Materials at High temperature , Creep

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Materials at High temperature , Creep
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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
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Time-dependent Mechanical Behavior - Creep
Creep A time-dependent and permanent
deformation of materials when subjected to a
constant load at a high temperature (gt 0.4 Tm).
Examples turbine blades, steam generators.
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Creep Testing
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Creep Curve
Typical creep curve under constant load
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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.
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Creep Curve Constant Stress
Comparison between constant load and constant
stress
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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.
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Parameters of Creep Behavior
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Power-Law Creep
Where n, the creep exponent, usually lies between
3 and 8. This sort of creep is called power-law
creep.
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Power-Law Creep
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Creep Stress and Temperature Effects
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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

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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?)
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Creep Stress and Temperature Effects
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Creep Stress and Temperature Effects
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Larson-Miller Relation for Creep
Since
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Larson-Miller Plot
Extrapolate low-temperature data from fast
high-temperature tests
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Creep Relaxation
Creep Relaxation At constant displacement,
stress relaxes with time.
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Creep Relaxation
(4)
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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.
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Creep Damage Creep Fracture
Void Formation and Linkage
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Creep Damage Creep Fracture
Damage Accumulation
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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
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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.
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Creep Damage Creep Fracture
Creep rupture Diagram
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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.

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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.

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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.

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Creep Resist Materials
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Creep Resist Materials
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Creep Resist Materials
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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.
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Case Study Turbine Blade
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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
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Turbine Blade Materials Nickel-base Superalloys
Composition of typical creep-resistant blade
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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

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Turbine Blade Materials Nickel-base Superalloys
Microstructures of the alloy
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Turbine Blade Development of Processing
Investment Casting of turbine blades
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Turbine Blade Development of Processing
Directional Solidification (DS) of turbine blades
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Turbine Blade Blade Cooling
Air-Cooled Blades