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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
What are geometrically closed packed phases?
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
26
What are dislocations?
the line defects that exist within all materials and govern their plastic deformation behaviour
27
What is meant by slip system and burgers vector
the combination of the plane and direction along which the dislocations move. The lattice displacement caused by the dislocation.
28
What is anomalous yield behaviour?
Superalloys experience increased strength at elevated temperatures
29
What effect does grain size have on the strength of superalloys
the smaller the grains are, the higher the strength of the alloy (hall petch)
30
What is solid solution strengthening?
a method through which we hinder dislocation motion by using different elements with different radii compared to Ni
31
how does the gamme prime phase affect strength of superalloys?
the higher the volume fraction of the gamma prime phase, the stronger the material - especially at higher temperatures
32
What mechanisms strengthen the alloy when alloying elements are added in terms of gamma prime phase?
- Solid solution strengthening ( like in gamma phase) - increasing the gamma prime APB energy (order strengthening)
33
How does precipitate size affect strength of superalloys?
generally finer precipitates provide increased strength
34
How does the lattice misfit between gamma and gamma prime phase affect superalloys?
increases the yield strength however it drives coarsening of gamma prime precipitates therefore a balance is required
35
What is dislocation creep?
The controlling mechanism at high Temp and higher stresses Deformation primary due to the movement of dislocations aided by vacancy diffusion
36
What is diffusion creep and what is it separated into?
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
What is coble creep?
Flow of vacancies from grain boundaries under tension to grain boundaries under compression smaller grains - higher creep strain rates
38
What is Nabarro-Herring creep?
Further adds bulk diffusion of vacancies
39
What strategies can be employed to reduce the creep deformation in superalloys?
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
What are the conditions for a aircraft engine?
Operating temp - 1450*C up to 300Mpa 10000rpm Fly 3 years 5 million miles
41
What elements are affected in the engine?
reduction in Co, Cr and Mo leads to increased fractions of deleterious phases TCP's
42
What should the volume fraction of gamma prime phase in turbine blades?
70% optimum for creep performance
43
What is the order of elements for increased strength potency?
Co->Cr->Ta->w->Re
44
What is the ideal lattice misfit for turbine application?
fairly small as close to zero as possible This is to avoid the coarsening of gamma prime precipitates
45
Why are processing improvements needed for turbine blades? Which is best?
Reducing or entirely eliminating grain boundaries limits Coble creep Single crystal best as can go the longest before increased creep strain
46
What affect do cooling techniques have on turbine blades?
Cooling channels and thermal barrier coating technologies allow turbines to operate in gas streams that well exceed the alloy melting points
47
What are the 8 steps for investment casting (lost wax)?
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
What is the Bridgman process?
used with investment casting alloy elongated grains along length of blade thermal gradient used to lower mould into coolant controlling withdrawal rate
49
How is single crystal obtained?
grain selector (pig tail) cross section smaller than grain size
50
What is casting segregation?
due to slow solidification massive segregation is observed therefore homogenisation or solution heat treatments used to redistribute elements
51
What are the steps of the cast and wrought process?
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
what are the methods of TMP?
rolling cogging upsetting - open die forging close die forging
53
What are the applications of IN718?
aerospace >50% of jet engines oil and gas high performance engines (CARS) nuclear/wind etc
54
how is IN718 different to most Ni based superalloys?
large concentrations of Nb with reduced Al gamma double prime phase provides strength
55
What is the structure of IN718?
body centred tetragonal 15-25% gamma double prime precipitates gamma double prime is coherent with gamma prime matrix
56
What are the limitations of IN718?
650*C service temperature metastable - prolonged time at extreme conditions forms gamma phase - brittle, degrades mechanical properties
57
What method is used for turbine disks and why?
Powder metallurgy much lower levels of elemental segregation finer grain structures reduced defect concentrations
58
How does powder metallurgy work?
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
what are the key aspects of the rim of a turbine disks?
support blades highest temp of component undergo creep
60
What are the key aspects of the bore in a turbine disk?
Highest stresses (rotation and weight) much lower temp than rim Tensile strength key
61
What are the issues that occur in a turbine disk, both rim and bore?
crack growth fatigue performance environmental resistance low density microstructural stability
62
what is the safe life of turbine disk?
number of stress cycles to cause 1 in 750 disks to fail by occurrence of a 0.75mm crack 95% confidence
63
What is meant by a scale?
formation of a protective layer, preventing further attack of the base material
64
What are the requirements for a scale to be protective?
- thermodynamic stability - Slow growth rates - Good adhesion - Thermal expansion matching with substrate - High melting temp - Component requirements
65
What is the oxidation of pure Ni and what does it do?
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
Is nickel oxide a good protection for superalloys?
no unsuitable
67
What do we rely on for the environmental protection of superalloys?
Cr2O3 and/or Al2O3 formation depends on the composition of the alloy
68
What does group 1 consist of?
low concentrations of Cr and Al, such that the dominant oxide forming in these alloys is NiO extensive sub-scale formation
69
What does group 2 consist of?
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
Which group do the majority of superalloys fall under?
group 2
71
What does group 3 consist of?
exclusively form an alumina Al2O3 scale no other oxides observed increases the fraction of the gamma prime phase
72
When do groups 2 and 3 form?
NiO forms first chromia and alumina form underneath NiO is brittle and spalls off leaving the chromia and alumina oxide providing environmental protection
73
What are the properties of chromia?
good overall protection some issues with volatilization at high temps not suitable >900*C
74
What are the properties of Alumina oxide as a protective layer?
superior compared to chromia needs to be balanced against overall fraction of gamma prime
75
In addition to oxidation what else is chromium good for protection wise?
Corrosion
76
What are the 3 coating technologies to protect superalloys
- Diffusion coatings - Overlay coating - Thermal Barrier Coating (TBCs)
77
What are diffusion coatings and how do they work?
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
What are overlay coatings and how do they work?
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
What is MCrAIX-type?
M - combination of Ni and Co X - Mixture of reactive elements such as Y, Si, and Hf
80
What is EB-PVD?
electron beam physical vapor deposition

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