25 Questions
What type of defects are formed when dislocations glide through g’ precipitates during plastic deformation?
Antiphase boundary defects
What contributes to the high-temperature strength of superalloys according to the text?
Ductility
In superalloys, what type of strengthening mechanism occurs when precipitates form coherent interfaces with the parent phase?
Coherency strengthening
What distinguishes single-crystal turbine blades from polycrystalline turbine discs in superalloy components?
Volume fraction of g’ phase
Which property of the g’ phase in superalloys is responsible for the strength increase with temperature anomaly?
Short-range ordering
Why must superalloys in harsh environments be adequately protected?
To prevent premature failure
What is one way to judge whether an oxide will be protective?
Oxide-metal volume ratio
Which of the following substances has low oxygen diffusivity in its oxide, making it a good candidate for protection?
Chromium
What type of coating volatilises at higher temperatures but provides good corrosion resistance?
Chromia coating
Which type of turbine blade coating is produced by plasma spraying and has a 'Herring bone' morphology?
Thermal barrier coatings
What caused the failure in the turbine disc mentioned in the text?
High operating temperature
Which processing route for disc manufacture is more expensive but allows for precise grain size control?
Powder metallurgy
What is the significance of the VIM→ESR→VAR method mentioned in the text?
It is used for producing high-purity metal ingots
What type of alloys are suitable for the Cast and Wrought processing route?
Alloys with lower g’ content
Why is it important that turbine discs are manufactured from high-purity alloys?
To ensure proper operational lifetime predictions
What process is used to dissolve all g’ precipitates and homogenize the composition in turbine blades?
Solution treatment
What is the purpose of the primary ageing treatment in the heat treatment of turbine blades?
Grow the desired distribution of g’ precipitates
Which of the following casting defects might result in a blade being scrapped according to the text?
Freckles
What operation involves applying a bond coat to turbine blades?
Deposit TBC
In the heat treatment process of turbine blades, what temperature is used for the secondary ageing treatment?
870°C
What is the purpose of the Vacuum Arc Remelting (VAR) process?
To further purify high-value alloy ingots
Why is Vacuum Arc Remelting (VAR) considered a slow and expensive process?
Because it requires multiple steps for purification
What advantage does Vacuum Arc Remelting (VAR) offer in terms of microstructural control?
Tight control over solidification rate
Why are some metals processed using Vacuum Arc Remelting (VAR) instead of open air furnaces?
To remove more volatile impurities like carbon and sulfur
What is the purpose of converting ingots into forged billets after the VIM+ESR+VAR stages?
To reduce their diameter and grain size for improved strength
Study Notes
- Order strengthening occurs due to the formation of antiphase boundary defects when dislocations glide through g’ precipitates during plastic deformation in superalloys.
- The g’ phase in superalloys is ductile compared to most intermetallics, exhibits a Yield Stress Anomaly where strength increases with temperature, precipitates from the γ phase via short-range ordering, and has a low lattice parameter mismatch forming coherent interfaces with the parent γ phase.
- Superalloys exhibit a combination of beneficial strengthening mechanisms including solution strengthening, order strengthening, precipitate strengthening, and coherency strengthening.
- Superalloy components can be single-crystals (SX) with high volume fraction of g’ suitable for turbine blades, or polycrystalline (PX) with smaller volume fraction of g’ suitable for turbine discs and combustors.
- Turbine blades made of single-crystal superalloys have complex geometries with internal cooling channels while turbine discs are around 20% of engine weight and require high strength but light materials.
- Processing routes for turbine disc manufacture include cast and wrought methods suitable for alloys with lower g’ content, and powder metallurgy method suitable for alloys with higher g’ content allowing precise grain size control.
- To ensure high quality, alloys used for turbine discs are produced using VIM→ESR→VAR method, where Vacuum Induction Melting (VIM) is used to produce primary alloy ingots, Electroslag Remelting (ESR) further purifies the ingots, and Vacuum Arc Remelting (VAR) achieves highest purity for mission-critical applications.
- Post-casting operations for turbine blades include heat treatments like solution treatment and ageing treatments to optimize microstructure, and finishing procedures like applying bond coats, depositing TBCs, laser drilling cooling holes, and undergoing NDT.
This quiz covers the formation of antiphase boundary defects when dislocations glide through g’ precipitates during plastic deformation in superalloys. It also discusses the special properties of the g’ phase contributing to high-temperature strength, such as ductility and strength increase with temperature.
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