Chapter 3: Superalloys Flashcards
What are the requirements inside the turbine engines
Mechanical forces – Creep – Fatigue – Thermomechanical fatigue • High temperature environment – Oxidation – Hot corrosion (due to contaminaAon)
Properties of Superalloys
Maintains high strength at high temperature
• Creep resistant a thigh temperatures
• Oxidation resistant at high temperatures
List three groups of superalloy
Ni-based
Ni-Fe based
Co based
Requirements of the Jet engine, gas turbine and rocket
Need high strength creep resistance
Oxidation and corrosion resistance
Fatigue resistance
Operate up to 18 hours continuously (nonstop) in flight at speed of 1000 km per hour
Explain why FCC metal is chosen as the base for the
development of superalloys
Superalloy components are designed to operate at elevated temperatures and are subjected to high stress.
FCC alloys have a lower diffusion rate
- Dislocation split and multiply, restricting further movement, thus strengthening the alloy
- No DBTT
- No Allotropic transformation
- Can dissolve more solutes for solid solution strengthening and precipitation strengthening
List the behaviours of the superalloy
Density is much greater than steel
Difficult to form or machine
The product can be made to final shape by investment casting
complex ingredients
Expensive
Limited to small parts where is not the determining factor
Describe how creep strength of superalloy is improved
Within grain
FCC solid solution
Fatigue/creep
Solution Strengthening
γ’ (Ni3Ti, Ni3Al)
Fatigue/Creep
Precipitation Hardening
Other precipitates
Fatigue
Precipitation Hardening
Carbides
Creep
Carbide Dispersion
Grain boundary
Carbides
Creep
Carbide Dispersion
Grain size
Fine
Fatigue
Grain size control
Coarse
Creep
Grain size control
What is a solid solution strengthening
Base metal dissolves other atoms in solid solution
In solid solution strengthening the amount of strengthening depends on
Amount of solute and size difference of the atoms
What is Dispersion hardening
Particles are added by mixing; no heat treatment
In Dispersion hardening the amount of strengthening depends on
-Dispersed particles must be stronger than the matrix
• Strength is not Time-dependent
• Strength is not temperature dependent
Strength obtained from distinct second-phase particles
dispersed throughout the material
How strength is obtained in Precipitation hardening
Strength is obtained from a non-equilibrium structure
produced by a three-step heat treatment
Discuss the formation and usefulness of gamma prime
(γ’) in superalloy.
- γ’ precipitates (Ni3Al or Ni3Ti compounds) formed when Al and Ti added to Ni by precipitation hardening heat treatment
- γ’ has the same structure (shape and size) as Ni matrix,
- This coherency between γ (=Nickel) and γ’ is maintained
- γ’ is very stable, difficult to dissolve and has high creep resistance.
How to obtain strength in Grain-size refinement
grain-size refinement can be used to increase strength
except at elevated temperatures, where failure is by grain-boundary diffusion-controlled creep mechanism
What is the function of the Helix
Helix to select ONLY
one (1) elongated grain
to grow into a single crystal
With sketches, describe how a single crystal turbine blade is
fabricated
Done by unidirectional solidification
The mould containing liquid metal progressively withdrawn from the furnace at a predetermined rate
A starter block give raise to the chill zone which promotes the solidification of columnar grains
A helix (spiral) attached between the starter block and turbine blade ensure that only a single crystal grows within the blade
Blades solidify progressively upwards from the chill as mould is drawn down into water-cooled lower chamber
What is a Single Crystal?
A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample with no grain boundaries
Advantages of Single Crystals
Relatively high strength at high temperatures
• Low creep at high temperature
• Anisotropic
• Improved corrosion resistance
Alloying produce strong, stable microstructure at high temperature using
Solid solution, dispersion, precipitation strengthening.
Compare carbide dispersion strengthening and
precipitation hardening in superalloy
Carbide Dispersion Hardening
The strength obtained from distinct second phase particles
dispersed throughout the materials
Dispersed particles must be stronger than the matrix
Add a little C and form TiC, Cr23C6, Mo6C, W6C etc
Network of fine, stable carbide particles
Interfere with dislocation movement
Strengthen grain boundaries
Prevent grain boundary sliding
Precipitation Hardening
Strength is obtained from a non-equilibrium structure
produced by a three-step heat treatment
Ni, Ni-Fe based containing Al & Ti
Especially compound of Ni with Al, Ti
Coherent precipitate gamma prime during ageing
FCC structure coherent with the matrix
Very stable and difficult to dissolve
Increase strength and resistance to creep at high temperature
Discuss the formation and usefulness of gamma prime
(γ’) in superalloy
-γ’ precipitates (Ni3Al or Ni3Ti compounds) formed when Al and Ti
added to Ni by precipitation hardening heat treatment
γ’ has same structure (shape and size) as Ni matrix,
This coherency between γ (=Nickel) and γ’ is maintained
γ’ is very stable, difficult to dissolve and high creep resistance.
