Chapter 7: Directions Of Movements

Question 1

What is slip? Describe what a slip plane directionand system is while including how it moves

Answer 1

• The movement of the dislocation is called slip.
The crystallographic plane along which the
dislocation line travels is the slip plane.
The direction it moves in is the slip direction.
Slip plane and slip direction form the slip system.
• The slip system is defined by the crystallographic
structure of the material
• The slip plane will move:
– Along the path of least resistance
– Along a crystal plane where the density of atoms
is highest

Question 2

Why do crystals slip?

Answer 2

Crystals slip due to a resolved shear stress, τR.
• Applied tension can produce such a stress.

Question 3

Describe slip motion in poly crystals

Answer 3

Slip plane and direction change from one crystal to another , tr will vary from one crystal to another
The crystal with the largest shear stress.
Stronger grain boundaries pin deformations.

Deformation of one grain constrained by neighbouring grains

Question 4

How can strength of metals and alloys be increased and explain how

Answer 4

If slip motion is made more difficult i.e

Reducing grain size: grain boundaries are barriers to slip, smaller grains means more barriers to slip.

Solid solutions: Impurity of the atoms distort the lattice and generate stress Stress can produce a barrier to dislocation motions. Large impurities concentrate at dislocation on low density side.

Cold working/strain hardening is when a material is plastically deformed, releasing the force and then straining again. Number of dislocation increase. Dislocation Motions become more difficult

Question 5

How do you calculate dislocation density?

Answer 5

Dislocation density (pd) = total dislocation length/unit volume