Nickel superalloys Flashcards
What is a superalloy?
an alloy developed for elevated temperature service usually based on Ni and Co, where relatively sever mechanical stressing is encountered
what 3 key factors define high temperature material?
- high operating temperature (near melting point) Thomologous > 0.6
- Resistance to mechanical degradation over long periods of exposure at high temps
- Tolerance of severe operating environments
.
How do you calculate the homolgous temperature?
Toperating/Tmelting
What are some applications of superalloys?
Industrial gas turbines/steam turbines
oil and gas: piping
exhaust assemblies
heat exchangers
rocket and jet engines
What are some key alloy requirements?
Mechanical properties
Oxidation and Corrosion resistance
Thermal Expansion
Density
Cost
Why is nickel suitable for high temperature?
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
Define Creep
The time-dependent plastic deformation of materials under constant load/stress
(that is below the yield stress of the
material)
When is creep particularily important?
0.4-0.5Tmelt
How is a creep test carried out?
Subjecting a specimen to a constant load @ constant temp
Deformation (strain) plot against time
What is primary creep/strain?
- Region of decreasing strain rate
- Creep resistance increases due to deformation (strain hardening)
- Low T and stress
What is the secondary creep/Steady-state creep?
- 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
What is Tertiary creep and Fracture?
- Accelerated damage accumulation
- Extensive cracking and cavitation
- Observed at high T and stress
Why is aluminium always added to superalloy?
to assist the formation of the strengthening precipitates gamma prime phase
Why is Chromium added to superalloys?
accelerates the formation of gamma prime
improves the environmental resistance of the material
Why is titanium added to some superalloys?
Strengthens the gamma prime phase
gamma prime phase must be already made
What is the gamma phase
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
What is the gamma prime phase?
Ordered structure NOT FCC
Looks FCC but isn’t
Al take up corners Ni take up faces
Not lattice site is equivalent
Are the gamma and gamma prime phases coherent?
Coherent
gamma a=3.58A
gamma prime a=3.6A
What are the solid solution strengthener’s and what do they do?
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
What are the grain boundary Strengtheners?
C, B, Zr
What elements help environmental resistance?
Cr and Al
What elements will occupy the Al sites in gamma prime phase?
Al, Ti, Ta, Nb, Hf
What do C, B, and Zr do to polycrystalline superalloys?
Increase the grain boundary strength by:
- Partitioning to the grain boundary
- Forming carbides/borides at the boundary
What are Topologically Closed Packed (TCP) phases?
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
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
What are dislocations?
the line defects that exist within all materials and govern their plastic deformation behaviour
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.
What is anomalous yield behaviour?
Superalloys experience increased strength at elevated temperatures
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)
What is solid solution strengthening?
a method through which we hinder dislocation motion by using different elements with different radii compared to Ni
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
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)
How does precipitate size affect strength of superalloys?
generally finer precipitates provide increased strength
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
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
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
What is coble creep?
Flow of vacancies from grain boundaries under tension to grain boundaries under compression
smaller grains - higher creep strain rates
What is Nabarro-Herring creep?
Further adds bulk diffusion of vacancies
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
What are the conditions for a aircraft engine?
Operating temp - 1450*C
up to 300Mpa
10000rpm
Fly 3 years
5 million miles
What elements are affected in the engine?
reduction in Co, Cr and Mo
leads to increased fractions of deleterious phases TCP’s
What should the volume fraction of gamma prime phase in turbine blades?
70%
optimum for creep performance
What is the order of elements for increased strength potency?
Co->Cr->Ta->w->Re
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
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
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
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
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
How is single crystal obtained?
grain selector (pig tail)
cross section smaller than grain size
What is casting segregation?
due to slow solidification massive segregation is observed
therefore
homogenisation or solution heat treatments used to redistribute elements
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
what are the methods of TMP?
rolling
cogging
upsetting - open die forging
close die forging
What are the applications of IN718?
aerospace >50% of jet engines
oil and gas
high performance engines (CARS)
nuclear/wind etc
how is IN718 different to most Ni based superalloys?
large concentrations of Nb with reduced Al
gamma double prime phase provides strength
What is the structure of IN718?
body centred tetragonal
15-25% gamma double prime precipitates
gamma double prime is coherent with gamma prime matrix
What are the limitations of IN718?
650*C service temperature
metastable - prolonged time at extreme conditions
forms gamma phase - brittle, degrades mechanical properties
What method is used for turbine disks and why?
Powder metallurgy
much lower levels of elemental segregation
finer grain structures
reduced defect concentrations
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
what are the key aspects of the rim of a turbine disks?
support blades
highest temp of component
undergo creep
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
What are the issues that occur in a turbine disk, both rim and bore?
crack growth
fatigue performance
environmental resistance
low density
microstructural stability
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
What is meant by a scale?
formation of a protective layer, preventing further attack of the base material
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
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
Is nickel oxide a good protection for superalloys?
no
unsuitable
What do we rely on for the environmental protection of superalloys?
Cr2O3 and/or Al2O3
formation depends on the composition of the alloy
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
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
Which group do the majority of superalloys fall under?
group 2
What does group 3 consist of?
exclusively form an alumina Al2O3 scale
no other oxides observed
increases the fraction of the gamma prime phase
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
What are the properties of chromia?
good overall protection
some issues with volatilization at high temps
not suitable >900*C
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
In addition to oxidation what else is chromium good for protection wise?
Corrosion
What are the 3 coating technologies to protect superalloys
- Diffusion coatings
- Overlay coating
- Thermal Barrier Coating (TBCs)
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
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
What is MCrAIX-type?
M - combination of Ni and Co
X - Mixture of reactive elements such as Y, Si, and Hf
What is EB-PVD?
electron beam physical vapor deposition
