High Temperature Materials

Front 1

What is Creep?

Back 1

A form of time dependent

slow mechanical deformation

when a material is exposed to a stress below yield strength

Irreversible

Front 2

What is the cause of Creep

Back 2

Diffusion

- movement of edge dislocation around obstacles

- Q Self diffusion ~ Q for creep

Front 3

What are the types of Creep?

Back 3

Hint 3

Ashby Map

Front 4

What is Diffusion creep?

Back 4

- Deformation by the diffusion of vacancies through the lattice structure

- Requires bonds to break and form -> Energy -> greater at higher Temp

- Stable when evenly distributed -> no concentration gradient - explained by Fick's law -> J = - D (delta C)/(delta x)

- And that's why the vacancies diffuse

- When mechanical stress applied: new vacancies created at the side perpendicular to the direction of the lowest principal stress

- vacancies start moving in direction of the crystal planes perpendicular to maximal stress (according to Fick's law - to reach even distribution)

- Vacancies either move through crystal (Grain) (Volume Diffusion)= Nabarro-Herring Creep

- Vacancies move along Grain boundaries = Coble Creep

Less stress needed than Dislocation creep

Front 5

What is Dislocation Creep?

Back 5

- Deformation involving the movement of dislocations through a crystal lattice

- every time disl. moves through crystal, part of crystal shifts by one lattice point along a plane

- the plane that separates the shifted and unshifted part is called the slip plane.

- along slip pane all ionic bonds must be broken (one by one) to allow for this shift

- this one by one/step by step process gives us a linear lattice defect (Edge or screw)

- climbing with diffusion of atoms and vacancies, concentrations here depend on particle distance

- if no particles, then subgrain boundaries

- if no subgrain boundaries, then dislocations effect each other - back stress

Front 6

Nabarro-Herring vs Coble Creep

Back 6

Nabaro-Herring:

- Volume diffusion

- strain rate ~ 1/d² (p =2) (d = grain sieze)

- predominates over high temperature dislocation dependent mechanisms only at low stresses, and then only for fine grained material

- increase linearly with stress increasing

- increase linearly with Grain boundary size decreasing

Coble Creep

- Grain boundary diffusion

- strain rate ~ 1/d³ (p=3) (d = grain size)

- Lower temp for Coble because activation energy is less for vacancy movement along grain boundary than through lattice

Front 7

Dislocation Climb

Back 7

- when obstacle (pt defect) gets in the way of a dislocation movement

- Vacancies required

- When vacancy arrives at place where dislocation is stuck

- dislocation can climb out of its glide plan and into another to overcome obstacle

- therefore vacancy diffusion dependent (in effect both dislocation creep and diffusion creep are diffusion controlled)

Front 8

Power Creep Law and Diffusion and Dislocation creep. What is n?

Back 8

- Diffusion: n = 1 -> creep power law tells us then that strain rate depends on stress linearly

- Dislocation: n = 3-8 -> creep power law tells us about a different relation between strain rate and stress

Front 9

When T < 0.4*Tm how does dislocation glide look for Ni and W?

Back 9

Ni - FCC

{111} 4 independent slip planes

<110> 3 independent slip directions

W - BCC

{110} 6 indep slip planes

{112} 12 indep slip planes

{321} 24 indep slip planes

<111>

For FCC hardly any strength reduction for increasing T

For BCC strong strength reduction for increasing T

At low Temp BCC is much stronger

Why?

1. BCC is less densely packed and thus requires more energy to casue a slip. The densest slip plan is less dense than for a FCC structure

FCC has a dense packing system - partial dislocation (only split into 2) - creates Area = Planar stacking Fault which needs a stacking fault energy

BCC - several splitting cases possible (split into 4 partial dislocations) much higher energy required for this dislocation -> higher temperature required for dislocations to move

Front 10

Slip Planes of FCC

Back 10

Front 11

BCC slip planes

Back 11

Front 12

Why Does the strength of Ni3Al increase with increasing Temperature?

Back 12

YSA (Yield Stress Anomaly) = Strength increases when Temp increases

Ni3Al

FCC: Al (cube) Ni (Face Centered) = L1_2

{111} slip plane

<110> slip direction

1. APB (Antiphase boundary) is a dislocation

cheaper to occupy a 'wrong' atom lattice space than to take the path of longer burger vectors to occupy the 'correct' lattice space.

