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AerospaceMetals > Nickel superalloys > Flashcards

Nickel superalloys Flashcards

1
Q

What is a superalloy?

A

an alloy developed for elevated temperature service usually based on Ni and Co, where relatively sever mechanical stressing is encountered

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2
Q

what 3 key factors define high temperature material?

A
  1. high operating temperature (near melting point) Thomologous > 0.6
  2. Resistance to mechanical degradation over long periods of exposure at high temps
  3. Tolerance of severe operating environments
    .
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3
Q

How do you calculate the homolgous temperature?

A

Toperating/Tmelting

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4
Q

What are some applications of superalloys?

A

Industrial gas turbines/steam turbines
oil and gas: piping
exhaust assemblies
heat exchangers
rocket and jet engines

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5
Q

What are some key alloy requirements?

A

Mechanical properties
Oxidation and Corrosion resistance
Thermal Expansion
Density
Cost

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6
Q

Why is nickel suitable for high temperature?

A

FCC crystal structure - tough and ductile

stable FCC structure from room temp till Tmelt
- No volumetric changes or dramatic
deformation mechanism changes

High tolerance for alloying/good solvent

low rates of self diffusion

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7
Q

Define Creep

A

The time-dependent plastic deformation of materials under constant load/stress
(that is below the yield stress of the
material)

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8
Q

When is creep particularily important?

A

0.4-0.5Tmelt

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9
Q

How is a creep test carried out?

A

Subjecting a specimen to a constant load @ constant temp
Deformation (strain) plot against time

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10
Q

What is primary creep/strain?

A
  • Region of decreasing strain rate
  • Creep resistance increases due to deformation (strain hardening)
  • Low T and stress
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11
Q

What is the secondary creep/Steady-state creep?

A
  • Constant strain rate due to balance between strain hardening and recovery
  • Average strain rate in this section corresponds to the minimum creep rate for the material
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12
Q

What is Tertiary creep and Fracture?

A
  • Accelerated damage accumulation
  • Extensive cracking and cavitation
  • Observed at high T and stress
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13
Q

Why is aluminium always added to superalloy?

A

to assist the formation of the strengthening precipitates gamma prime phase

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14
Q

Why is Chromium added to superalloys?

A

accelerates the formation of gamma prime
improves the environmental resistance of the material

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15
Q

Why is titanium added to some superalloys?

A

Strengthens the gamma prime phase
gamma prime phase must be already made

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16
Q

What is the gamma phase

A

Nickel solid solution (A1)
FCC crystal structure
Stable up to Tmelt
Random solid solution of Ni,Co,Cr,Fe,Mo,W,Re,etc
Each lattice site is equivalent

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17
Q

What is the gamma prime phase?

A

Ordered structure NOT FCC

Looks FCC but isn’t

Al take up corners Ni take up faces

Not lattice site is equivalent

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18
Q

Are the gamma and gamma prime phases coherent?

A

Coherent
gamma a=3.58A
gamma prime a=3.6A

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19
Q

What are the solid solution strengthener’s and what do they do?

A

Co, Cr, No, W, Fe and Re
- preferentially partition to the gamma phase
or
- if present in the gamma prime phase, will occupy Ni sites

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20
Q

What are the grain boundary Strengtheners?

A

C, B, Zr

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21
Q

What elements help environmental resistance?

A

Cr and Al

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22
Q

What elements will occupy the Al sites in gamma prime phase?

A

Al, Ti, Ta, Nb, Hf

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23
Q

What do C, B, and Zr do to polycrystalline superalloys?

A

Increase the grain boundary strength by:
- Partitioning to the grain boundary
- Forming carbides/borides at the boundary

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24
Q

What are Topologically Closed Packed (TCP) phases?

A

complicated crystal structures, kinetically sluggish (i.e. they appear during service)
- Degrade mechanical properties - crack propogation
- Deplete Cr from matrix - reduce environmental resistance
σ phase adopts a BCT structure with general formula: (Cr, Mo)x(Ni,Co)y , where x and y
are approximately equal
* µ phase, Rhombohedral with general formula (Ni,Co)7(Mo,W)6

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25
Q

What are geometrically closed packed phases?

A

Precipitate at the expense of gamma prime
δ or η phase
They can be used to control the grain boundaries
during thermomechanical processing operations!
Small discreet particles in a blocky morphology
* BUT if excessive needle like precipitation, detrimental
to mechanical properties

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26
Q

What are dislocations?

