Strengthening of crystals

Question 1

What is the underlying principle for how materials can be strengthened and hardened?

Answer 1

By impeding dislocation movement.

Question 2

Based on shear stress, when does slip occur?

Answer 2

When the resolved shear stress on a favourable slip plane exceeds the critical resolved shear stress.

Question 2

What are the methods that hinder dislocation motion and strengthen the crystal material (increase yield strength, ultimate tensile strength, and hardness)?

Answer 2

Grain size reduction; solid solution strengthening; work hardening.

Question 2

What is the relationship between grain size and yield strength and which equation demonstrates that relationship?

Answer 2

According to the Hall-Petch equation, a material’s yield strength increases as the grain size decreases.

Question 2

Why must grain boundaries impede dislocation motion?

Answer 2

1. Dislocations must change directions due to grain misorientation
2. Atomic disorder at grain boundary causes discontinuity of slip planes

Question 3

What types of solute can be used for the solid-solution strengthening method?

Answer 3

Substitutional solute and interstitial solute.

Question 3

What is the substitutional solute?

Answer 3

A solute where the alloy atom has a similar size as host atom, and
replaces them in the crystal structure

Question 3

What is the interstitial solute?

Answer 3

Alloy atom is smaller than the host atom, and fits into
spaces (interstices) between host atoms

Question 4

What do substitutional atoms impose that impede dislocation motion?

Answer 4

Lattice strain

Question 5

What are the differences between a smaller substitutional atom and a large substitutional atom?

Answer 5

Smaller substitutional atom:
* Tensile stresses in lattice
* Alloy atoms are attracted to
the upper side of the dislocation

Larger substitutional atom:
Compressive stresses in lattice
* Alloy atoms are attracted to
lower side of dislocation

Question 6

What are the two types of interaction between the substitutional atom and dislocation?

Answer 6

1. Alloy atoms diffuse to dislocation to pin it down
2. Strain field interactions between a moving
   dislocation and substitutional atom

Question 7

When is the work-hardening equation applicable?

Answer 7

From the onset of yielding up to necking.

Question 8

What happens to the strength and hardness of a ductile metal during work hardening?

Answer 8

A ductile metal becomes stronger and harder as it is plastically deformed (work hardened), as
evidenced by increase in true stress beyond yield point

Question 9

What is work hardening also referred to and why?

Answer 9

Work hardening is also called cold hardening because it occurs below a material’s melting
temperature; often at room temperature

Question 10

How does work hardening impact dislocation density and dislocation-dislocation interaction?

Answer 10

Work hardening involves an increase in dislocation
density and dislocation-dislocation interaction with plastic deformation.

Question 11

What are examples of dislocation formation sites?

Answer 11

Existing dislocations
* Internal defects
* Surface defects (scratches and nicks)

Question 12

How do dislocation-dislocation strain interactions impact dislocation motion?

Answer 12

Dislocation-dislocation strain
interactions make dislocation
motion more difficult

Question 13

According to the Hall-Petch equation,
what is the relationship between a
material’s yield strength and its grain size?

Answer 13

Yield strength increases as grain size
decreases

Question 14

What is the relationship between grain size, grain boundary area, and the impediment to dislocation motion?

Answer 14

Decreased grain size results in increase in grain
boundary area, which increases the impediment
to dislocation motion

Question 15

Why are pure metals weaker and softer than their alloys? (Written Response)

Answer 15

• Because impurity atoms in solid
  solution impose lattice strains on the surrounding host atoms.
• Lattice strain field interactions between dislocations and these impurity atoms result and dislocation movement is restricted.

Question 16

Why do alloy atoms diffuse to dislocation to pin it down and what is required for this to happen?

Answer 16

• Alloy atom diffuses to a location to reduce
  strain imposed by dislocation
• Energy is required to dissociate from low-strain regions