dislocation. There are two types of these motions as follows:
      1. Glide motion
      2. Climb motion

    Twinning:

    Twinning mechanism diagram
    Twinning Mechanism Diagram
    1. In the twinning mechanism, the atomic arrangement of one part looks like an exact mirror image of another part.
    2. The boundary of the plane which acts like a mirror is known as a twinning plane or a composition plane.
    3. It shows an indirect contribution to plastic deformation.
    4. Twinning which is created by the deformation process is called deformation twinning.
    5. Twinning which is created by the heat treatment process is called annealing twinning.
    6. Twin planes and twin directions for typical crystal structures are given below in table form.
    Crystal Structure Twin Plane Twin Direction
    B.C.C. (1 1 2) [ 1 1 1 ]
    F.C.C. (1 1 1) [1 1 2 ]
    H.C.P. (1 0 1 2) [ 1 0 1 2 ]
    7. Time and stress required for twinning is more than slip
    This is a whole detail of the mechanism of plastic deformation. From the above information, we can conclude the difference between slip and twinning.

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