A

the line defects that exist within all materials and govern their plastic deformation behaviour

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27
Q

What is meant by slip system and burgers vector

A

the combination of the plane and
direction along which the dislocations
move.

The lattice displacement
caused by the dislocation.

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28
Q

What is anomalous yield behaviour?

A

Superalloys experience increased strength at elevated temperatures

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29
Q

What effect does grain size have on the strength of superalloys

A

the smaller the grains are, the higher the strength of the alloy (hall petch)

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30
Q

What is solid solution strengthening?

A

a method through which we hinder dislocation motion by using different elements with different radii compared to Ni

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31
Q

how does the gamme prime phase affect strength of superalloys?

A

the higher the volume fraction of the gamma prime phase, the stronger the material
- especially at higher temperatures

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32
Q

What mechanisms strengthen the alloy when alloying elements are added in terms of gamma prime phase?

A
  • Solid solution strengthening ( like in gamma phase)
  • increasing the gamma prime APB energy (order strengthening)
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33
Q

How does precipitate size affect strength of superalloys?

A

generally finer precipitates provide increased strength

34
Q

How does the lattice misfit between gamma and gamma prime phase affect superalloys?

A

increases the yield strength
however
it drives coarsening of gamma prime precipitates

therefore a balance is required

35
Q

What is dislocation creep?

A

The controlling mechanism at high Temp and higher stresses

Deformation primary due to the movement of dislocations aided by vacancy diffusion

36
Q

What is diffusion creep and what is it separated into?

A

mechanism that can occur at different temps and stresses nut usually at lower stresses where dislocation glide is less favourable

separated into
coble creep
Nabarro-Herring creep

37
Q

What is coble creep?

A

Flow of vacancies from grain boundaries under tension to grain boundaries under compression

smaller grains - higher creep strain rates

38
Q

What is Nabarro-Herring creep?

A

Further adds bulk diffusion of vacancies

39
Q

What strategies can be employed to reduce the creep deformation in superalloys?

A

strengthening the matrix through solid solution

Precipitate strengthening

Most strengthening mechanisms that improve yield strength (not grain refinement)

In extreme environments we can eliminate grain boundaries

40
Q

What are the conditions for a aircraft engine?

A

Operating temp - 1450*C
up to 300Mpa
10000rpm
Fly 3 years
5 million miles

41
Q

What elements are affected in the engine?

A

reduction in Co, Cr and Mo
leads to increased fractions of deleterious phases TCP’s

42
Q

What should the volume fraction of gamma prime phase in turbine blades?

A

70%
optimum for creep performance

43
Q

What is the order of elements for increased strength potency?

A

Co->Cr->Ta->w->Re

44
Q

What is the ideal lattice misfit for turbine application?

A

fairly small
as close to zero as possible
This is to avoid the coarsening of gamma prime precipitates

45
Q

Why are processing improvements needed for turbine blades? Which is best?

A

Reducing or entirely eliminating grain boundaries
limits Coble creep

Single crystal best as can go the longest before increased creep strain

46
Q

What affect do cooling techniques have on turbine blades?

A

Cooling channels and thermal barrier coating technologies allow turbines to operate in gas streams that well exceed the alloy melting points

47
Q

What are the 8 steps for investment casting (lost wax)?

A

assembly - wax model

investing - ceramic shell created by dipping into ceramic slurries

stuccoing - larger ceramic particles added

dewaxing - wax melted in furnace low temp

firing - baked to strengthen

pouring - superalloy poured under vacuum into mould

knockout - solidified and mould removed

finishing - machining

48
Q

What is the Bridgman process?

A

used with investment casting alloy elongated grains along length of blade

thermal gradient used to lower mould into coolant controlling withdrawal rate

49
Q

How is single crystal obtained?

A

grain selector (pig tail)
cross section smaller than grain size

50
Q

What is casting segregation?

A

due to slow solidification massive segregation is observed
therefore
homogenisation or solution heat treatments used to redistribute elements

51
Q

What are the steps of the cast and wrought process?

A

vacuum induction melt - melting ingredients

electro-slag refine - reduce elemental segregation and defects (refine metal)

vacuum arc remelt - control over solidification rate

anneal - remove remnant segregation

upset and draw - thermomechanical processing (TMP)

forge

52
Q

what are the methods of TMP?

A

rolling
cogging
upsetting - open die forging
close die forging

53
Q

What are the applications of IN718?