APB increases stacking fault energy in system => dislocation hindered

2. Kear Wilsdorf Lock: when Screw dislocations cross slip (partial dislocation from (111) to (010) = immobilisation (LOCK)

- first disl slides transversely from (111) to (010)

- second disl would unlikely cross slip at exactly same position

This cross slip is thermally activated, and requires a double kink to be formed which also needs energy

Front 13

Anti Phase Boundary

Back 13

Front 14

YSA (above and below peak)

Back 14

Yield Strength Anomaly

Below peak YSA

- octahedral slip: (111) [-101]

- strength nearly independent from strain rate

- After deformation: long immobile screw dislocations

Above peak YSA

- 700 800 900 deg celsius

- cube slip: (001) [-101] (exception: [001] orientated crystals)

- strength depends on loading rate

-> same b vector but different slip systems

Kear Wilsdorf Lock: Immobilisation of [-110] screw dislocation after cross slip of partial dislocation from (111) -> (010) planes

This cross slip is thermally activated

cross slip requires a double kink -> Kear Wilsdorf Lock (also need high energy to form which means we need an increase in temp)

NB! Only screw dislocations work with this because they do not have a defined slip plane

So this Lock makes movement of dislocations more difficult

Front 15

Why does the strength of Ni3Al drop again after very high temperatures after it increases at higher temperatures?

Back 15

Diffusion sets in and the APB dissolve

Energetically cheaper when stacking errors are undone

Front 16

Why does FeAl increase in strength with increasing Temperature?

Back 16

B2

BCC (Fe - Cube, Al Body centered)

YS increases, but only at 400 deg C

YSA comes through the vacancies

Hardening by thermal vacancies

H_formation < H_movement (special case for FeAl)

These vacancies cause distortion in the lattice -> hardening

Front 17

What happens to FeAl if heated to 700 degrees celsius (lots of vacancies) and then cooled?

Back 17

The Vacancies remain in the material since the vacancy transportation energy is very high

Front 18

Dislocation Climb

Back 18

Front 19

What does the stress depend on in creep?

Back 19

sigma proprtional to sqrt(rho) = 1/L

L = dist between particles or obstacles (dislocation distance - translated)

tau = Gb/L

Obstacles (dislocations) have to be within a Grain Boundary, so L would need to be smaller than Grain Boundaries.

For high Temp Grain Boundaries: about 100 um

So L = about 10 um

rho is the density of obstacles (dislocations - translated)

Front 20

Draw a Nickel Stress Strain Curve at RT and HT

Back 20

Front 21

What elements do you need for a Nickel Base Super Alloy?

Back 21

- Co, Mo, Re, W, Ta for solid solution hardening

- Al (Al2O3), Cr (Ni2CrO4) for oxidation resistance (Al for precipitation hardening so Cr used here)

- Al and Ti for y' precipitation hardening

Attention: y is a mixed crystal, y' is an ordered crystal

Front 22

Draw the y and y' phase unit cell

Back 22

Front 23

What do you need for a MA6000?

Back 23

A Dispersoid such as Y2O3 (yttrium oxide)

Dispersoids inhibit recrystallization and Grain growth by pinning grain boundaries during subsequent heat treatments

Uncrystallized grain structures provide higher strength

Front 24

What is the difference between a presipitate and a Dispersoid?

Back 24

Precipitates:

- components formed during melting phase

- a second phase, typically present in smaller amounts

- acts as strong obstacle to slip of dislocations

- should be small and high density

Dispersoids:

- finely divided particles that can improve recrystallization resistance by pinning GB

- insoluble particles

- formed during homogenization of large parts by a solid-solution reaction of modest solubility elements with transition metals

Front 25

What do you need for precipitation hardening?

Back 25

- A supersaturated mixed crystal

Front 26

What happens at primary, secondary and tertiary creep

Back 26

1. Primary

Decreasing strain rate over time

- dislocations come from cooling after removed from the melt

- increasing dislocation density, or increasing grain size

- in ss- hardened materials: decreases because thinning of solute drag

2. Secondary

Strain rate constant

- Dislocations and GS reached equilibrium

- Dislocation Formation = Dislocation Annihilation

3. Tertiary

exponentially increases

- necking

- cracking

- voids

- all decrease CS area and increase true stress

= fracture

Front 27

How does cancelling of dislocations work

Back 27

If 2 dislocations with opposite signs are on the same sliding plane they cancel each other out

Front 28

What is important for dislocation climb and what does it depend on?

Back 28

It depends on vacancy diffusion. Because for a climb to take place vacancies need to be present.

Diffusion is proportional to the arrhenius equation: D ~ exp(-Q/(RT))

Q = activation energy (Made up of formation and movement enthalpy)

Front 29

If I increase Stress but keep T constant that means Diff coeff also remains constant. Why does the creep rate still increase

Back 29

Dislocation density is higher

Higher strain rate means more dislocation formation and recovery takes place simultaneously. So the dislocation density is higher = diffusion paths for dislocation climbing are shorter (L is shorter)

sigma ~ spt(rho)

Front 30

Draw the Creep Curve for 2 strains. What remains constant.

Back 30

Front 31

What are the contributions to overall deformation

Back 31

Front 32

What is the norton plot?

