Creep of materials

1. Materials at High temperature , Creep

2. Materials at High Temperature
Microstructure Change – Stability of Materials
Grain growth
Second-phase coarsening
Increasing vacancy density
Mechanical Properties Change
Softening
Increasing of atoms mobility
Increasing of dislocations mobility (climb)
Additional slip systems

3. Time-dependent Mechanical Behavior
- Creep
Creep: A time-dependent and permanent deformation
of materials when subjected to a constant load at a high
temperature (> 0.4 Tm). Examples: turbine blades, steam
generators.

4. Creep Testing

5. Creep Curve
Typical creep curve under constant load

6. Creep Curve
1. Instantaneous deformation, mainly elastic.
2. Primary/transient creep. Slope of strain vs.
time decreases with time: work-hardening
3. Secondary/steady-state creep. Rate of
straining is constant: balance of work-hardening
and recovery.
4. Tertiary. Rapidly accelerating strain rate up to
failure: formation of internal cracks, voids, grain
boundary separation, necking, etc.

7. Creep Curve – Constant Stress
Comparison between constant load and constant stress

8. Parameters of Creep Behavior
The stage secondary/steady-state creep is of
longest duration and the steady-state creep
rate is the most important parameter of the
creep behavior in long-life applications.
Another parameter, especially important in
short-life creep situations, is time to rupture,
or the rupture lifetime, tr.

9. Parameters of Creep Behavior

10. Power-Law Creep
By plotting the log of the steady creep-rate ss, against log
(stress, ), at constant T, in creep curve, we can establish

ss = Bn

Where n, the creep exponent, usually lies between 3 and
8. This sort of creep is called “power-law” creep.

11. Power-Law Creep

12. Creep: Stress and Temperature Effects

13. Creep: Stress and Temperature Effects
With increasing stress or temperature:
The instantaneous strain increases
The steady-state creep rate increases
The time to rupture decreases

14. Creep: Stress and Temperature Effects
The stress/temperature dependence of the steady-
state creep rate can be described by
where Qc is the activation energy for creep, K2 is
the creep resistant, and n is a material constant.
(Remember the Arrhenius dependence on temperature for
thermally activated processes that we discussed for diffusion?)

15. Creep: Stress and Temperature Effects

16. Creep: Stress and Temperature Effects

17. Larson-Miller Relation for Creep
exp( / )
ln( ) ln( )
(ln( ) ln( ))
s
s
s
A G RT
G
A
RT
G
T A
R



  

 

  
tan
s r
t Cons t
  
Since
( ln( ))
( log( ))
r
r
G
T B t
R
LMP T C t

  
  

18. Larson-Miller Plot
Extrapolate low-temperature data from fast high-
temperature tests

19. Creep Relaxation
Creep Relaxation: At constant displacement, stress
relaxes with time.

20. Creep Relaxation
tot = el + cr (1)
But el = /E (2)
and (at constant temperature)
cr = Bn (3)
Since tot is constant, we can differentiate (1) with respect
to time and substitute the other two equations into it give

(4)

21. Creep Relaxation
Integrating from  = i at t = 0 to  =  at t = t gives
As the time going on, the initial elastic strain i/E is slowly
replaced by creep strain, and the stress relaxes.
(5)

22. Creep Damage & Creep Fracture
Void Formation and Linkage

23. Creep Damage & Creep Fracture
Damage Accumulation

24. Creep Damage & Creep Fracture
Since the mechanism for void growth is the same as
that for creep deformation (notably through diffusion),
it follows that the time to failure, tf, will follow in
accordance with:

25. Creep Damage & Creep Fracture
As a general rule:
ss  tf = C
Where C is a constant, roughly 0.1. So, knowing the
creep rate, the life can be estimated.

26. Creep Damage & Creep Fracture
Creep – rupture Diagram

27. Creep Design
In high-temperature design it is important to make sure:
(a) that the creep strain cr during the design life is
acceptable;
(b) that the creep ductility f
cr (strain to failure) is adequate
to cope with the acceptable creep strain;
(c) that the time-to-failure, tf, at the design loads and
temperatures is longer (by a suitable safety factor) than
the design life.

28. Creep Design
Designing metals & ceramics to resist power-law creep
(a)Choose a material with a high melting point
(b)Maximize obstructions to dislocation motion by alloying
to give a solid solution and precipitates; the precipitates
must be stable at the service temperature
(c)Choose a solid with a large lattice resistance: this means
covalent bonding.

29. Creep Design
Designing metals & ceramics to resist diffusional flow
(a)Choose a material with a high melting point
(b)Arrange that it has a large grain size, so that diffusion
distances are long and GBs do not help diffusion much
(c)Arrange for precipitates at GBs to impede GB sliding.

30. Creep
Resist
Materials

31. Creep Resist Materials

32. Creep Resist Materials

33. Case Study – Turbine Blade
General Electric TF34 High Bypass Turbofan Engine
For (1) U.S. Navy Lockheed S-3A anti submarine warfare aircraft
(2) U.S. Air Force Fairchild Republic A-10 close support aircraft.

34. Case Study – Turbine Blade

35. Case Study – Turbine Blade
Alloy requirements for turbine blades
(a) Resistance to creep
(b) Resistance to high-temperature oxidation
(c) Toughness
(d) Thermal fatigue resistance
(e) Thermal stability
(f) Low density

36. Turbine Blade Materials –
Nickel-base Superalloys
Composition of typical creep-resistant blade

37. Turbine Blade Materials –
Nickel-base Superalloys
Microstructures of the alloy:
(1)Has as many atoms in solid solution as possible ( Co,
W, Cr)
(2) Forms stable, hard precipitates of compounds like
Ni3Al, Ni3Ti, MoC, TaC to obstruct the dislocations
(3) Forms a protective surface oxide film of Cr2O3 to
protect the blade itself from attack by oxygen

38. Turbine Blade Materials –
Nickel-base Superalloys
Microstructures of the alloy

39. Turbine Blade –
Development of Processing
Investment Casting of turbine blades

40. Turbine Blade –
Development of Processing
Directional Solidification (DS) of turbine blades

41. Turbine Blade – Blade Cooling
Air-Cooled Blades