Creep Flashcards
Define creep
Time-dependent plastic deformation at a constant stress
Define yielding
Instantaneous plastic deformation i.e. time-independent
What does creep depend on
thermal activation energy and strain rate at a given stress level - is very temperature sensitive
If __ and __ are held fixed, plastic flow can occur
if σ and T are held fixed, plastic flow can occur
Give the plastic flow equation
dƐ/dt = A . exp(-Q / kT)
Q and A are constants
What happens at Stage-0 of creep, give cause
- Initial (mostly) elastic strain upon loading
What happens at Stage-1 of creep, give cause
- dƐ/dt decreases with time and strain
- the material strengthens due to increase in dislocation density and sub-grains formation
What happens at Stage-2 of creep, give cause
- 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
What happens at Stage-3 of creep, give cause
- strain rate increase with time and strain; fracture occurs soon after
- re-crystallisation, coarsening of secondary phases, and formation of internal cracks and voids
Give some reasons why not all 3 stages of creep can be seen in experimental tests
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
Give equation for steady state creep - region II only
Known as ‘power-law’ creep
dƐ/dt = Aσ^m . exp(-Q / RT)
A and m - material constants (m around 4.5)
Q - activation energy
R - gas constant
Give value of m and type of creep mechanism it indicates
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)
Explain Nabarro-Herring creep
usually at elevated temperature, it is diffusion of vacancies to grain boundaries through the grains
Explain Coble creep
usually at lower temperature, it is diffusion along grain boundaries
When designing for creep failure, when should you use dƐ/dt and when should you use time to failure
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
How is high creep strength suceeded
minimising dislocation glide, recovery and diffusion
What is recovery
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.
How do edge dislocations differ from screw dislocations with respect to movement/escaping
edge dislocations - by climb
screw dislocations - by cross-slip
Give equation for Nabarro-Herring creep
dƐ/dt(NH) = Anh(Dl/d^2)(σΩ/KT)
Dl - lattice self diffusion coefficient
d - grain size
Ω - atomic volume
Give equation for Coble creep
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
How can Grain boundary sliding effect grains
GBS may allow a material to elongate with no net change in grain volume during superplastic deformation
When does GBS occur
together with NH and Coble creep
What happens to creep rate if a fine dispersion of non-deforming particles are at grain boundaries
creep rate is lowered.
The particles arrest Grain boundary sliding by pinning grain boundaries and reducing recrystallisation
Larger particles are more effective
