5. Defects: Good or Bad?

Question 1

MICROSTRUCTURAL CONTROL

Answer 1

YAAY

Question 2

WHAT DOES “moving” mean in this context?

Answer 2

Change in location

Question 3

How can you cause transformations?

Answer 3

Vacancies! More you have, the faster it happens

Question 4

Are vacancies good or bad?

Answer 4

They cause transformations to happen faster, s they’re good in general

Question 5

What is the effect of vacancies on atoms? Electrons? Why?

Answer 5

Vacancies encourage atom movement, and discourage electron movement.

Atoms need somewhere to go, electrons are waves and depend on periodicity of material

Question 6

Generality 2: fill in the blanks:

Electron motion is ____ by all defects.

Atom motion is ____ by all defects.

Answer 6

Electron motion: inhibited

Atom motion: enhanced

Question 7

If you heat a copper alloy, what will happen to its conductivity?

Answer 7

NOTICE!! ALLOY!! Depends on the microstructure. And alloying composition, sure, but yea.

Question 8

RELATION BETWEEN ATOMIC BONDING AND PLASTICITY

Answer 8

YEAH

Question 9

What’s the theoretical yield strength of a material?

Answer 9

1/10th of E, for perfect crystals.

Question 10

Why didn’t the theory work?

Answer 10

Defects weaken the material. Specifically, strained bonds at edge dislocations.

Question 11

What is slip?

Answer 11

The motion of a dislocation through the material

Question 12

GEN 3: Dislocation motion is ______ by all defects.

Why?

Answer 12

Inhibited. Except for vacancies.

Interaction of stress fields.

Question 13

How does solid solution strengthening work?

Answer 13

Interstitials and solutes will migrate to dislocations (minimize energy) and reduce their strain.

Reduction in strain means more shearing required to break bonds at dislocation.

Question 14

How do planar defects inhibit slip?

Answer 14

The change in plane gets in the way!

Question 15

What is the Hall-Petch equation?

Answer 15

An equation relating yield strength to grain size.

Sigma y is inversely proportional to sqrt(d) (in um)

Question 16

What is work hardening, under the context of packing defects?

Answer 16

Dislocations building up and interacting with each other to reduce strain energy!

Increased number of dislocations make it harder to plastically deform the metal.

Question 17

Explain why dislocations are important for plasticity.

Answer 17

Dislocations reduce the strain energy required to break bonds.

Absence of dislocations would simply lead to fracture.

However, they need to move in order to help.

Question 18

How does hardness and ductility change with increasing numbers of dislocations?

Answer 18

Ductility needs dislocations, but get lots of them and you end up with work hardening.