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
Since

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
(5)
As the time going on, the initial elastic strain i/E is slowly replaced by creep strain, and the stress relaxes.

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:
that the creep strain cr during the design life is acceptable;
that the creep ductility fcr (strain to failure) is adequate to cope with the acceptable creep strain;
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
Choose a material with a high melting point
Maximize obstructions to dislocation motion by alloying to give a solid solution and precipitates; the precipitates must be stable at the service temperature
Choose a solid with a large lattice resistance: this means covalent bonding.

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

30. Creep
Resist
Materials

31. Creep Resist Materials

32. Creep Resist Materials

33. General Electric TF34 High Bypass Turbofan Engine
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:
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. Development of Processing
Turbine Blade –
Development of Processing
Investment Casting of turbine blades

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

41. Turbine Blade – Blade Cooling
Air-Cooled Blades