Deformation and Dislocations

Question 1

What is:
1 - A Tensile Load
2 - A Compressive Load

Answer 1

A tensile load produces an elongation and positive linear strain

A compressive load produces contraction and a negative linear strain

Question 2

What is Poisson’s Ratio?

Answer 2

Poisson’s Ratio is the ratio of lateral and axial strains

Question 3

How do Bonds act between atoms?

Answer 3

The forces which hold atoms together (interatomic bonds) act like springs, liking one atom to the next in the solid state.

Question 4

What Factors affect the modulus in metals?

Answer 4

Temperature - Increased temperature increases interatomic spacing, reducing the binding forces between atoms

Alloying elements - Large (poorly fitting) substitution elements stretch bonds and hence reduce modulus

Smaller elements contract bonds and hence increase module

Anisotropy/Loading Direction - Different crystallographic directions have different bond radii and different combinations of tensile and shear components. Hence different loading direction relative to these directions produce can have different moduli.

Question 5

How to we achieve plastic deformation

Answer 5

Slip - Movement of a plane of arms relative to their neighbours. Slip always occurs under the action of a shear stress and preferably on planes of the closest atomic packing and in direction of closest atomic packing.

Question 6

Where will slip occur?

Answer 6

On the system experiencing maximum shear stress. And when the RSS exceed the CRSS.

Question 7

What Orientation experiences the maximum shear stress

Answer 7

Grains with their preferred slip systems orientated at 45deg to the tensile axis will experience the maximum shear stress

Question 8

What factors affect yield stress in metals?

Answer 8

Temperature - Increased temperature increases atomic vibrations making slip easier

Grain Size - As grain size decreases, yield stress increases. Hall-Petch Relationship

Loading Direction/Anisotropy - Different crystallographic slip systems have different CRSS values. This can affect the macroscopic yield stress of a material.

Question 9

What is the correct model for slip?

Answer 9

Lower energy process acting - starts in a localised region (pre-existing dislocations in the crystal) and then progresses over the slip plane; reducing the number of atomic bonds broken at any time.

Question 10

What does the Burger’s Vector Define?

Answer 10

The magnitude and direction of slip

Question 11

How is an edge dislocation formed?

Answer 11

Half plane of atoms created above slip plane, tweak a few bonds, half plane then rebonds, a new plane is formed, and the dislocation moves through the lattice.

Line of Edge Dislocation = PERPENDICULAR to slip

Question 12

What direction is Screw Dislocation?

Answer 12

Parallel to dislocation line

Question 13

Draw Diagrams of left hand, and right hand screw dislocations.

Answer 13

PP2 - P 37

Question 14

Can dislocations end within a crystal?

Draw the Diagram of a dislocation loop

Answer 14

No - dislocations will exist as loops comprised of edge and screw components.

The Burgers vector for any part of the loop will be constant

Dislocations opposite sides of the loop have opposite polarity.

Question 15

Describe the Frank Network

Answer 15

A Frank Network is a 3D network of dislocations that is formed during solidification. Each length of dislocation ends at a node, where three or more dislocations meet.

Question 16

What is Annihilation and Repulsion?

Answer 16

Annihilation - Positive and negative dislocations will vanish as they meet, displacing the block by the Burger’s vector B

Repulsion - If 2 edge dislocations join (both positive) the distortion of the lattice is impossibly large. Therefore the 2 like dislocations repel each other.

Question 17

What is the Frank-Reed Source?

Answer 17

Since dislocations of opposite polarity repel each other, each new loops drive the former loop outwards

Sloip therefore generally occurs by the expansion of a series of loops originating from isolated sources in this fashion

Draw the diagram - P46

Question 18

What is the Peierls-Nabarro Force?

Answer 18

tau(b) - Force per unit dislocation length - Much less than the theoretical sheet stress because of distortions in the lattice around the dislocation.

Question 19

What is strain hardening

Answer 19

A metal contains a network of dislocations prior to loading.

During a tensile test, the material will deform elastically (region 1) where atomic bonds are stretched but remain intact. The stress is insufficient to move dislocations.

When the yield condition is reached, some dislocations start to move on favourable planes - Schmid Criterion exceed

New dislocations must be generated to move through the material via Frank-Reed sources -> produces more strain deformation into region 3

Draw graph.

Question 20

What does strain hardening cause?

Answer 20

If the movement of dislocations is stopped by an obstacle, the next loop moves towards the leading loop but is repelled. This pattern continues until the last loop to emerge from the source is stopping and the stress field of the last loop stops at the source.

Question 21

What does dislocation density increase with?

Answer 21

Dislocation density increases with increasing strain. The higher the dislocation density, the lower the distance between dislocations.

Question 22

What is cross slip?

Answer 22

A dislocation can only move easily on a plane which contains both dislocation line and Burger’s vector. Screw dislocation is cylindrically symmetrical about it’s axis and can therefore transfer to any plane which contains the line and Burger’s vector/

If planes X And Y are on the same line and b, cross slip occurs.

Question 23

What is an edge dislocation?

Answer 23

An edge dislocation, the line must be perpendicular to the Burger’s vector. Transfer of an edge to the other planes must be accomplished by the loss or gain of atoms.

For an edge dislocation, movement normal to its plane is caused by climb.

Positive climb - Occurs by diffusion of vacancies to to interstitials away from the line.

Negative climb occurs by diffusion of vacancies away from to interstitial to the line.

Climb can only occur at temperatures where diffusion can occur. >0.4Tm (melting point)