Back 32

Front 33

Influence on Stress on Vacancies and Strain

Back 33

More stress means more vacancies formed means higher strain rate

n=1

strain rate ~ Stress

Front 34

What does creep rate depend on

Back 34

External:

Temperature

Stress

Diffusion of Vacancies

Internal:

Grain Size

SS strengthening

Particle strengthening (precipitate)

Front 35

Why does Dislocation creep depend on grain size when the grain size is less than 10 um?

Back 35

Because then the grain diameter (d) has similar length to average dislocation distance (L) -> less recovery

Front 36

How does the creep test work? What do you need?

Back 36

An oven (for heating)

A Constant load (extension meter)

Heat up Temperature of oven and sample. Then apply the constant stress and measure the change in length of the Sample through the LVDT

Front 37

What does high temperature mean at what is significant about it?

Back 37

0.4 to 0.5 * Tm

Where diffusion plays a role

Front 38

Explain the constant load on the sample

Back 38

Stress does not remain constant because the cross section changes. That is why we use true stress

Front 39

How are dislocations formed

Back 39

Frank Reed Source:

-mechanism explaining generation of multiple dislocations in specific well spaced slip planes in the crystals when they are deformed

- When a crystal is deformed, in order for a slip to occur, dislocations must be generated in the material

- cold working increases the number of dislocation by Frank Reed Mechanism

Front 40

Orowan

Back 40

strain rate = b*rho*v

Front 41

How Can you determine the volume fraction of the present phases in a material?

Back 41

Lever Rule

or

Calculate from SEM or TEM images

Front 42

What is the difference between AlCu and Ni Superalloys in terms of precipitates?

Back 42

AlCu Precipitate percentage very small < 10% in AlCu

NiAl Precipitate a very high percentage - lever rule says even above 50%

Front 43

How does Yield Strength of Ni3Al and Ni-crystal depend on Temperature? Why?

Back 43

Front 44

At lower temperatures dislocations are much more prevalent in an Ni-Crystal than in Ni3Al. Why?

Back 44

At lower temperatures there is not enough energy for the APB to occur in the Ni3Al crystal structure. And the burger vector from Al to Al lattice site is too far and thus requires far to much energy.

Ni crystal is much softer and dislocations can form with less activation energy.

Front 45

What is important for dislocation climbing?

Back 45

Vacancies present and an external stress

Front 46

What is crucial for diffusion creep?

Back 46

GB size

Activation of Vacancies (Diffusion Coefficient)

D = D_o * exp(-Q/(RT))

Graph: increasing straight line for log(D) vs T

Front 47

How can the Diffusion rate be influenced

Back 47

- Temperature Obviously

- D_o can be influenced by lattice structure (packing density - BCC open better for Diffusion, FCC close packed)

- can also be influenced by enthalpy of formation and migration

Front 48

Is enthalpy formation or migration usually higher and where is the exception?

Back 48

Normally: H_vac,formation > H_vac,migration

Exception: FeAl => relatively stable vacancies

- Once vacancies form at higher temperatures they remain in the crystal when cooled = SS strengthening effect

Front 49

Why is diffusion creep relatively difficult to measure?

Back 49

Test must be carried out at high temp and for a long time.

-> structural change, coarsening = no stationary creep rate value

Grain Coarsening happens when recrystallization and recovery have occurred and then the only way now to minimize energy is by reducing the total length of the grain boundaries thereby increasing grain size = coarsening

Front 50

What are Ni-base super alloys? And how are they formed?

Back 50

2 phase alloys.

Ni3Al (y') precipitate in Ni-crystal (y) structure

Quite a high percentage of the precipitate > 50 %

1) Solution Annealing

2) Quenching

3) Age hardening at elevated temperature

Front 51

Which phase in the Ni-Al phase diagram is interesting at high temperatures? What are the advantages? And what is the Structure?

Back 51

NiAl - Structured BCC B2

Advantages

- Tm = 1638 deg C

- low density = 5.9 g/cm³

- high thermal conductivity

- good oxidation resistance

Disadvantage

- Low RT ductility

- Low fracture toughness

- low creep resistance (best solution is adding Mo through fibres 2nd best is particle strengthening)

Front 52

Draw the precipitate in the Ni solution

Back 52

Front 53

Why is there good oxidation resistance of NiAl? And what other protection layers are possible?

Back 53

Al2O3 forms on the surface

Chromium Oxide and Silicon Oxide

Front 54

How would a creep curve look like if there are small Grains and no dislocations

Back 54

primary region left out

Front 55

How do the dislocations behave in an y/y' super alloy?

Back 55

Dislocations form in y-matrix.

Path limited by y'

When enough energy given = cutting into y'

APB must be created because of the L1_2 structure