Chapter 7 - Dislocations

Question 1

what is the difference between a edge and a screw dislocation?

Answer 1

Edge: dislocation line moves in the direction of the applied shear stress

Screw: dislocation line is perpendicular to the stress direction

Question 2

Which type of material is dislocation motion the hardest?

Answer 2

ionic ceramics
covalent ceramics
metals

Question 3

do dislocations tend to happen in polymers?

Answer 3

no, therefore slip is brittle failure is more likely then slip

Question 4

why do ceramics fail brittley rather than by dislocation slip?

Answer 4

In ceramics, it is most likely to fail because of flaws rather than sufficient stress to cause dislocation movement.

Question 5

dislocations in Carefully solidified metal crystals:

Answer 5

10^3 mm-2

Question 6

dislocations in Heavily deformed metals:

Answer 6

10^9 to 10^10 mm-2

Question 7

dislocations in Ceramic materials:

Answer 7

10^2 to 10^4 mm-2

Question 8

dislocations in Silicon single crystals:

Answer 8

0.1 to 1 mm-2

Question 9

what is strain energy?

Answer 9

Pot. energy that is gained during elongation.

Question 10

what is a slip plane ?

Answer 10

A crystallographic plane on which slip occurs most easily.

Question 11

What is slip direction ?

Answer 11

A crystallographic direction along which slip occurs most easily.

Question 12

what does the formula 𝜏R = F’/A’?

Answer 12

𝜏R = tensile component of shear stress
F’ = force acting along the slip direction.
A’ = area of the slip plane

Question 13

What is this formula: F’ = F * cos(λ)?

Answer 13

F’: Force along the slip direction
F: Applied tensile force
λ (lambda): Angle between the force direction and slip direction

Question 14

What is this formula: A’ = A / cos(φ)?

Answer 14

A’: Area of the slip plane
A: Cross-sectional area perpendicular to the tensile force
φ (phi): Angle between the normal to the slip plane and the tensile force direction

Question 15

What is this formula: 𝜏R = (F / A) * cos(λ) * cos(φ)?

Answer 15

𝜏R: Resolved shear stress
F: Applied tensile force
A: Cross-sectional area perpendicular to the tensile force
λ (lambda): Angle between the force direction and slip direction
φ (phi): Angle between the normal to the slip plane and the tensile force direction

Question 16

What is this formula: 𝜏R = σ * cos(λ) * cos(φ)?

Answer 16

𝜏R: Resolved shear stress
σ (sigma): Applied tensile stress (F / A)
λ (lambda): Angle between the force direction and slip direction
φ (phi): Angle between the normal to the slip plane and the tensile force direction

Question 17

When is resolved shear stress at its maximum

Answer 17

when phi and lambda is at 45 degrees

Question 18

what is slip?

Answer 18

a small fraction of the metallic bonds across the interface needs to be a broker at a time.

Question 19

does increasing dislocation mobility increase or decrease the strength

Answer 19

decreases

Question 20

does reducing grain size increase or decrease dislocation motion?

Answer 20

it decreases

Question 21

what is this formula: σyield = σ0 +ky d^-1/2

Answer 21

σyield: This represents the yield stress of the material. It is the stress at which the material begins to deform plastically.

σ0: This is the intrinsic yield strength of the material, which is the yield strength in the absence of any grain boundaries.

ky: constant given

d: This represents the average diameter of grain

Question 22

What is this formula: d^n - d0^n = K * t?

Answer 22

d: Average grain size after heat treatment
d0: Initial average grain diameter before heat treatment
n: Exponent, typically around 2
K: Material constant (depends on temperature, T, but independent of time, t)
t: Heat treating time

Question 23

what is solid solution strengthening, and what does it do to dislocation motion?

Answer 23

it introduces impurity atoms that distort the lattice, and act as barrier to dislocation motion

Question 24

what is strain hardening?

Answer 24

plastically defomring metals at room temp, makes them harder and stronger. while introducing dislocations.

Question 25

what is heat treating?

Answer 25

warming up cold worked metals to bring back their ductile properties