What is plastic deformation? Distinguish between Slip and Twin Mechanism.
1 Answer

1) Plastic deformation is a process in which permanent deformation is caused by a sufficient load. It produces a permanent change in the shape or size of a solid body without fracture, resulting from the application of sustained stress beyond the elastic limit.

2) Here the deformation occurs after stress exceeds the elastic point, and thus makes permanent changes in the metal structure, and the forms some amount of dislocations.

3) In ductile material plastic deformations causes due to dislocations and in brittle material deformation occurs due to slippage of micro cracks.

4) Plastic deformation is always an irreversible process where elastic deformation is reversible.

Difference between Slip and Twin Mechanism:

Crystal slip is a line defect. Twinning is a surface defect grain boundary defect
During slip, all atoms in a block move the same distance During twinning, the atoms in each successive plane in a block move through different distances proportional to their distance from twinning plane
Slip is commonly observed in Body-centered Cubic (BCC) and Face Centered Cubic (FCC) metals Twinning is commonly observed in Hexagonal Close Packing (HCP) metals
After the slip, the crystal axis remains the same. After twinning, the crystal axis is deformed
The slipped crystal lattice has the same orientation. The twinned crystal lattice is the minor image of the original lattice
The stress required for slip is comparatively low. The stress required for twinning is comparatively more
The stress necessary to propagate slip is usually higher than the stress required to start the slip. The stress necessary to propagate twinning is lesser than that required starting it.
Slip can be seen as thin lines when viewed under microscope. Twinning can be seen as broad lines when viewed under microscope.
For slipping to occur, a threshold value of stress called critical resolved shear stress is required. For twinning to occur, no such threshold value of stress is required.
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