Föreläsning 4 - plasticity

Question 1

What is plasticity?

Answer 1

The propensity of a solid material to undergo permanent deformation under load.

Has to do with things such as yield stress, plastic strain, elongation.

Question 2

What is hardness?

Answer 2

In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion.

• Measured with indentation
• Several different methods
• Linked to yield strength

Question 3

What are some types of imperfections in crystals?

Answer 3

• Vacancies = missing atoms
• Solute atoms = atoms of different kinds
• Dislocations = extra atomic planes
• Grain bounadries

Question 4

What are dislocations, and how do they move?

Answer 4

Extra atomic planes.

• Dislocations move when the load is higher than the yield stress
• The dislocation slip gives a plastic strain with the direction and size of the slip vector.
• They are driven by shear strains
• Dislocations move in small steps

=> permanent deformation of the crystal

• They move easiest in directions close to 45 degrees towards the load direction
• Plasticization takes place at constant volume
• Gives rise to shear bands = bands of plastacized material.

There are edge and screw dislocations

Question 5

What are some hardening mechanisms?

Answer 5

• Solution hardening
• Precipitation hardening
• Strain hardening
• Grain boundary hardening

*Defects become obstacles for dislocation movement.
* Change in microstructure to make dislocation movement more difficult.
* Higher yield stress, higher hardness
* Often lower extension

see hardening of copper in slides

Question 6

Describe solution hardening

Answer 6

• atoms of a different kind dissolve into the crystal
• the stress field around the atoms prevents dislocation movement
• greater effect with greater concentration and greater difference in atomic size
• takes place with alloy in melt

Question 7

Describe precipitation hardening

Answer 7

• Particles of another phase are formed in the material
• Particles prevent dislocation movement
• Takes place with alloying of the melt and heat treatment in solid form

Question 8

Describe strain hardening

Answer 8

• Dislocations prevent other dislocations from moving (locking each other)
• The amount of dislocations increases sharply with plastic deformation => plastic hardening
• Takes place during cold working (pressing rolling, forging…)

Question 9

Describe grain boundary hardening

Answer 9

• Grain boundaries prevent dislocations
• Small grains provide harder materials
• Important for BCC metals (some steels)

Question 10

How do we use multiple material indices?

Answer 10

-Make chart with 1/M_1 against 1/M_2
- Draw the pareto surface
- Select material on or near pareto surface