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Mechanical & Aerospace Engineering West Virginia University Materials at High temperature, Creep.

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Presentation on theme: "Mechanical & Aerospace Engineering West Virginia University Materials at High temperature, Creep."— Presentation transcript:

1 Mechanical & Aerospace Engineering West Virginia University Materials at High temperature, Creep

2 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Creep Testing

5 Mechanical & Aerospace Engineering West Virginia University Creep Curve Typical creep curve under constant load

6 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Creep Curve – Constant Stress Comparison between constant load and constant stress

8 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Parameters of Creep Behavior

10 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Power-Law Creep

12 Mechanical & Aerospace Engineering West Virginia University Creep: Stress and Temperature Effects

13 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University 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, K 2 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 Mechanical & Aerospace Engineering West Virginia University Creep: Stress and Temperature Effects

16 Mechanical & Aerospace Engineering West Virginia University Creep: Stress and Temperature Effects

17 Mechanical & Aerospace Engineering West Virginia University Larson-Miller Relation for Creep Since

18 Mechanical & Aerospace Engineering West Virginia University Larson-Miller Plot Extrapolate low-temperature data from fast high- temperature tests

19 Mechanical & Aerospace Engineering West Virginia University Creep Relaxation Creep Relaxation: At constant displacement, stress relaxes with time.

20 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Creep Damage & Creep Fracture Void Formation and Linkage

23 Mechanical & Aerospace Engineering West Virginia University Creep Damage & Creep Fracture Damage Accumulation

24 Mechanical & Aerospace Engineering West Virginia University 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, t f, will follow in accordance with:

25 Mechanical & Aerospace Engineering West Virginia University Creep Damage & Creep Fracture As a general rule:  ss  t f = C Where C is a constant, roughly 0.1. So, knowing the creep rate, the life can be estimated.

26 Mechanical & Aerospace Engineering West Virginia University Creep Damage & Creep Fracture Creep – rupture Diagram

27 Mechanical & Aerospace Engineering West Virginia University 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, t f, at the design loads and temperatures is longer (by a suitable safety factor) than the design life.

28 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Creep Resist Materials

31 Mechanical & Aerospace Engineering West Virginia University Creep Resist Materials

32 Mechanical & Aerospace Engineering West Virginia University Creep Resist Materials

33 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Case Study – Turbine Blade

35 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Turbine Blade Materials – Nickel-base Superalloys Composition of typical creep-resistant blade

37 Mechanical & Aerospace Engineering West Virginia University 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 Mechanical & Aerospace Engineering West Virginia University Turbine Blade Materials – Nickel-base Superalloys Microstructures of the alloy

39 Mechanical & Aerospace Engineering West Virginia University Turbine Blade – Development of Processing Investment Casting of turbine blades

40 Mechanical & Aerospace Engineering West Virginia University Turbine Blade – Development of Processing Directional Solidification (DS) of turbine blades

41 Mechanical & Aerospace Engineering West Virginia University Turbine Blade – Blade Cooling Air-Cooled Blades


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