Transcript Slide 1

CREEP
 CREEP
Mechanical Metallurgy
George E Dieter
McGraw-Hill Book Company, London (1988)
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
Plastic
deformation
Fatigue
Fracture
Twinning
Slip
Chemical /
Electro-chemical
degradation
Creep
Microstructural
changes
Twinning
Wear
Corrosion
Phase transformations
Grain growth
Particle coarsening
* Beyond a certain limit
Physical
degradation
Oxidation
Erosion
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
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.
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
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 > 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.
Constant load creep curve
II
Strain () →
I
III
0 → Initial instantaneous strain
0
t →
 The distinguishability of the three stages strongly depends on T and 
Constant Stress creep curve
II
Strain () →
I
III
t →
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
Strain () →
Effect of stress
 →
Elastic strains
Increasing stress
0 increases
 0  0'  0''
 →
0
t →
Strain () →
Effect of temperature
 →
E↓ as T↑
Increasing T
0 increases
0
 0'  0''
 →
0
t →
As decrease in E with temperature is usually small the 0 increase is also small
Creep Mechanisms of crystalline materials
Cross-slip
Dislocation related
Climb
Glide
Harper-Dorn creep
Coble creep
Creep
Grain boundary diffusion controlled
Diffusional
Nabarro-Herring creep
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
Harper-Dorn creep
Phenomenology
Power Law creep
Creep can be classified based on
Mechanism
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)]
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
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.


Flow of vacancies
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
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
Creep
resistance
Dispersion hardening → ThO2 dispersed Ni (~0.9 Tm)
Solid solution strengthening
Single crystal / aligned (oriented) grains
 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