CREEP

CREEP

Mechanical MetallurgyGeorge E Dieter

McGraw-Hill Book Company, London (1988)

Plastic

deformation

Mechanisms / Methods by which a can Material can FAIL

FractureFatigue

CreepChemical /

Electro-chemical

degradation

Physical

degradation

Wear

Erosion

Microstructural

changes

Phase transformations

Twinning

Grain growth

Elastic deformation

Particle coarsening

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.

* Beyond a certain limit

Corrosion

Oxidation

Slip

Twinning

Review

Slip

(Dislocation

motion)

Plastic Deformation in Crystalline Materials

Twinning Phase Transformation Creep Mechanisms

Grain boundary sliding

Vacancy diffusion

Dislocation climb

+ Other Mechanisms

Note: Plastic deformation in amorphous materials occur by other mechanisms including flow (~viscous fluid) and shear

banding

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):

Grain rotation

Review

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.

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 precipitates and precipitate coarsening decrease in strength

The material may creep (time dependent elongation at constant load/stress).

Enhanced oxidation and intergranular penetration of oxygen

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

Etc.

In some sense creep and superplasticity are related phenomena: in creep we can think of

damage accumulation leading to failure of sample; while in superplasticity extended plastic

deformation may be achieved (i.e. damage accumulation leading to failure is delayed).

Creep is permanent deformation of a material under constant load (or constant stress) as a

function of time. (Usually at high temperatures lead creeps at RT).

Creep

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.

Creep can be classified based on

Mechanism

Phenomenology Harper-Dorn creep

Power Law creep

t

Str

ain

()

Constant load creep curve

I

III

II

The distinguishability of the three stages strongly depends on T and

0 0 Initial instantaneous strain

Constant load creep curve

t

Str

ain

()

Constant Stress creep curve

I II

III

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

Effect of stress

t

Str

ain

()

0

'

0''

00

Elastic strains

Increasing stress

0in

crea

ses

Effect of stress

Effect of temperature

t

Str

ain

()

0

'

0''

00

Increasing T

E as T

As decrease in E with temperature is usually small the 0 increase is also small

0in

crea

ses

Effect of temperature

Creep

Dislocation related

Diffusional

Grain boundary sliding

Creep Mechanisms of crystalline materials

Nabarro-Herring creep

Coble creep

Lattice diffusion controlled

Grain boundary diffusion controlled

Dislocation core diffusion creep

Climb

Cross-slip

Glide

Diffusion rate through core of edge dislocation more

Harper-Dorn creep

Interface-reaction controlled diffusional flow

Accompanying mechanisms: creep with dynamic recrystallization

Creep can be classified based on

Mechanism

PhenomenologyHarper-Dorn creep

Power Law creep

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

Diffusional creep

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

Coble creep low T Due to GB diffusion

Nabarro-Herring creep high T lattice diffusion

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.

Creep

resistance

Dispersion hardening ThO2 dispersed Ni (~0.9 Tm)

Solid solution strengthening

High melting point E.g. Ceramics

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