Choose Aerospace Nickel base superalloys
Nickel base super alloys for aerospace engineering are a special class of materials offering high strength at the elevated temperature limits.
Nickel based alloys are suitable for use above 500oC in the corrosive and oxidizing media.
Aircraft engine compressor, combustion chamber, turbine discs and blades that need high stress and high temperature tolerable properties, high yield strength and creeping resistance are needed in addition of fatigue resistance and hot corrosion resistance and performance up to 750oC.
Nickel superalloys have fcc structure, offer good ductility, dispersion strengthening at the elevated temperature and stable performance at the high temperature.
Role of alloying agents in Nickel base super alloys and their main functions
|Nickel||Fcc matrix, develops gamma prime phase|
|Cobalt||Solution strengthening affects gamma prime, influencing carbides|
|Chromium||Oxidation resistance and solution|
|Molybdenum||Solution strengthening affects gamma prime, affecting carbides|
|Titanium||Produces gamma prime form and develops carbides|
|Zirconium||Develops metal carbides, enhances grain boundary strength|
|Niobium||Produces gamma double prime, develops metal carbide|
|Tantalum||Solution strengthening develops metal carbide, affecting gamma prime|
|Aluminum||Develops gamma prime, offers oxidation resistance|
The super alloys are categorized as:
- Solution and carbide hardened
- Gamma prime precipitation hardened
- Gamma double prime precipitation hardened
- Oxide dispersion reinforced
Solution processing is followed in all of above alloy classes.
- Solution hardened Nickel based super alloys contain chromium, cobalt and molybdenum as prime alloying agents
- Precipitation hardened Nickel base super alloys contain titanium and aluminum as main ingredients
- Gamma double prime hardened alloys contain titanium, aluminum and columbium (or niobium) as main alloying elements
Heat and thermomechanical treatment
The objectives of heat treatment are:
- Offer precipitation hardening
- To obtain required precipitation carbide
- To remove the embrittling effect of mechanical treatment in wrought alloys by recrystallization and grain development
- To develop required grain size by grain growth and recrystallization and grain escalation with mechanical deformation hence known as thermomechanical treatment.
Grown grain size offers
Enhanced creeping resistance, prevents creeping extension to damage, minimize short term strength and damage
Mechanical processing of wrought super alloys
- To shape the material by forging, rolling etc
- To attain uniform size of microstructures by eliminating segregation of alloying inclusions after casting and distribution of MC carbides
Super alloys are induction melted in vacuum and cast into ingot in vacuum. Remelting decreases the extent of segregation. Vacuum arc remelting and electrostag remelting methods are used.
Aerospace Applications of Nickel base super alloys
Turbine Blades: A key use of nickel based super alloys is in the production of aerospace engine turbine blades in the zones where the temperature rises above 400oC.
Turbine Discs: The blades are connected with disc in order to connect with the turbine shaft. The characteristics needed for aeroengine discs are unlike to that required for turbine because material works at lower temperature. The discs should prevent damage by fatigue. They are cast and then forged into shape. They are polycrystalline.
Turbochargers: An internal combustion engine normally employs a stoichmetric ratio of air to fuel. It is an equipment to force air into engine, permitting corresponding larger magnitude of fuel to be burnt in every stroke. It improves the power output of engine. The turbocharger comprises of two parts- a turbine that is operated by exhaust gases from engine. It spins at the rate of 100 to 150,000 rotations per minute. As the turbocharger is empowered by exhaust gases, it experiences extremely hot temperatures and needs very high oxidation resistance and large strength.
Melt Working: The super alloys comprise of reactive elements like aluminum and titanium. Therefore alloy melting is essential in vacuum with the included benefit of removing the harmful trace element through evaporation. Vacuum induction melting is normally followed as the inductive stirring improves evenness and offers to show more of the liquid to the melt vacuum interface. It results into better removal of unwanted gases and volatile contaminants.
Several alloys are then vacuum arc remelted to obtain larger purity and improved solidification microstructure.
Nickel super alloy blades are normally constructed by investment casting procedure. forming a wax model surrounding which a ceramic is poured to produce the mould. The wax is eradicated from the solid ceramic and molten melt poured into fill the mould. The practical procedure is more complex due to an intricate shape of the blade, its quenching channels and other characteristics.