superalloys

1.  INTRODUCTION :
 A Superalloy is an alloy which can be used at high temperatures, often in excess of 0.7 of the
absolute melting temperature (Tm).
 A Superalloy or high-performance alloy exhibits several key characteristics such as:
1. Excellent mechanical strength
2. Resistance to thermal creep deformation
3. Good surface stability
4. Resistance to corrosion or oxidation
5. High fracture toughness
6. High fatigue resistance.
Examples: Haste alloy, Inconel, Waspalloy, Rene alloys, Haynes alloys, Incoloy, etc.

2. Ni-BASED SUPERALLOYS :
Composition : 38-76% Ni, up to 27% Cr, up to 20% Co.
Example : Nimonic, Inconel, Waspalloy.
Fe-Ni-BASED SUPERALLOYS:
Composition : 32-67% Fe, 9-38% Ni, 15-22% Cr .
Example : Incoloy series
Co-BASED SUPERALLOYS:
Composition : 30-65% Co, 19-30% Cr, up to 35% Ni .
 General Composition:

3.  Why is Ni used as a matrix for superalloys ?
1) Due to FCC structure it provides good ductility
2) It has low linear thermal coefficient of expansion (13x10-6 /ºC at 20ºC)
3) Low volume thermal coefficient of expansion (39x10-6 /ºC at 20ºC)
4) Ni-matrix does not undergo phase transformation up to Tm
 Ni-BASED SUPERALLOYS :
Ni-based superalloys are more strong and have more corrosion resistance.
They are the most commonly used superalloys generally used above
500ºC in oxidising and corrosive environment. Example: Turbine Blades.

4. • The Ni-based superalloys contain high Cr with Ti, Al to form precipitates and
additions of Mo, Co, Fe, C etc.
• On the basis of variation of additional alloying elements Ni-based superalloys
are generally of three types :
i) Inconel
Composition: 15% Cr - 6.75% Fe - 2.5% Ti - 0.8% Al - 0.85% Co
ii) Nimonic
Composition: 20% Cr - 18% Co - 2.5% Ti - 1.5% Al - 0.05% C
iii) Waspalloy
Composition: 19.5% Cr - 13.5% Co - 2% Fe - 4.25% Mo - 1.3% Al - 3% Ti - 0.1% C

5.  PROCESSING OF SUPERALLOYS
There are many forms of superalloy present within a gas turbine engine, and the
processing methods vary widely depending on the necessary properties of each
specific part. Some of them are:
 Casting and Forging
 Powder Metallurgy
 Directional Solidification
 Single Crystal Growth

6.  Conventional Casting: Initially the Superalloys were produced through conventional casting
process. But superalloys having high alloying elements, ingot casting caused a high amount of
shrinkage along with blowholes, porosities etc. and also had dendritic structure. Vacuum melting
process is used to remove the blowholes and gas porosities, specifically oxygen. Dendritic
structure and shrinkage is eliminated by deformation by rolling or forging. Here
thermomechanical treatment gives uniform deformation of phases but still causes segregation in
weaker regions. So they have short service life.
 Powder Metallurgy: This is a class of modern processing techniques in which metals are first
converted into a powder form, compacted and then formed into the desired shape by sintering
below the melting point. This is in contrast to casting, which occurs with molten metal.
Superalloy manufacturing often employs powder metallurgy because of its material efficiency -
typically much less waste metal must be machined away from the final product—and its ability
to facilitate mechanical alloying. But it also facilitates higher cost of production.

7.  Directional solidification: This process uses a thermal gradient to promote nucleation of
metal grains on a low temperature surface, as well as to promote their growth along the
temperature gradient. This leads to grains elongated along the temperature gradient, and
significantly greater creep resistance parallel to the long grain direction.
 Single Crystal Growth: In this process, a single
crystal is manufactured by removing all the grain
boundaries resulting in a sharp increase in strength
and creep properties. Single crystal growth starts
with a seed crystal which is used to template
growth of a larger crystal. This involves highly
controlled and therefore relatively slow
crystallization. They have long service life.

