High Performance Alloys Flashcards
What is the three phases occurring within Nickel Superalloys? How do the phase contribute to its strengthening
properties?
γ (gamma). This is a continuous matrix of FCC austenite. It is solid solution strengthened by alloying elements such as, Cr, Mo, Ti, W, Co, Fe and Al. The slow diffusing elements such as Mo and W are
good for enhancing high temperature creep resistance.
γ’ (gamma prime): This is the major precipitable phase (Ni3Al).
A coherent precipitate (there is only about 0.1% mismatch
between γ and γ’). Coherency is maintained by tetragonal distortion.
Carbides: There are various types, but mainly “M”23C6 and “M”C. (where M represent the metal).
“M”C Carbides (Monocarbides): “M” is usually Ti, Ta, Nb or W. They are very stable and form just below solidification temperature. They
restrict grain growth during solution treatment. Blocky, needle like form, distributed well at grain boundaries.
“M”23C6 Carbides: “M” is usually Cr but can be Fe, W, Mo or Co. These carbides form at lower temperature heat treatments, and in
service, at temperatures in the range 760 – 980°C. Finer particles at grain boundaries and at γ channels.
“M” 6C Carbides: Form at temperatures in the range 815 – 980 oC, they are similar to “M”23C6 and form when Mo and W contents are high. Eg. in M252 alloy, which contains 6 – 8wt% Mo or W; “M” 6C and “M”23C6 form on grain boundaries. Fine and dispersive “M”6C particles can pin down dislocation movement, prevent γ’ phase to be sheared and thus strengthen the superalloy.
What is the metallographic structure of Cobalt Superalloys and how does it contribute to its properties?
Wrought Co superalloys contain ~40% Co and increased Ni.
Cobalt based alloys are not as high strength as Nickel based but they maintain their properties to higher temperatures.
There is a γ matrix and carbide precipitates.
The strength therefore comes from solid solution strengthening and carbide precipitation.
The carbides are particularly important as there is no γ’ precipitate, hence the carbon content is relatively high at ~0.1 - 1wt%C.
Co have a lower strength at intermediate temperatures due to the lack of coherent γ’ precipitation which all Ni superalloys have.
Co based only has carbides and solid solution strengthening, hence it cannot compete in the high stress applications.
It is used for low stress, high temperature and long life applications.
How does standard casting, directionally solidified and single crystal casting differ from one another?
Traditional casting: uses a ceramic mould, and solidification begins at the mould walls and the grain structure is formed by many nucleation sites starting grain growth.
The growth continues until growing grains impinge on each other.
Directional casting: a mould is heated and supported on a water-cooled chill plate. The mould is then slowly lowered out of the furnace. Grains nucleate at the chilled end and grow upward, eliminating traverse boundaries. The resulting casting is then stronger in the direction of the centrifugal forces.
Single crystal: The mould is similar to directional casting but has a constriction (corkscrew or pig’s tail) which is designed to let only the most favourably oriented grain grow through. All others are intercepted at the walls of the constriction. Again the mould is slowly lowered to prevent other grains nucleating at the mould walls.
The lack of grain boundaries make them more resistant to CREEP and THERMAL SHOCK.
What is the casting process used for?
The casting process can be applied in the fabrication of turbine blades.
What is the hot corrosion effect? Describe the process.
The formation of condensed phases on the surface of alloys under high temperatures.
Hinders the passivation layer on the alloy
During combustion in the gas turbine, sulphur from the fuel reacts with sodium chloride from ingested air at elevated temperatures to form sodium sulphate.
The sodium sulphate then deposits on the hot-section components, such as nozzle guide vanes and rotor blades, resulting in accelerated oxidation (or sulphidation) attack.
This is commonly referred to as “hot corrosion”.
Condensed alkali metal salts (e.g. NA2SO4) are thought to be primarily responsible.
Sources could be ingestion of sea salt in the marine environment or combustion of fuels.
Explain diffusion protection coatings and how they operate. Provide an example.
A reservoir with a very high concentration of the element that will enrich the material is created at the surface either in the gas phase or in a solid/liquid state.
For example, with Nb, Mo or W, silicon is deposited onto the surface and diffused into the substrate to form a silicide e.g. MoSi2.
This will then oxidise to form Si rich oxides that subsequently provide protection.
Alternatively, Al could be deposited and diffused to form aluminides e.g. NbAl3, which subsequently forms Al rich oxides.
Diffusion coatings involve and rely on the substrate for their formation and therefore the composition of the coatings is limited.
If the majority of the diffusion is from the coating to the substrate it is described as “net inward”.
If the majority of the diffusion is from the substrate to the coating it is described as “net outward”.
Explain Overlay protection coatings and how they operate. Provide and example.
A layer is applied to a material and diffusion is not the primary formation mechanism. Coatings are relatively thick (~50-150μm) compared to diffusion coatings.
Overlay coatings can be produced by processes like PVD, thermal spraying or cladding.
The protective life increases in proportion to the coating thickness. Thicker coatings are more prone to cracking as the cyclically induced stresses are higher.
One of the main types used is based on “M”-Cr-Al-Y.
The “M” (Ni or Co) is the matrix of the coating. The matrix is ductile to provide better resistance to thermal fatigue.
The Cr content is high to provide corrosion resistance. The Al is provided for Al2O3 formation. Yttrium is included to enhance the oxide adherence.
For what type of application might you use Cobalt based superalloys as opposed to Nickel based?
Compare the composition and strengthening mechanism of cobalt vs nickel superalloys
Cobalt based superalloys are not as high strength as Nickel based superalloys, but maintain their properties to higher temperatures.
Thus they are good for LONG LIFE STATIC PARTS at low stress, such as nozzle guide vanes.
Nickel based superalloys are made up of γ, γ´, and carbide precipitates, whereas Co based is only y phase and carbon precipitates. Strength relies heavily on carbide precipitation therefore the carbon content of the Co based are relatively higher (0.1-1wt%Co).
