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GP3 > Structural Failure of Materials - Creep > Flashcards

Structural Failure of Materials - Creep Flashcards

1
Q

What is creep

A

The time dependent increase of inelastic strain at constant applied stress.

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2
Q

How can creep lead to failure?

A

With increasing time, damage can accumulate in the material, thus accelerating the rate of deformation. Eventually, failure occurs when the state of damage reaches a critical value.

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3
Q

When do materials creep?

A

In metallic components at temperatures in excess of about a third of their true melting point.
In ceramics at about half their true melting point.
Some materials such as polymers creep at ambient temperatures and ice creeps close to 0°C.

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4
Q

Describe a simple creep test.

A

Apply a constant stress and measure how strain evolves with time.

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5
Q

On loading a sample for a creep test, what happens over the course of the test?

A

Elastic and time independent straining occurs, followed by primary, secondary and tertiary creep.

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6
Q

What happens in the primary creep regime?

A

The strain-rate gradually decreases with increasing time

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7
Q

What happens in the secondary creep regime?

A

The strain rate eventually reduces to a constant rate. This is characterised by the steady state creep rate.

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8
Q

What happens in the tertiary creep regime?

A

Strain rate gradually increases as damage in the material accumulates.

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9
Q

State Norton law for steady state creep.

A

Check L1 S13

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10
Q

State the Arrhenius relation for steady state creep

A

Check L1 S13

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11
Q

What are the main classes of mechanisms of creep deformation?

A

Diffusional creep

Dislocation (or Power law) creep

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12
Q

What paths can material be transferred through for diffusional creep

A

Through the bulk crystal (Herring-Nabarro)

Along grain boundaries (Coble)

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13
Q

Derive the Herring-Nabarro creep equation.

A

Check L1 S20

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14
Q

State a relation between the activation energies for diffusional creep methods.

A

Q>Q(dislocation core)>Q(grain boundary)>Q(surface)

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15
Q

State the Coble creep equation

A

Check L1 S22

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16
Q

Give a total creep rate equation from Herring-Nabarro and Coble

A

Check L1 S22 (contributions are additive)

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17
Q

Although diffusionaln creep methods are additive, does one tend to dominate?

A

Yes, in high T Herring-Nabarro is dominant but at low T Coble is.

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18
Q

Give an expression for the characteristic spacing of dislocations when a shear stress τ is applied to a material.

A

Check L1 S29

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19
Q

Is spacing of dislocations characterised by cell size or grain size in the steady state?

A

Cell size.

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20
Q

State an expression for creep in the dislocation regime controlled by self diffusion.

A

Check L1 S31

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21
Q

State an expression for creep in the dislocation regime controlled by dislocation core diffusion

A

Check L1 S34

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22
Q

When comparing mechanisms for creep on a graph of logε’ and logσ which dominates?

A

The mechanism with the higher strain rate.

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23
Q

State an expression that represents the time hardening response primary part of creep. (From the Andrade creep model)

A

Check L1 S39

This is time hardening response

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24
Q

State an expression for the strain hardening response in the primary creep regime.

A

Check L1 S40

25
Q

Which method of predicting creep response is more accurate in the proimary regime, strain or time hardening?

A

Strain hardening is more accurate however both should be treated as limits.

26
Q

Give a steady state expression for creep taking into account the recovery of a material at raised temperature.

A

Check L1 S44

Robinson model

27
Q

What types of damage in a material can contribute towards the ternary state of creep being reached?

A

Cavities at gbs that can’t nuclear grow and coalesce.
Precipitates can coarsen becoming less effective giving rise to time softening.
Dislocations becoming mobile at higher T giving rise to strain softening.
Oxidation or corrosion can result in degradation of creep strength.

28
Q

What is superplastiocity?

A

The ability of certain p[olycrystalline materiasls to exhibit extensive neck free elongations during deformation at elevated temperatures (and small grain size).

29
Q

What are the necessary requirements for superplastic deformation?

A

Fine equiaxed microstructures
Very slow deformation velocities
Relatively high forming T
Good resistance to grain coarsening and void nucleation (stable microstructure)

30
Q

Compare superplastic and plastic deformation.

A

Superplastic:
Grains remain equiasxed with deformation
Grain neighbours continue to change
Texture weakens with straining

Plastic:
Grains elongate along the tensile direction
Grain neighbours remain unchanged
Texture increases with straining

31
Q

What creep regime is superplasticity considered under?

A

High T creep regime

32
Q

Which equation best represents steady state superplasticity?

A

Dorn’s creep equation

33
Q

State Dorn’s creep equation

A

Check L2 S11

34
Q

Which mechanism is most applicable for the superplastic effect?

A

Grain boundary sliding (n≈2)

35
Q

What are the two main theories for how grain boundary sliding occurs?

