Superalloys - Lifing - Environment and coatings - Future

Question 1

What is the UK’s safe life policy for critical components?

Answer 1

Life to first crack

Question 2

What is the Life to first crack?

Answer 2

The number of cycles until 1 out of 750 components fail by a 0.75mm crack.

Question 3

What is size of the crack required for failure to be assumed in the ‘Life to first crack’ approach?

Answer 3

0.75mm

Question 4

What are the neagtives of the Life the first crack method?

Answer 4

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.

Question 5

What is the ‘Damage-tolerant’ approach to lifing?

Answer 5

this method depends of the fracture mechanics of fatigue and non destructive inspection.

Question 6

What is the procedure for damage-tolerant lifing?

Answer 6

-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.

Question 7

What is a scale?

Answer 7

A protective layer formed due to oxidation with a base metal.

Question 8

A scales effectiveness depends on what?

Answer 8

• thermodynamic stability
• slow growth rates
• good adhesion
• thermal expansion matching substrate
• high melting temp

Question 9

Why is the Nickel oxide unsuitable?

Answer 9

Nickel Oxide has very fast growth kinetics, making it unstable and therefore unsuitable. Superalloys rely on Aluminium or Chromium oxides.

Question 10

What are the 3 scale groups relating to superalloys?

Answer 10

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.

Question 11

How are Group 2 and 3 scales formed?

Answer 11

Both Chromium and Aluminium oxide grow under Nickel oxide and so force the NiO to crack off.

Question 12

Why is chromium important even with group 3 scales?

Answer 12

Chromium can accelerate the growth of Alumina meaning less wt% aluminium is required.

Question 13

What is corrosion?

Answer 13

Salts or other chemicals reacting with the material.

Question 14

What oxide is most valuable in terms of corrosion?

Answer 14

Chromium Oxide.

Question 15

What are the 3 types of coating used with superalloys in order of protection?

Answer 15

Diffusion Coating
Overlay Coating
Thermal Barrier Coating

Question 16

What is Diffusion Coating?

Answer 16

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.

Question 17

What is Overlay Coating?

Answer 17

Where a M Cr Al X coating is applied via vaccum plasma spraying or electron beam physical vapour deposition. the X represents reactive elements.

Question 18

What is a thermal barrier coating?

Answer 18

Cermamic layer applied by electron beam physical vapour deposition. Consists of a bonding layer (basically overlay coating) and a Zirconia top layer.

Question 19

What are the 5 future trends associated with superalloys?

Answer 19

Co based Superalloys
Titanium Alumnides
Refractory Superalloys
High Entropy Superalloys
Additive Manufacturing

Question 20

Why is interest in Co based superalloys picking back up?

Answer 20

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.

Question 21

How are Co based superalloys traditionally strengthened?

Answer 21

Not by gamma prime phase but through carbide networks. Gamma prime phase possible however in terniary system.

Question 22

What are Titanium Alumnides?

Answer 22

roughly 50/50 titanium aluminium. These have two lamellar phases (TiAl and Ti(3)Al)

Question 23

What are the positives of Titanium Alumnides?

Answer 23

• good mechanical properties at temperatures above 600 celcius.
• Excellent corrosion resistance
• Very light (half that of Ni superalloys)

Question 24

What are the negatives of Titanium Alumnides?

Answer 24

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.

Question 25

Where are Titanium Alumnides currently used?

Answer 25

In GE’s GEnX engine, turbine section, cast.

Question 26

What are Refractory superalloys?

Answer 26

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.

Question 27

What are High entropy alloys?

Answer 27

They have no base elements are are usally made from equiatonic combinations of 5 or more elements.

Question 28

What are the 4 core principles of High entropy alloys?

Answer 28

• 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.

Question 29

What are the benefits of Additive manufacturing?

Answer 29

Access to complex parts/shapes
Elminiation of joints/fastening
Lower lead time
Potentially easier repair through directional laser depsoition.

Question 30

What are the disadvantages of Additive manufacturing with superalloys?

Answer 30

Simply hard to use superlalloys in AM as they’re not very weldable.