Nickel Based High Performance Alloys

Question 1

What is a super alloy?

Answer 1

A superalloy is a high temperature, heat resistant alloys that is able to retain their mechanical strength & creep resistance at high temperatures.

Question 2

What is “Nimonic 80”?

Answer 2

One of the earliest alloys, based on:
➢ Ni – Balance
➢ Cr – 20 wt%;
➢ Ti – 2.25 wt%;
➢ Al - 1.0 wt%

Question 3

What do the elements provide in Nimonic 80?

Answer 3

➢ The Cr is largely in substitutional solid solution and contributes to strength in this way, it also contributes to oxidation resistance.

➢ The Ti and Al allow the formation of Ni3Ti and Ni3Al precipitates after age hardening treatments.

➢ Modern day improvements on the base alloy include additions of Fe, Mo, Nb, Zr, B, Co and others.

Question 4

What are the major phases present in nickel based super alloys?

Answer 4

1. γ(gamma). This is a continuous matrix of fcc austenite.
2. γ’ (gamma prime). This is the major precipitable phase (Ni3Al).
3. “M” Carbides. There are various types, but mainly “M”23C6 and “M”C. (where M represent the metal).

Question 5

Elaborate on γ (gamma) phase.

Answer 5

➢ This is the matrix, it is a nickel-based austenite. It is solid solution
strengthened by some of the other alloying elements such as, Cr, Mo, Ti, W, Co, Fe and Al.
➢ The slow diffusing elements such as Mo and W are good for enhancing high temperature creep resistance

Question 6

Elaborate on γ’ (gamma prime) phase.

Answer 6

➢ This is a high nickel “A3B” type compound. “A” is a relatively electronegative element eg. Ni, “B” is a relatively electropositive element eg. Al, Ti,

➢ γ’ is a coherent precipitate (there is only a about 0.1% mismatch
between γ and γ’), coherency is maintained by a tetragonal distortion (crystal stretches). More desirable for high temp
materials owing to ‘order’ hardening effects.

➢ γ’ is a coherent precipitate (there is only a about 0.1% mismatch between γ and γ’), coherency is maintained by a tetragonal distortion (crystal stretches). More desirable for high temp materials owing to ‘order’ hardening effects

Question 7

What are different types of interface between solids?

Answer 7

• Coherent - there is perfect registry of the lattices.
• Coherent with strain - it is quite likely for there to be some strain with the interface, due to imperfect matching. The strain energy increases with the size of the growing particle, and there is a transition to a semi-coherent interface.
• Semi-coherent interface - the introduction of dislocations reduces the strain energy (but they themselves contribute to the energy of the system).
• Incoherent - there is no matching of the interface

Question 8

What are the types of “M” carbides?

Answer 8

3.1 “M”C Carbides (Monocarbides)
➢ “M” is usually Ti, Ta, Nb or W.
➢ They are very stable and form just below solidification temperature.
➢ They restrict grain growth during solution treatment.
➢ Blocky, needle like form, distributed sparsely well at GB.

3.2 “M”23C6 Carbides
➢ “M” is usually Cr but can be Fe, W, Mo or Co.
➢ These carbides form at lower temperature heat treatments, and in
service, at temperatures in the range 760 – 980oC.
➢ Finer particles at GB and at  channels.

Note: “M”C & “M”23C6 Carbide at grain boundaries often inhibit grain boundary migration during deformation and thus enhance the fatigue performance of superalloys.

3.3 “M”6C Carbides
➢ Form at temperatures in the range 815 – 980 oC, they are similar to “M”23C6 and form when Mo and W contents are high.
➢ Eg. in M252 alloy, which contains 6 - 8wt% Mo or W;
➢ “M” 6C and “M”23C6 form on grain boundaries.
➢ Fine and dispersive “M” 6C particles can pin down dislocation movement, prevent γ’ phase to be sheared and thus strengthen
the superalloy

Question 9

Where do the carbides form?

Answer 9

Carbides form on the grain boundaries and within the grains. Since carbides are hard and brittle, the distribution on the grain boundaries will particularly affect the high temperature strength, the ductility and the creep properties of the alloy.

1. If there are no carbides on grain boundaries the excess grain boundary sliding can promote creep.
2. Excess (continuous) carbide films on grain boundaries provide a brittle fracture path (crack propagation) and result in low (poor) impact properties.
3. The optimum is a discontinuous carbide chain on the grain boundaries.