A

aerospace >50% of jet engines
oil and gas
high performance engines (CARS)
nuclear/wind etc

54
Q

how is IN718 different to most Ni based superalloys?

A

large concentrations of Nb with reduced Al
gamma double prime phase provides strength

55
Q

What is the structure of IN718?

A

body centred tetragonal
15-25% gamma double prime precipitates
gamma double prime is coherent with gamma prime matrix

56
Q

What are the limitations of IN718?

A

650*C service temperature
metastable - prolonged time at extreme conditions
forms gamma phase - brittle, degrades mechanical properties

57
Q

What method is used for turbine disks and why?

A

Powder metallurgy

much lower levels of elemental segregation

finer grain structures

reduced defect concentrations

58
Q

How does powder metallurgy work?

A

Vacuum induction melt - melt ingredients

remelt and atomise - argon gas, disintegrate metal into spherical particles

sieve - desired size

can - stainless steel can

Degas and seal - under vacuum

hot isostatic press - consolidate powder high pressure and temp

extrude

forge

59
Q

what are the key aspects of the rim of a turbine disks?

A

support blades

highest temp of component

undergo creep

60
Q

What are the key aspects of the bore in a turbine disk?

A

Highest stresses (rotation and weight)

much lower temp than rim

Tensile strength key

61
Q

What are the issues that occur in a turbine disk, both rim and bore?

A

crack growth
fatigue performance
environmental resistance
low density
microstructural stability

62
Q

what is the safe life of turbine disk?

A

number of stress cycles to cause 1 in 750 disks to fail by occurrence of a 0.75mm crack 95% confidence

63
Q

What is meant by a scale?

A

formation of a protective layer, preventing further attack of the base material

64
Q

What are the requirements for a scale to be protective?

A
  • thermodynamic stability
  • Slow growth rates
  • Good adhesion
  • Thermal expansion matching with substrate
  • High melting temp
  • Component requirements
65
Q

What is the oxidation of pure Ni and what does it do?

A

Nickel oxide
p-type semiconductor oxide
oxidation proceeds at the oxide/gas interface as a function of Ni-cation diffusion outwards

i.e. the growth of the oxide are very fast when concentration of O2 is high

66
Q

Is nickel oxide a good protection for superalloys?

A

no
unsuitable

67
Q

What do we rely on for the environmental protection of superalloys?

A

Cr2O3 and/or Al2O3
formation depends on the composition of the alloy

68
Q

What does group 1 consist of?

A

low concentrations of Cr and Al, such that the dominant oxide forming in these alloys is NiO
extensive sub-scale formation

69
Q

What does group 2 consist of?

A

formation of a continuous protective Cr2O3 scale with characteristic Al2O3 fingers extending into the bulk

NiO and spinels form at early stages until protective chromia scale has forms

70
Q

Which group do the majority of superalloys fall under?

A

group 2

71
Q

What does group 3 consist of?

A

exclusively form an alumina Al2O3 scale
no other oxides observed
increases the fraction of the gamma prime phase

72
Q

When do groups 2 and 3 form?

A

NiO forms first
chromia and alumina form underneath
NiO is brittle and spalls off leaving the chromia and alumina oxide providing environmental protection

73
Q

What are the properties of chromia?

A

good overall protection
some issues with volatilization at high temps
not suitable >900*C

74
Q

What are the properties of Alumina oxide as a protective layer?

A

superior compared to chromia
needs to be balanced against overall fraction of gamma prime

75
Q

In addition to oxidation what else is chromium good for protection wise?

A

Corrosion

76
Q

What are the 3 coating technologies to protect superalloys

A
  • Diffusion coatings
  • Overlay coating
  • Thermal Barrier Coating (TBCs)
77
Q

What are diffusion coatings and how do they work?

A

Simplest and most widely used
Al deposited on the surface by chemical vapor deposition (aluminization)
suitable heat treatments allow interdiffusion and promotes adhesion
This leads to Al2O3 on surface
Layer of Pt corrosion resistance

78
Q

What are overlay coatings and how do they work?

A

Used when additional protection needed than aluminization
MCrAIX-type
Deposited using air or vacuum plasma spraying or EB_PVD
Performance does not depend on substrate

79
Q

What is MCrAIX-type?

A

M - combination of Ni and Co
X - Mixture of reactive elements such as Y, Si, and Hf

80
Q

What is EB-PVD?

A

electron beam physical vapor deposition

AerospaceMetals (3 decks)

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