A

Lifshitz

Rachinger

36
Q

Describe the Lifshitz method for grain boundary sliding.

A

Assumes boundary sliding is a consequence of diffusional deformation.

Diagram L2 S13

37
Q

Describe the Rachinger method for grain boundary sliding.

A

Assumes that grains stayed equiaxed and the net number increases along the tensile direction. However, there must be some accommodation within the grains in order to avoid the opening of gaps.

Diagram L2 S13

38
Q

What mechanisms exist to accommodate for grain boundary sliding.

A 
Diffusion accommodation
Dislocation based accommodation
Grain boundary migration
Accommodation by void formation
Core and mantle accommodation
Dislocation creeo accommodation.
39
Q

What is the single most important property for the description of superplastic materials?

A

The strain rate sensitivity parameter, m

m is the inverse of creep exponent n

40
Q

Why is the strain rate sensitivity so important?

A

It provides information regarding the regime of deformation and the operative characteristic.
It can be used to depict the deformation stability, critical to optimise the amount of deformation that a material can take before failure.

41
Q

Show that increasing m causes more stability in a material being superplastically deformed.

A

Check L2 S17

42
Q

State Considere’s and Hart’s criterion’s.

A

Check L2 S19

43
Q

What are desirable characteristics of high temperature materials?

A

Ability to withstand loading at an operating T close to melting T.
A substantial resistance to mechanical degradation over extended periods of time.
Tolerance of severe operating environments (hot corrosion/oxidation resistance)

44
Q

Why have nickel based alloys have emerged for high T applications?

A

fcc crystal structure, stable from ambient to T(m) with no phase transitions
Tough and ductile due to a considerable cohesive energy from bonding provided by outer d-electrons.
Low rates of thermally activated processes such as creep and diffusion (diffusion rates are low for fcc metals such as Ni)
Other suitable fcc metals are expensive and rare (eg Pt, Pd, Ir)

45
Q

State the Larson-Miller expression for ranking creep performance in the steady state.

A

Check L3 S10

46
Q

State an expression for time to rupture in creep performance.

A

Check L3 S12

47
Q

Along what direction is there a tendency to solidify for Ni alloys anf what is the advantage of this?

A

100 direction

Allows for single crystal growth of turbine blades

48
Q

Why do we want to remove grain boundaries in Ni superalloys for turbine blades?

A

gbs are a source of failure as voiding and nucelation are prevalent. By removing them creep performance is much improved.

49
Q

Compare how single crystals and polycrystals deform by creep

A

Single crystal:
No substantial secondary regime.
Creep rate increases progressively over life
No signs of damage by creep cavitation or cracking.
Norton equation does not apply as predicts a physically unrealistic activation energy.
Dislocation density increases progressively but at all stages dislocation densities are very low.

50
Q

Derive an expression for damage in single crystals that relates tom damage by dislocation density not cavitation.

A

Check L3 S22

51
Q

How does the γ/γ’ structure affect dislocations.

A

Dislocations are assumed to be free to glide in the γ matrix or are trapped in the γ’ matrix.
Trapped dislocations climb at a rate controlled by vacancy emission and absorption.
Rate of unpinning gives rise to the dependence of creep deformations on chemistry.

52
Q

Derive expressions for shear strain rate and creep strain rate in single crystal superalloys.

A

Check L3 S24/5

53
Q

Why does γ’ in a superalloy trap dislocations?

A

In order for passage of a dislocation into the γ’ an anti phase boundary must form at a significant entry penalty.

54
Q

What is a Kear-Wilsdorf lock?

A

When dislocations are travelling in pairs to lower the energy of the AFB in γ’ but one of the dislocations cross slips due to the elevated T.

55
Q

What is the Rhenium effect in Ni superalloys?

A

Rh is the slowest diffuser of all elements in d-bloc transition metals.

56
Q

State en expression showing the diffusivity of rhenium in Ni

A

Check L3 S43

57
Q

Why are significant quantities of Al and Cr added to ni superalloys?

A

They are desinged to form alumina or chroma protective scales by a process of selective oxidation that provide a thermodynamically stable protection at high T and grows over time as oxygen and metal ions slowly diffuse through the outer oxide (scale grows slowly).

58
Q

How are turbine blades temperature managed?

A

Silica casting cores care carefully inserted inside and held using Pt pins.
They are removed after casting with acidic solvents.
High pressure cooled air from the compressor flows out over the surface of the blade (parasitic cooling)
Serpentine cooling refers to the shape of the internal cooling passages.
Cooled air sticks to the blade as a boundary layer and provides effective insulation.

59
Q

How can thermal barrier coatings be used to protect turbine blades?

A

Modern blades coated in yttria stabilised zirconia by EB-PVD.
Bond coated to minimise diffusion with underlying Ni superalloy.
Removes need for parasitic cooling from the compressor.

GP3 (5 decks)

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