1. CREEP
CREEP
Dr. Mohammed Abdulrazzaq
Materials Engineering Department

4. Mechanisms / Methods by which a can Material can FAIL
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-chemical degradation
Creep
Physical degradation
Fatigue
Plastic
deformation
Fracture
Microstructural changes
Twinning
Wear
Slip
Twinning
Corrosion
Erosion
Phase transformations
Oxidation
Grain growth
Particle coarsening
* Beyond a certain limit

5. Plastic Deformation in Crystalline Materials
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

6. 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.

7. 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.

8. Constant load creep curveI II
Strain () →
III 0
0 → Initial instantaneous strain
t →
The distinguishability of the three stages strongly depends on T and 

9. Constant Stress creep curveI II
Strain () →
III  
t →

10. 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

11. Effect of stress Strain () →  → Increasing stress 0 increases 0
 →
 →
Elastic strains
Increasing stress
0 increases 0
t →

12. Effect of temperature
E↓ as T↑
Strain () →
Increasing T
 →
0 increases 0
 →
t →
As decrease in E with temperature is usually small the 0 increase is also small

13. Creep Mechanisms of crystalline materials
Cross-slip
Dislocation related
Climb
Glide
Harper-Dorn creep
Coble creep
Creep
Grain boundary diffusion controlled
Diffusional
انتشاري
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

14. Creep can be classified based on
Harper-Dorn creep
Phenomenology
Power Law creep
Creep can be classified based on
Mechanism

15. 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)]

16. 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

17.  Flow of vacancies Diffusional creep
Nabarro-Herring creep → high T → lattice diffusion
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

18. 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

19. 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

20. Cost, fabrication ease, density etc
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