CREEP

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CREEP

• CREEP

Mechanical Metallurgy George E Dieter
McGraw-Hill Book Company, London (1988)
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Review
If failure is considered as change in desired
performance- which could involve changes in
properties and/or shape then failure can occur
by many mechanisms as below.
Mechanisms / Methods by which a can Material can
FAIL
Elastic deformation
Chemical /Electro-chemicaldegradation
Creep
Physicaldegradation
Fatigue
Plastic deformation
Fracture
Microstructuralchanges
Twinning
Wear
Slip
Twinning
Erosion
Corrosion
Phase transformations
Oxidation
Grain growth
Particle coarsening
Beyond a certain limit
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Review
Though plasticity by slip is the most important
mechanism of plastic deformation, there are other
mechanisms as well (plastic deformation here
means permanent deformation in the absence of
external constraints)
Plastic Deformation in Crystalline Materials
Slip(Dislocation motion)
Twinning
Phase Transformation
Creep Mechanisms
Grain boundary sliding
Other Mechanisms
Vacancy diffusion
Grain rotation
Dislocation climb
Note Plastic deformation in amorphous materials
occur by other mechanisms including flow
(viscous fluid) and shear banding
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High-temperature behaviour of materials

• Designing materials for high temperature
  applications is one of the most challenging tasks
  for a material scientist.
• Various thermodynamic and kinetic factors tend to
  deteriorate the desirable microstructure.
  (kinetics of processes are an exponential
  function of temperature).
• Strength decreases and material damage (void
  formation, creep oxidation) tends to accumulate.
• Cycling between high and low temperature will
  cause thermal fatigue.

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High temperature effects (many of the effects
described below are coupled)

• Increased vacancy concentration ? at high
  temperatures more vacancies are thermodynamically
  stabilized.
• Thermal expansion ? material will expand and in
  multiphase materials/hybrids thermal stresses
  will develop due to differential thermal
  expansion of the components.
• High diffusion rate ? diffusion controlled
  processes become important.
• Phase transformations can occur ? this not only
  can give rise to undesirable microstructure, but
  lead to generation of internal stresses.?
  Precipitates may dissolve.
• Grain related? Grain boundary weakening ? may
  lead to grain boundary sliding and wedge
  cracking. ? Grain boundary migration ?
  Recrystallization / grain growth ? decrease in
  strength
• Dislocation related ? these factors will lead to
  decrease in strength? Climb? New slip systems
  can become active? Change of slip system ?
  Decrease in dislocation density
• Overaging of precipitate particles and particle
  coarsening ? decrease in strength
• The material may creep (time dependent elongation
  at constant load/stress).
• Enhanced oxidation and intergranular penetration
  of oxygen

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CREEP ? Permanent deformation of a material
under constant load (or constant stress) as a
function of time

• Normally, increased plastic deformation takes
  place with increasing load (or stress)
• In creep plastic strain increases at constant
  load (or stress)
• Usually appreciable only at T gt 0.4 Tm ? High
  temperature phenomenon.
• Mechanisms of creep in crystalline materials is
  different from that in amorphous materials.
  Amorphous materials can creep by flow.
• At temperatures where creep is appreciable
  various other material processes may also active
  (e.g. recrystallization, precipitate coarsening,
  oxidation etc.- as considered before).

• Creep experiments are done either at constant
  load or constant stress.

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Constant load creep curve
I
II
Strain (?) ?
III
?0 ? Initial instantaneous strain
?0
t ?

• The distinguishability of the three stages
  strongly depends on T and ?

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Constant Stress creep curve
II
I
Strain (?) ?
III
?
?
t ?
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Stages of creep
I

• Creep rate decreases with time
• Effect of work hardening more than recovery

II

• Stage of minimum creep rate ? constant
• Work hardening and recovery balanced

III

• Absent (/delayed very much) in constant stress
  tests
• Necking of specimen start
• specimen failure processes set in

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Effect of stress
Strain (?) ?
Increasing stress
?0 increases
?0
t ?
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Effect of temperature
Strain (?) ?
E? as T?
Increasing T
? ?
?0 increases
?0
? ?
t ?
As decrease in E with temperature is usually
small the ?0 increase is also small
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Creep Mechanisms of crystalline materials
Cross-slip
Climb
Dislocation related
Glide
Harper-Dorn creep
Coble creep
Creep
Grain boundary diffusion controlled
Nabarro-Herring creep
Diffusional
Lattice diffusion controlled
Dislocation core diffusion creep
Diffusion rate through core of edge dislocation
more
Interface-reaction controlled diffusional flow
Grain boundary sliding
Accompanying mechanisms creep with dynamic
recrystallization
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Harper-Dorn creep
Phenomenology
Power Law creep
Creep can be classified based on
Mechanism
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Cross-slip

• In the low temperature of creep ? screw
  dislocations can cross-slip (by thermal
  activation) and can give rise to plastic strain
  as f(t)

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Dislocation climb

• Edge dislocations piled up against an obstacle
  can climb to another slip plane and cause plastic
  deformation as f(t), in response to stress
• Rate controlling step is the diffusion of
  vacancies

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Nabarro-Herring creep ? high T ? lattice diffusion
Diffusional creep
Coble creep ? low T ? Due to GB diffusion

• In response to the applied stress vacancies
  preferentially move from surfaces/interfaces (GB)
  of specimen transverse to the stress axis to
  surfaces/interfaces parallel to the stress axis?
  causing elongation.
• This process like dislocation creep is controlled
  by the diffusion of vacancies ? but diffusional
  does not require dislocations to operate.

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Grain boundary sliding

• At low temperatures the grain boundaries are
  stronger than the crystal interior and impede
  the motion of dislocations
• Being a higher energy region, the grain
  boundaries melt before the crystal interior
• Above the equicohesive temperature grain
  boundaries are weaker than grain and slide past
  one another to cause plastic deformation

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Creep Resistant Materials

• Higher operating temperatures gives better
  efficiency for a heat engine. Hence, there is a
  need to design materials which can withstand high
  temperatures.

High melting point ? E.g. Ceramics
Dispersion hardening ? ThO2 dispersed Ni (0.9 Tm)
Creep resistance
Solid solution strengthening
Single crystal / aligned (oriented) grains
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• Cost, fabrication ease, density etc. are other
  factors which determine the final choice of a
  material
• Commonly used materials ? Fe, Ni (including
  superalloys), Co base alloys
• Precipitation hardening (instead of dispersion
  hardening) is not a good method as particles
  coarsen (smaller particles dissolve and larger
  particles grow ? interparticle separation ?)
• Ni-base superalloys have Ni3(Ti,Al) precipitates
  which form a low energy interface with the matrix
  ? low driving force for coarsening
• Cold work cannot be used for increasing creep
  resistance as recrystallization can occur which
  will produced strain free crystals
• Fine grain size is not desirable for creep
  resistance ? grain boundary sliding can cause
  creep elongation / cavitation? Single crystals
  (single crystal Ti turbine blades in gas turbine
  engine have been used)? Aligned / oriented
  polycrystals

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