9.2 - Dislocations & burger vectors

What are dislocations?

Where do they come from?

• dislocations = 1D lattice defects representing linear perturbation of the lattice

Origins:

• Crystal growth (formation of crystal lattice)

• Recrystallisation (after recovery in the annealing process)

• Result of residual stresses

• Plastic deformation

What are the 2 types of dislocations?

Draw them

• EDGE dislocation

→ extra half-plane of storms that is inserted into the crystal lattice & has no continuation on the opposite side

→ burger vector perpendicular to the dislocation line

→ move in the same direction as the applied force

• SCREW dislocation

→ spiral planar ramp (step) resulting from shear deformation

→ burger vector // to the dislocation line

→ move perpendicular to the applied force

What happens at the macroscopic scale when dislocations move?

It produces lasting (permanent) deformation

How would elastic & plastic deformations be for a perfect crystal (no defects)?

How do defects overcome this?

Perfect crystal:

• plastic deformation

→ would be very difficult to plastically deform it because it would imply to move all the atoms at once (break all atomic bonds at once)

→ would require a very high stress

• elastic deformation

→ would be very easy to elastically deform because there is no defect to hinder atomic displacement

Real crystal:

→ easier to deform plastically because the required stress is reduced as atoms slide progressively (one row at a time)

→ dislocations make that possible, and make the material stronger in the mean time by causing internal stresses

Explain the movement of an edge dislocation

→ the edge dislocation moves in the same direction as the applied force but perpendicular to its dislocation line

→ bonds break and reform as the dislocation moves until it pops out at the end

Explain the movement of a screw dislocation

→ the screw dislocation moves perpendicular to the applied force & // to the dislocation line

→ goes on until it reaches the end of the material

What’s a burger vector? What does it do?

How do we get it?

Burger vector characterises dislocations → quantifies the displacement of atoms around a dislocation line

Method

→ draw a closed loop in the undistorted lattice moving from atom to atom, by the same number of steps in each direction

→ draw the same loop in the distorted lattice (including the dislocation) (the loop won’t be closed)

→ gap required to close the 2nd loop = burger vector

Edge dislocations:

→ burger vector perpendicular to dislocation line

Screw dislocation:

→ burger vector // to dislocation line

What’s the difference between single crystals & polycrystals?

What does it involve for each concerning deformation?

• single crystal = the entire structure is continuous, unbroken lattice, uniform orientation, no grain boundaries

→ displacement is localised to most favourably I oriented slip system, leading t non-uniform deformation

• polycrystal = composed of many, small, individual crystals (grains) with different orientations, sperated by grain boundaries

→ displacement is distributed across multiple grains

→ deformation starts in favourably oriented grains and spreads to less favourable ones as the load increases