Superalloys - Lifing - Environment and coatings - Future Flashcards
What is the UK’s safe life policy for critical components?
Life to first crack
What is the Life to first crack?
The number of cycles until 1 out of 750 components fail by a 0.75mm crack.
What is size of the crack required for failure to be assumed in the ‘Life to first crack’ approach?
0.75mm
What are the neagtives of the Life the first crack method?
Choice of crack size is very important, real critical crack size may be much larger.
Calculation requires spin rig tests which are incredibly expensive.
Might be too conservative.
What is the ‘Damage-tolerant’ approach to lifing?
this method depends of the fracture mechanics of fatigue and non destructive inspection.
What is the procedure for damage-tolerant lifing?
-The actual critical crack length is found using fracture mechanics
-Then the number of cycles from the crack to grow from being visible to non destructive inspection, to the critical crack length, is found
-Using the paris law the number of cycles to failure can be found.
This process is repeated at each inspection.
What is a scale?
A protective layer formed due to oxidation with a base metal.
A scales effectiveness depends on what?
- thermodynamic stability
- slow growth rates
- good adhesion
- thermal expansion matching substrate
- high melting temp
Why is the Nickel oxide unsuitable?
Nickel Oxide has very fast growth kinetics, making it unstable and therefore unsuitable. Superalloys rely on Aluminium or Chromium oxides.
What are the 3 scale groups relating to superalloys?
Group 1 (worst) - Nickel Oxide dominates with Aluminium and Chromium oxide subscale. Group 2 (most common) - Continuous protective Chromium oxide with Alumonium oxide subscale fingers. Can be volatile at high temperatures. Group 3 (best) - Exclusively Almunium oxide scale. Large wt% al required.
How are Group 2 and 3 scales formed?
Both Chromium and Aluminium oxide grow under Nickel oxide and so force the NiO to crack off.
Why is chromium important even with group 3 scales?
Chromium can accelerate the growth of Alumina meaning less wt% aluminium is required.
What is corrosion?
Salts or other chemicals reacting with the material.
What oxide is most valuable in terms of corrosion?
Chromium Oxide.
What are the 3 types of coating used with superalloys in order of protection?
Diffusion Coating
Overlay Coating
Thermal Barrier Coating
What is Diffusion Coating?
Where an Aluminium layer is deposited onto a surface via chemical vapour deposition and then heat treated. Platinum can be added to imprive corrosion resistance.
What is Overlay Coating?
Where a M Cr Al X coating is applied via vaccum plasma spraying or electron beam physical vapour deposition. the X represents reactive elements.
What is a thermal barrier coating?
Cermamic layer applied by electron beam physical vapour deposition. Consists of a bonding layer (basically overlay coating) and a Zirconia top layer.
What are the 5 future trends associated with superalloys?
Co based Superalloys Titanium Alumnides Refractory Superalloys High Entropy Superalloys Additive Manufacturing
Why is interest in Co based superalloys picking back up?
Interest died due to Ni superalloys and that Co sourcing was becoming unstable due to war in Congo.
Co based superalloys show great hot corrosion resistance. They also can form a gamma prime phase if in a ternirary system which is good as Co has a higher Tm than Ni, increasing it’s service temperature.
Positive lattice misfit surprisingly does not affect creep resistance.
How are Co based superalloys traditionally strengthened?
Not by gamma prime phase but through carbide networks. Gamma prime phase possible however in terniary system.
What are Titanium Alumnides?
roughly 50/50 titanium aluminium. These have two lamellar phases (TiAl and Ti(3)Al)
What are the positives of Titanium Alumnides?
- good mechanical properties at temperatures above 600 celcius.
- Excellent corrosion resistance
- Very light (half that of Ni superalloys)
What are the negatives of Titanium Alumnides?
The main problem is a very low ductility (1-2% at rm temp)
This means that there are many problems in manufacturing. However additive manufacturing looks promising.
Where are Titanium Alumnides currently used?
In GE’s GEnX engine, turbine section, cast.
What are Refractory superalloys?
Refractory superalloys use BCC elements as their base such as Nb, Mo, Ta, W. Increase base Tm from Ni’s 1450 to 2500. However have very poor creep resistance.
What are High entropy alloys?
They have no base elements are are usally made from equiatonic combinations of 5 or more elements.
What are the 4 core principles of High entropy alloys?
- HEA’s exhibit slow diffusion kinetics (good for creep)
- Crystal lattices are severely strained
- High Entropy stabalises simple solid solutions rather than intermetallic phases.
- Complexity of combination creates ‘cocktail effect; which can produce some unusual behaviours.
What are the benefits of Additive manufacturing?
Access to complex parts/shapes
Elminiation of joints/fastening
Lower lead time
Potentially easier repair through directional laser depsoition.
What are the disadvantages of Additive manufacturing with superalloys?
Simply hard to use superlalloys in AM as they’re not very weldable.
