Chapter 6: Plastic Deformation in Metals

Question 1

Shear stress for slip in pure, close-packed metals

Answer 1

1MPa

Question 2

How is plastic deformation produced?

Answer 2

Relative motion of one part of material against another

Question 3

Why is yield stress so high in metals?

Answer 3

Formation and motion of dislocations due to shear stress.

Question 4

Schmid’s law for slip (resolved shear stress)

Answer 4

𝜏R = σcosλcosΦ
σ = applied tensile stress
λ = angle between the slip direction and the stress axis Φ = angle between the normal to the slip plane and stress axis

Question 5

Critical resolved shear stress

Answer 5

Shear stress required to cause a dislocation to move in a pure single crystal of a metal. Maximum at 45°

Question 6

Relationship of plane roughness to motion

Answer 6

As the planar density decreases, resistance to motion in shear increases

Question 7

How many slip systems for good ductility?

Answer 7

5

Question 8

Number of slip systems in BCC

Answer 8

12

Question 9

Number of slip systems in FCC

Answer 9

12

Question 10

Number of slip systems in HCP

Answer 10

3

Question 11

What does strengthening do?

Answer 11

Reduce plastic deformation

Question 12

Grain boundary strengthening

Answer 12

Strengthening materials by decreasing their average crystal (grain) size. Described by the Hall-Petch equation.

Question 13

Hall-Petch equation

Answer 13

σys = σ0 + kd^(-1/2)
σ0 = 3𝜏
k = Hall-Petch coefficient (MNm^(-3/2))
d = grain size (m)

Question 14

Solid solution strengthening

Answer 14

Strengthening a metal by adding solutes of different size and them on the lattice (substitutional solid solution) or between the lattice points (interstitial solid solution) to achieve strengthening through interaction with defects (such as dislocations).

Question 15

Solid solution strengthening equation

Answer 15

𝜏ys = 𝜏0 + βGsqrt(c)
β = property of the solvent/solute combination
c = concentration of solute

Question 16

Strain/work/cold hardening

Answer 16

Permanent deformation to increase the strength of the metal

Question 17

Processing methods to strain harden a metal (4)

Answer 17

1. Forging (molding)
2. Rolling
3. Drawing
4. Extrusion

Question 18

Dislocation density and intersections equation

Answer 18

𝜏ys = 𝜏0 + αGbsqrt(p)
b = Burger’s vector of a dislocation (m)
p = dislocation density (m/m^3)

Question 19

Precipitation hardening

Answer 19

Strengthening a metal by growing precipitates or adding small and hard (strong) precipitates.

Question 20

Precipitates vs dispersoids strengthening

Answer 20

Precipitates: grown in situ, shearable when small Dispersoids or particles: added, not shearable