Lecture 1: Strengthening Mechanisms

Question 1

What is recovery?

Answer 1

Strain energy is relieved due to dislocation motion and decrease in number of dislocations

Question 2

What is recrystallisation?

Answer 2

New grains form as small nuclei and then grow. Depends on time and temperature

Question 3

What is grain growth?

Answer 3

Energy is minimised through GB motion. GB’s move through short range motion of atoms. Begins in highly stressed regions

Question 4

What is grain size hardening?

Answer 4

• Adjacent grains have different crystallographic orientations
• Grain boundaries (GB) act as barriers for dislocations because of grain
misorientations and discontinuity in slip planes
• If the misorientation is large, dislocations pile up at GB. Pile-ups increase
stress concentration at GB and generate new dislocations in adjacent grains.
• A fine grained material is harder and stronger (higher yield stress) than a
coarse grained material.
Because the fine grained material has a large GB area that can obstruct
dislocation motion.

Question 5

What is solid solution Hardening?

Answer 5

Atoms of alloying elements distort the lattice and create lattice strain fields.
• These strain fields interact with the strain fields of dislocations.
• This leads to pinning of dislocations. In order to move these “pinned
dislocations higher stress is required increase in yield strength.
• Substitutional solid solution - requires atoms of similar size
• Interstitial solid solution - requires much smaller atoms

Question 6

What is Work Hardening?

Answer 6

• Strain hardening: a ductile metal becomes harder and stronger as it
is plastically deformed.
• Plastic deformation causes dislocations to move and to multiply, i.e.
new dislocations will form.
• Eventually, dislocations become progressively hindered from moving
because they become blocked by other dislocations, yield stress increases

Question 7

What is precipitation Hardening?

Answer 7

Fine particles (precipitates) increase the strength and hardness of
metallic alloys.
• Precipitates are formed through heat treatment called precipitation
hardening.
• Precipitates obstruct dislocation motion, strength increases.
• Fine precipitates are better than the coarse ones.

Question 8

What is strengthening due to martensite formation?

Answer 8

Martensite, a high strength (0.8 - 1 GPa) phase is formed in steels
when rapidly quenched (e.g. during welding) or applied stress or
plastically deformed (e.g. rolling of steels).
• TRIP (Transformation-induced plasticity) effect takes place.
• Select an appropriate quenching process depending on the steel
composition, to achieve the desired amount of martensite and
retained austenite, therefore to achieve the desired amount of strength and ductility.

Question 9

What is the critical resolved shear stress?

Answer 9

• Dislocations move along slip plane and slip direction.
• Even though the applied stress is tensile or compressive, shear
components exist, known as resolved shear stresses
• 𝜏𝑅 𝑚𝑎𝑥 = 𝜎 𝑐𝑜𝑠∅cos 𝜆 Max. shear stress along the most favourable slip system.
• Slip commences when 𝜏𝑅 𝑚𝑎𝑥 = 𝜏𝑐𝑟𝑠𝑠 and when 𝜎 = 𝜎yield
• Critical resolved shear stress:
𝜏𝑐𝑟𝑠𝑠 = 𝜎𝑦𝑖𝑒𝑙𝑑 𝑐𝑜𝑠∅cos 𝜆