8. Fig: Showing variation of Creep rate with Time for differently cast Superalloys. Single
Crystal gives more creep life, hence can work for prolonged time compared to Directionally
Solidified and Conventionally Cast Superalloys

9. γ-PHASE
The continuous matrix is a FCC Ni-based austenitic phase
(γ-phase) that usually contains a high percentage of solid-
solution elements such as Co, Cr, Mo and W.
γ'-PHASE
The primary strengthening phase in Ni-based
superalloys is Ni3(Al, Ti) or the γ'-phase. It is a
coherently precipitating phase (i.e., the crystal planes
of the precipitate are in registry with the γ-matrix)
with an ordered FCC crystal structure.
 PROPERTIES & MICROSTRUCTURE :

11. CARBIDE PRECIPITATION CONTD…….

12. Transmission Electron micrograph showing cuboidal
γ' (Ni3(Al,Ti)) (FCC-L12) precipitate in γ (FCC)
(A1)matrix.
There are also the carbide formers (Cr, W, and Ti)
present.
• The carbides tend to precipitate at grain
boundaries and hence reduce the tendency for
grain boundary sliding.
• Cobalt, iron, chromium, niobium, tantalum,
molybdenum, tungsten, vanadium, titanium and
aluminium are also solid-solution strengtheners,
both in γ and γ'.
• Cr, Co, here, partitions into γ whereas Ti
partitions into γ'. W may partition both into γ and
γ'.
• Dislocations in γ find it difficult to penetrate into
atomically ordered γ' and leads to strengthening.
Ni-9.7 Al-1.7 Ti-17.1 Cr-6.3 Co-2.3 W (TEM Imaging)

13.  STRENGTHENING MECHANISMS :
There are three prior strengthening mechanisms for Ni-based superalloys ;
 Solid Solution Hardening
 Coherent Precipitate Hardening
 Hardening by Carbide precipitation on grain
boundaries

14.  Solid Solution Strengthening:
Cr, Mo, Al, Nb, Ti and other alloying elements go to the solution and replace the solvent
atoms in their lattice positions. So local stress fields are formed that interact with those of
the dislocations, impeding their motion and causing an increase in the yield stress of the
material, which means an increase in strength of the material.
 Precipitation Strengthening:
It happens mostly due to Al & Ti, when they form γ' or Ni3(Al,Ti) with the solvent atoms. This
γ' phase impede the movement of dislocations and defects in a crystal lattice which leads to
increase of strength.
 Hardening by Carbide Precipitates:
Presence of carbide precipitates (like M23C6, M6C or MC) which are hard and brittle in nature
causes strengthening of superalloys.
 Oxide dispersion strengthening is another method of strengthening in which Yttrium oxides
are dispersed on the matrix making the material suitable for high temperature applications.

15. Fig: Alloying element effects in Nickel based superalloys.
 Effect of Alloying Elements :

16. MICROSTRUCTURES…….

17. CONTD…..

18. CONTD…….

19. CARBIDE PRECIPITATION IN
SUPERALLOYS

20. CARBIDE PRECIPITATION
CONTD…..

21. CONTD…. …

22. CONTD… …

23. Fig: Variation of Yield Stress with Temperature for different
Ni-based superalloys

24.  APPLICTIONS :
Ni-based superalloys are widely used in load bearing structures to the highest homologous
temperature 0.9 Tm
Superalloys are used in,
Aerospace
 Turbine Blades and Jet/Rocket engines
 Marine Industry
 Submarines
 Nuclear Reactors
 Heat Exchanger Tubing
 Industrial Gas Turbines and Combustion Engine Valves
 Petrochemical Equipments
 Hot working Tooling and Dies

25. Fig. Jet Engines
Fig. Rocket Engine
Fig. Turbine Blades

26. Fig. Gas Turbine for marine propulsion
Fig. Gas Turbine for Thermal power plant

28. CONTD……

29. CONTD……