Superalloys - Cast and Wrought Flashcards
What processing improvements have been made in the context of turbine blades?
The old equiaxed structure can now be columated structure of even better single crystal structure. This reduces and eliminates grain boundaries from the product and so eliminates coble creep.
What advances mean that turbine blades can withstand temperatures above their melting points?
Thermal coatings and cooling channels.
What is investment casting?
When a ceramic mould is created using a wax model.
What are the steps of investment casting?
Assembly Investment Dewaxing Firing Pouring (slowly in vacuum) Knockout Finishing
How is a columated structure achieved?
By combining investment casting with the bridgeman process. Here a thermal gradient is used where the poured casting is slowly lowered into coolant.
How is a single crytsal structure achieved?
A pig tail is added to the mould. This means that as the mould is lowered through a thermal gradient only one grain orientation is allowed through the pig tail and into the turbine section. Incredibly slow withdrawal rate.
How does the ‘pig tail’ work?
The ‘pig tail’ grain selector must be smaller than the grain size to prevent grains of different orientations growing.
What are the consequences of a super slow casting process and how are they rectified?
Massive segregation and dendrites. Solution is heat treatment such as homogenisation.
What elements do single crystal alloys lack?
Carbon, Boron, Zirconium.
Why must thermo-mechanical processes be chosen very carefully?
You don’t want to destroy the material.
What is the most popular superalloy?
Inconel 718 - accounts for over 50% of all superalloys by volume.
What are some characteristics of Inconel 718?
Uses Iron as a solid solution strengthener as it’s cheaper.
Has a high percentage of Niobium which is responsible for a new phase gamma prime prime (omega), increases strength more.
What are some characteristics of the omega (gamma prime prime) phase in Inconel 718.
Body centred tetragonal crystal structure
chemically ordered
coherent with the gamma phase.
the omega (gamma prime prime) phase in Inconel 718 is metastable. What does that mean?
It changes crystallography at certain temps. It can produce a brittle phase that degrades properties.
What is the service temperature of Inconel 718?
650 celcius
