Nickel base Superalloys

Question 1

When are superalloys used

Answer 1

high temperature applications

i.e. in jet engines - ducts, cases, liners, turbine blades

Question 2

Nickel crystal structure

Answer 2

FCC - Face centred cubic

Question 3

Properties of Nickel

Answer 3

• good high and low temperature strength
• high oxidation resistance
• good corrosion resistance

Question 4

What range of temperatures found in jet engines

Answer 4

700 - 1500 C

Question 5

What happens when gas stream is converted to shaft power

Answer 5

This is used to power the compressor

The energy removed by expansion process which results in a decrease of temperature and pressure

Question 6

What are the material properties required in a jet engine

Answer 6

• good strength at high temperatures
• good ductility to tolerate creep deformation and resist low-cycle fatigue deformation
• good oxidation resistance
• resist surface degradation by hot corrosion
• coatings also used`

Question 7

What is oxidation

Answer 7

the reaction of an alloy with oxygen in the presence of products of combustion of ‘clean’ fuels

Question 8

what are clean fuels

Answer 8

fuels that do not contain contaminants such as sulphur, sodium and vanadium

Question 9

How is good oxidation resistance achieved

Answer 9

via the formation of a continuous surface scale that acts as a diffusion barrier and does not spall off during thermal cycling

Question 10

How is hot corrosion resistance achieved

Answer 10

requires resistance to a combination of oxidation and reaction with sulphur, sodium, vanadium contained in the fuel or ingested with the inlet air.

Achieved using Cr content in Ni base superalloys

Question 11

What are the 2 major solutes in Ni base superalloys

Answer 11

Aluminium and Titanium

usually less that 10wt%

Question 12

What are the two major phases in Ni base superalloys microstructure

Answer 12

γ (gamma)

γ’ (gamma-prime)

Question 13

What microstructure is largely responsible for the elevated-temperature strength of the material and its resistance to creep formation

Answer 13

γ’

Question 14

How does γ’ perform so well at elevated temperatures

Answer 14

• The flow stress of γ’ increases with temperature and exhibits a maximum at 900C.
• By interaction with dislocations γ’ contributes to strengthening via APB (anti-phase boundary) formation
• This also prevents severe embrittlement

Question 15

What dictates amount of γ’ in Nickel base superalloys

Answer 15

the chemical composition and temperature

Question 16

What happens to γ’ as temperature increases. How can one retain γ’

Answer 16

fraction of γ’ decreases as temperature increased. Thus γ’ can be dissolved at a sufficiently high temperature (solution treatment), followed by ageing at a low temperature in order to generate a uniform and fine dispersion of strengthening precipitates

Question 17

What atomic arrangement does γ have

Answer 17

A solid solution with a random distribution of the different species of atoms

Question 18

What atomic arrangement does γ’ have

Answer 18

• nickel atoms are at the face-centres
• Al/Ti atoms at the cube corners
• Typical chemical formula Ni3Al

Question 19

Are Ni-Al-Ti systems always stoichiometric

Answer 19

No, there may exist an excess of vacancies on one of the sub-lattices which leads to deviations from stoichiometry

Question 20

What are solute elements that promote solid solution strengthening that partition to γ

Answer 20

Cr, Mo, Re, W

Question 21

What are solute elements that promote precipitation hardening that partition to γ’

Answer 21

Al, Nb, Ta, Ti

Question 22

What are solute elements that segregate to the boundaries and provide grain boundary strengthening. Why is this good

Answer 22

B, Zr, C

Good as means better creep strength and ductility

Question 23

What does the addition of Re (Rhenium) achieve

Answer 23

• improvement in the creep strength (due to rafting)
• occurs as clusters in the γ phase
• claimed that it reduces overall diffusion rate in Ni base superalloys

Question 24

For typical alloy compositions what does 1,2,3,4 indicate when placed after the name

Answer 24

1 - conventional wrought disc composition
2 - directionally solidified blade alloy
3 - single crystal alloy
4 - powder metallurgy disc material

Question 25

List typical carbides and where they would be found in microstructure

Answer 25

M23C6 - usually forms along grain boundaries

MC - usually forms as large blocky spherical particles

Question 26

Why are σ, μ and Laves phases undesirable

Answer 26

They are intrinsically brittle and their precipitation depletes the matrix from valuable elements which are added for different purposes

Question 27

What does Re promote and what needs to be compensated

Answer 27

promotes TCP (Topologically closed packed) formation

therefore alloys with Re must reduce levels of Cr, Co, W or Mo to compensate.
Although Cr is good for oxidation resistance, this can be achieved by coating the blades.

Question 28

Why is TCP formation bad

Answer 28

TCP phases (which include σ, μ and Laves phases) promote lower rupture strength and ductility

Question 29

Describe dislocation movement from γ to γ’ phases

Answer 29

• γ and γ’ both have cubic lattices with similar lattice parameters
• this means they have a special cube-cube relationship
• γ’ is coherent with the γ when the precipitate size is small
• dislocation moving in the γ would have to cut through the ordered γ’
• the order interferes with dislocation motion and hence strengthens the alloy

Question 30

Describe the misfit parameter δ, between γ and γ’

Answer 30

The misfit parameter is low, this means low interfacial energy between the two phases and translate to a stable microstructure.
This then translates to very good high temperature applications

Question 31

What happens to γ’ when temperature elevated to 0.6T(melting)

Answer 31

The γ’ phase coarsens allowing dislocations to bypass the γ’ precipitates.
The rate of coarsening is controlled through changes in parameter ‘B’
B is proportional to DγCe

D - Diffusion coefficient
γ - surface tension of γ and γ’
Ce - equilibrium concentration at temp

Question 32

What alloying elements reduces coarsening rate of γ’

Answer 32

Cr, Co, Mo, Mo + W - due to reduction in coherency strains

Nb - has low Ce and D values which are more influential then reducing coherency strains

Question 33

What is the γ’ coarsening driven by

Answer 33

The γ’ coarsening is driven by the minimisation of the interfacial free energy per unit volume in the absence of an applied stress

Question 34

What happens when γ’ coarsens in the presence of stress

Answer 34

The γ’ particles coalesce to form layers known as rafts

This is influenced by the size and sign if the misfit parameter δ

Question 35

How can misfit parameter δ be controlled

Answer 35

By the chemical composition, especially Al/Ti ratio

Question 36

What happens when δ > 0

Answer 36

• This means γ’ has a larger lattice parameter than γ

Question 37

What happens when δ < 0

Answer 37

The means rafts of γ’ form, which are essentially layers of γ’ phase in a direction normal to the applied stress.

• γ will therefore be in compression in the vicinity of the interface with γ’ and there will be compensating tensile stresses in the γ’

Question 38

What do rafts do with respect to creep rate

Answer 38

Rafts can help slow creep rate

Question 39

What effects the rate of rafting

Answer 39

The magnitude of applied stress and the lattice misfit between γ and γ’ crystals
also the chemical composition of the alloy

Question 40

Describe heat treatment of superalloys

Answer 40

• solution treatment:
  raised to high temperature where all γ’ dissolve
• air cooled
• second solution treatment occurs
• high temp ageing treatment:
  leads to coarser precipitates of γ’
• low temp ageing treatment:
  leads to a finer, secondary dispersion of γ’
• this has a net result of a bimodal distribution of γ’
• Time and temperature of these treatments determine amount and grain size of γ’

Question 41

What carbides are formed during solid solution treatment

Answer 41

MC carbides only

Question 42

Define flow stress

Answer 42

The instantaneous stress required for a material to continue to plastically deform