Nickel base Superalloys Flashcards
When are superalloys used
high temperature applications
i.e. in jet engines - ducts, cases, liners, turbine blades
Nickel crystal structure
FCC - Face centred cubic
Properties of Nickel
- good high and low temperature strength
- high oxidation resistance
- good corrosion resistance
What range of temperatures found in jet engines
700 - 1500 C
What happens when gas stream is converted to shaft power
This is used to power the compressor
The energy removed by expansion process which results in a decrease of temperature and pressure
What are the material properties required in a jet engine
- 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`
What is oxidation
the reaction of an alloy with oxygen in the presence of products of combustion of ‘clean’ fuels
what are clean fuels
fuels that do not contain contaminants such as sulphur, sodium and vanadium
How is good oxidation resistance achieved
via the formation of a continuous surface scale that acts as a diffusion barrier and does not spall off during thermal cycling
How is hot corrosion resistance achieved
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
What are the 2 major solutes in Ni base superalloys
Aluminium and Titanium
usually less that 10wt%
What are the two major phases in Ni base superalloys microstructure
γ (gamma)
γ’ (gamma-prime)
What microstructure is largely responsible for the elevated-temperature strength of the material and its resistance to creep formation
γ’
How does γ’ perform so well at elevated temperatures
- 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
What dictates amount of γ’ in Nickel base superalloys
the chemical composition and temperature
What happens to γ’ as temperature increases. How can one retain γ’
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
What atomic arrangement does γ have
A solid solution with a random distribution of the different species of atoms
What atomic arrangement does γ’ have
- nickel atoms are at the face-centres
- Al/Ti atoms at the cube corners
- Typical chemical formula Ni3Al
Are Ni-Al-Ti systems always stoichiometric
No, there may exist an excess of vacancies on one of the sub-lattices which leads to deviations from stoichiometry
What are solute elements that promote solid solution strengthening that partition to γ
Cr, Mo, Re, W
What are solute elements that promote precipitation hardening that partition to γ’
Al, Nb, Ta, Ti
What are solute elements that segregate to the boundaries and provide grain boundary strengthening. Why is this good
B, Zr, C
Good as means better creep strength and ductility
What does the addition of Re (Rhenium) achieve
- 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
For typical alloy compositions what does 1,2,3,4 indicate when placed after the name
1 - conventional wrought disc composition
2 - directionally solidified blade alloy
3 - single crystal alloy
4 - powder metallurgy disc material
List typical carbides and where they would be found in microstructure
M23C6 - usually forms along grain boundaries
MC - usually forms as large blocky spherical particles
Why are σ, μ and Laves phases undesirable
They are intrinsically brittle and their precipitation depletes the matrix from valuable elements which are added for different purposes
What does Re promote and what needs to be compensated
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.
Why is TCP formation bad
TCP phases (which include σ, μ and Laves phases) promote lower rupture strength and ductility
Describe dislocation movement from γ to γ’ phases
- 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
Describe the misfit parameter δ, between γ and γ’
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
What happens to γ’ when temperature elevated to 0.6T(melting)
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
What alloying elements reduces coarsening rate of γ’
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
What is the γ’ coarsening driven by
The γ’ coarsening is driven by the minimisation of the interfacial free energy per unit volume in the absence of an applied stress
What happens when γ’ coarsens in the presence of stress
The γ’ particles coalesce to form layers known as rafts
This is influenced by the size and sign if the misfit parameter δ
How can misfit parameter δ be controlled
By the chemical composition, especially Al/Ti ratio
What happens when δ > 0
- This means γ’ has a larger lattice parameter than γ
What happens when δ < 0
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’
What do rafts do with respect to creep rate
Rafts can help slow creep rate
What effects the rate of rafting
The magnitude of applied stress and the lattice misfit between γ and γ’ crystals
also the chemical composition of the alloy
Describe heat treatment of superalloys
- 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’
What carbides are formed during solid solution treatment
MC carbides only
Define flow stress
The instantaneous stress required for a material to continue to plastically deform
