Hardness, Plastic Deformation, and Strengthening Mechanisms

Question 1

Define true strain

Answer 1

ln(length / original length)

Question 2

What is elastic strain recovery?

Answer 2

The amount of strain recovered during elastic unloading of plastic deformation.

Question 3

Define Hardness

Answer 3

A material’s resistance to local deformation

Question 4

How do we define hardness qualitatively?

Answer 4

Mohr scale

Question 5

How do we define hardness quantitatively?

Answer 5

Rockwell, Brinell, Knoop Microharndess test, Vickers Microharndess test

Question 6

Describe a Rockwell Hardness test

Answer 6

A hardened steel ball or diamond cone is pushed into a material with a minor load of 10 kg and a major load of 60, 100, and 150 kg. The hardness is the depth between these two penetrations.
A thin specimen would use a minor load of 3 kg and major loads of 15, 30, and 45 kg.

Question 7

Describe a Brinell hardness test

Answer 7

A hardened steel ball at loads of 500-3000kg.

Question 8

Describe a microhardness test

Answer 8

A diamond pyramid makes a microscopic indent.

Question 9

Why do we use hardness tests?

Answer 9

Inexpensive
Non-destructive
Other properties can be calculated (such as tensile strength is 3.45HB)

Question 10

Why might measured hardness vary?

Answer 10

test method
operator bias
fabrication procedure
apparatus calibration
inhomogenous material

Question 11

What causes plastic deformation?

Answer 11

Dislocation movement

Question 12

What factors are required for dislocation movement?

Answer 12

Shear stress
Slip planes
Slip directions

Question 13

What determines a slip plane or direction?

Answer 13

The plane or direction with the most densly packed atoms.

Question 14

Define slip system

Answer 14

Combination of slip plane and direction

Question 15

How does the number of slip systems in a material affect ductility?

Answer 15

The more slip systems, the easier it is to plastically deform, the more ductile the material.

Question 16

When does slip begin in a material?

Answer 16

When the shear stress in the slip systems reaches the critical resolved shear stress.

Question 17

Is critical resolved shear stress a material property?

Answer 17

Yes

Question 18

What is the angle between two directions?

Answer 18

D1 dot D2 / [|D1| |D2|]

Question 19

How do you strengthen a material?

Answer 19

Make dislocation motion difficult

Question 20

What are four strengthening processes?

Answer 20

Grain size reduction
Solid Solution Strengthening
Precipitation hardening
Strain Hardening (cold work)

Question 21

Describe grain size reduction as a strengthening process.

Answer 21

Grain lines trap dislocations by changing slip planes and causing dislocation to change direction. More grain lines = more strength.

Question 22

Describe solid solution strenghening.

Answer 22

Adding other elements (alloying). This results in points of lattice strain, which attracts and ties down dislocations, which also have strain.
Smaller impurity atoms add tensile strain, while larger impurity atoms cause compressive strain.

Question 23

Describe precipitation hardening.

Answer 23

Adds particles of different phase to prohibit dislocation movement through them, and these have lattice strains at the borders, trapping dislocations.

Question 24

Describe strain hardening

Answer 24

Uses plastic deformation to increase the dislocation density, which gives each dislocation less ‘room to move’.

Question 25

Describe the stages of annealing heat treatment.

Answer 25

Recovery releases internal energy and restores thermal and electrical conductivity, but does not change grains.
Recrystallization forms new strain free grains, and is driven by the amount of internal energy in the material.
Grain growth amalgomates grains into large ones, and is caused by short range diffusion of atoms across grain boundaries.

Question 26

What is hot working?

Answer 26

Plastic deformation above the recrystallization temperature. Has no associated strain, and allows for large deformations.