2: Twinning

Twinning in Metals

  • Overview of Twinning

    • Twinning is more frequently observed in metals with limited slip systems.

    • Low symmetry lattices are prone to twinning, with hexagonal close packed (HCP) structures being the most common.

Example: Pure Tin

  • Deformation Behavior

    • Pure tin exhibits extreme twinning due to its tetragonal lattice structure.

    • During plastic deformation, twinning can be so prevalent that it produces a crackling sound as twins are formed.

Demonstration by Dr. Jessica Wynne

  • Lecture Context

    • Dr. Wynne from the University of Cambridge demonstrates twinning with two bars of freshly cast tin.

    • Tin's tetragonal structure complicates dislocation motion, leading to favoring deformation twinning.

  • Experimental Process

    • One tin bar is heated with boiling water, while the other is bent at room temperature.

    • The sounds generated during the bending illustrate twinning activity.

Types of Twinning in Metal Lattices

  • Preferred Lattice Types for Twinning

    • Deformation twinning primarily occurs in:

      1. Hexagonal Close Packed (HCP)

      2. Body-Centered Cubic (BCC)

      3. Rarely in Face-Centered Cubic (FCC) unless under specific conditions.

  • Conditions Favoring Twinning

    • High-speed deformation processes are conducive to twinning due to quicker deformation rates.

    • Low temperatures hinder movement of dislocations, promoting twinning as an alternative deformation mode.

Pseudoelastic and Shape Memory Alloys

  • Nickel Titanium Alloys

    • Examples include Nitinol, Flexinol, or muscle wires; fundamental characteristics are due to twinning.

    • Shape memory effect is limited to approximately 7-8% strain, correlating with maximum twinning strain in crystalline materials.

Interaction Between Twinning and Dislocation Motion

  • Reorientation of Lattice

    • Twinned regions experience changes in lattice orientation relative to the applied load.

    • This reorientation affects the resolved shear stress on slip systems, potentially activating previously inactive systems.

  • Cooperative Effect

    • Active slip systems within the twin allow for greater plastic deformation beyond twinning.

    • Enhanced plastic strain occurs when dislocations activate concurrently with twinning under the same applied stress.

Graphical Representation of Twinning Effects

  • Slip Systems and Applied Shear Stress

    • The graph illustrates that before twinning, the resolved shear stress is insufficient to activate dislocations.

    • Post-twinning, a new slip direction is established, increasing resolved shear stress enabling dislocation movement.

Conclusion

  • In summary, today's lecture elaborated on the importance and implications of twinning in metals, featured practical demonstrations, and highlighted its interactions with dislocation mechanics.