Creep

Question 1

Define creep

Answer 1

Time-dependent plastic deformation at a constant stress

Question 2

Define yielding

Answer 2

Instantaneous plastic deformation i.e. time-independent

Question 3

What does creep depend on

Answer 3

thermal activation energy and strain rate at a given stress level - is very temperature sensitive

Question 4

If __ and __ are held fixed, plastic flow can occur

Answer 4

if σ and T are held fixed, plastic flow can occur

Question 5

Give the plastic flow equation

Answer 5

dƐ/dt = A . exp(-Q / kT)

Q and A are constants

Question 6

What happens at Stage-0 of creep, give cause

Answer 6

• Initial (mostly) elastic strain upon loading

Question 7

What happens at Stage-1 of creep, give cause

Answer 7

• dƐ/dt decreases with time and strain

- the material strengthens due to increase in dislocation density and sub-grains formation

Question 8

What happens at Stage-2 of creep, give cause

Answer 8

• steady state creep: microstructure and strain rate remains constant
• Increase in dislocation density from deformation is balanced by decrease in dislocation density due to recovery

Question 9

What happens at Stage-3 of creep, give cause

Answer 9

• strain rate increase with time and strain; fracture occurs soon after
• re-crystallisation, coarsening of secondary phases, and formation of internal cracks and voids

Question 10

Give some reasons why not all 3 stages of creep can be seen in experimental tests

Answer 10

No stage II or III - because too long a time is required (moderate stress, low T)

No stage III - because of cavitation failure

No stage II - because specimen necks & ruptures before steady state

Inverted primary creep - solid solution strengthening

Question 11

Give equation for steady state creep - region II only

Known as ‘power-law’ creep

Answer 11

dƐ/dt = Aσ^m . exp(-Q / RT)

A and m - material constants (m around 4.5)
Q - activation energy
R - gas constant

Question 12

Give value of m and type of creep mechanism it indicates

Answer 12

m = 5
dislocation climb/creep which occurs as metal deforms

m = 3
dislocation glide, metal deformation with glide. glide is slower thus rate controlling

m = 2
metal deformation owing to weakening of grain boundaries, grains slide over each other

m = 1
diffusion creep, metal deformation via diffusion of vacancies to grain boundaries through the grains (Nabarro-Herring creep) or at lower temperatures, via diffusion along grain boundaries (Coble creep)

Question 13

Explain Nabarro-Herring creep

Answer 13

usually at elevated temperature, it is diffusion of vacancies to grain boundaries through the grains

Question 14

Explain Coble creep

Answer 14

usually at lower temperature, it is diffusion along grain boundaries

Question 15

When designing for creep failure, when should you use dƐ/dt and when should you use time to failure

Answer 15

dƐ/dt - use when long life and dimensional tolerances are critical i.e. jet turbine blades

time to failure - use when creep deformation is tolerable i.e. rocket casing

Question 16

How is high creep strength suceeded

Answer 16

minimising dislocation glide, recovery and diffusion

Question 17

What is recovery

Answer 17

Recovery is a process by which deformed grains can reduce their stored energy by the removal or rearrangement of defects in their crystal structure. These defects, primarily dislocations, are introduced by plastic deformation of the material and act to increase the yield strength of a material.

Question 18

How do edge dislocations differ from screw dislocations with respect to movement/escaping

Answer 18

edge dislocations - by climb

screw dislocations - by cross-slip

Question 19

Give equation for Nabarro-Herring creep

Answer 19

dƐ/dt(NH) = Anh(Dl/d^2)(σΩ/KT)

Dl - lattice self diffusion coefficient
d - grain size
Ω - atomic volume

Question 20

Give equation for Coble creep

Answer 20

dƐ/dt(c) = Ac(Dgb x δ’/d^3)(σΩ/KT)

Dgb - grain boundary diffusion coefficient
d - grain size
δ’ - grain boundary width
Ω - atomic volume

Question 21

How can Grain boundary sliding effect grains

Answer 21

GBS may allow a material to elongate with no net change in grain volume during superplastic deformation

Question 22

When does GBS occur

Answer 22

together with NH and Coble creep

Question 23

What happens to creep rate if a fine dispersion of non-deforming particles are at grain boundaries

Answer 23

creep rate is lowered.
The particles arrest Grain boundary sliding by pinning grain boundaries and reducing recrystallisation
Larger particles are more effective