Light and High Performance Alloys Flashcards
Phases within Nickel superalloy
Y(gamma) - continuous matrix of fcc austinite. Solid solution strengthened by Cr, Mo, Ti, W, Co, Fe, and Al.
Y’(gamma) - large square-shaped grains within the alloy. It shows a high coherency (a 0.1 % miss match) which is maintained by tetragonal distortion. This hardens the material making it more durable at high temperatures.
Carbides - “M”C and “M”23 C6
“M”C - M is usually Ti, Ta, No, W. Very stable below solidification temp. They restrict grain growth during solution treatment.
“M”23 C6 - M is usually Fe, W, Mo, Co. These form at lower temperature heat treatment.
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 Ni based but they maintain their properties to higher temperatures. There is basically a y matrix and carbide precipitates. The strength therefore comes from solid solution strengthening and carbide precipitation. There is no y’ precipitate, so Co have lower strength at intermediate temperatures, hence, the carbon content is relatively high at ~ 0.1 – 1.0wt%. Co based only has carbides and solid solution strengthening, hence it cannot compete in the high stress applications. So it is used for low stress, high temperature, long life applications.
Standard casting, directionally solidified and single crystal casting
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 - The 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 transverse boundaries. The resulting casting is then stronger in the direction of the centrifugal forces.
Single Crystal: The mould is similar to DC 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 in these castings makes them more resistant to CREEP and THERMAL SHOCK.
What is the casting process used for
The processes can be applied in the fabrication of turbine blades
What is the hot corrosion effect and describe the process?
The formation of condensed phases on the surface of alloys under high temperatures, hinders the passivation layer on the alloy protecting it. DURING COMBUSTION in the gas turbine, sulfur from the fuel reacts with sodium chloride from ingested air at elevated temperatures to form sodium sulfate. The sodium sulfate then deposits on the hot-section components, such as nozzle guide vanes and rotor blades, resulting in accelerated oxidation (or sulfidation) attack.
Explain diffusion protection coatings and how they operate? Provide an example?
A reservoir with a very high concentration of the element that should be enriched is created at the surface either in the gas phase or in a solid/liquid state. 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”.
For example, with Nb, Mo or W, silicon is deposited onto the surface and diffused into the substrate to form a silicide eg. MoSi2, this will then oxidize to form Si rich oxides that subsequently provide protection. Alternatively Al could be deposited and diffused to form aluminides eg. NbAl3, which subsequently forms Al rich oxides.
Explain Overlay protection coatings and how they operate? Provide an example?
A layer is applied to a material and diffusion is not the primary formation mechanism. Coatings are relatively thick (~ 50 – 150um) compared to diffusion coatings. Overlay coatings are produced by one of the following processes: Physical vapour deposition (PVD) process, thermal spraying process, or Overlay welding. 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 (where M = Ni or Co). 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 included for Al2O3 formation. The Y (Yttrium) is included to enhance the oxide adherence
Titanium Properties
High strength to weight ratio
Low density
High corrosion resistance
biocompatible
Titanium application
aerospace
chemical pressing
automotive
watches/cameras/jewelleries
biomedical
architectural
Titanium Stabilisers
Pure titanium undergoes a transformation from hexagonal close-packed (α –at lower temperature) to body centred cubic (β – at higher temperature) by increasing the temperature up to 882oC.
titanium alpha stabilisers
Substitutional elements such as Al, Sn (Tin), Ga, Ge;
Interstitial elements such as O;N and C.
➢ Generally non heat treatable
➢ Medium strength
➢ Good notch toughness
➢ Good high Temperature creep resistance
titanium beta stabilisers
They make the b phase stable to lower temperatures.
The main elements are vanadium (V) and molybdenum (Mo).
What are a/β Ti alloys?
α/β Ti alloys contain both the alpha and beta phases. They can form if alloyed with beta stabilisers or alpha stabilisers after a certain concentration and temperature.
Meta-stable β alloys
The temperature “martensite start” of phase transition β → α is decreased below room temperature.
Ageing metastable β phase to produce some α phase. (Ageing causes precipitation of fine α phase (dispersion strengthening)).
How are B stable alloys formed and what property has changed to induce this?
Increasing the content of β stabilising elements causes the Beta transus temperature to decreased below RT. After cooling to room temperature, β phase remains stable.
