Chapter 3: Superalloys

Question 1

What are the requirements inside the turbine engines

Answer 1

Mechanical	forces	
– Creep	
– Fatigue	
– Thermomechanical fatigue	
• High temperature environment	
– Oxidation	
– Hot corrosion (due to	contaminaAon)

Question 2

Properties of Superalloys

Answer 2

Maintains high strength at high temperature
• Creep resistant a thigh temperatures
• Oxidation resistant at high temperatures

Question 3

List three groups of superalloy

Answer 3

Ni-based
Ni-Fe based
Co based

Question 4

Requirements of the Jet engine, gas turbine and rocket

Answer 4

Need high strength creep resistance
Oxidation and corrosion resistance
Fatigue resistance
Operate up to 18 hours continuously (nonstop) in flight at speed of 1000 km per hour

Question 5

Explain why FCC metal is chosen as the base for the

development of superalloys

Answer 5

Superalloy components are designed to operate at elevated temperatures and are subjected to high stress.

FCC alloys have a lower diffusion rate

• Dislocation split and multiply, restricting further movement, thus strengthening the alloy
• No DBTT
• No Allotropic transformation
• Can dissolve more solutes for solid solution strengthening and precipitation strengthening

Question 6

List the behaviours of the superalloy

Answer 6

Density is much greater than steel

Difficult to form or machine

The product can be made to final shape by investment casting

complex ingredients
Expensive
Limited to small parts where is not the determining factor

Question 7

Describe how creep strength of superalloy is improved

Answer 7

Within grain
FCC solid solution
Fatigue/creep
Solution Strengthening

γ’ (Ni3Ti, Ni3Al)
Fatigue/Creep
Precipitation Hardening

Other precipitates
Fatigue
Precipitation Hardening

Carbides
Creep
Carbide Dispersion

Grain boundary

Carbides
Creep
Carbide Dispersion

Grain size
Fine
Fatigue
Grain size control

Coarse
Creep
Grain size control

Question 8

What is a solid solution strengthening

Answer 8

Base metal dissolves other atoms in solid solution

Question 9

In solid solution strengthening the amount of strengthening depends on

Answer 9

Amount of solute and size difference of the atoms

Question 10

What is Dispersion hardening

Answer 10

Particles are added by mixing; no heat treatment

Question 11

In Dispersion hardening the amount of strengthening depends on

Answer 11

-Dispersed particles must be stronger than the matrix
• Strength is not Time-dependent
• Strength is not temperature dependent
Strength obtained from distinct second-phase particles
dispersed throughout the material

Question 12

How strength is obtained in Precipitation hardening

Answer 12

Strength is obtained from a non-equilibrium structure

produced by a three-step heat treatment

Question 13

Discuss the formation and usefulness of gamma prime

(γ’) in superalloy.

Answer 13

• γ’ precipitates (Ni3Al or Ni3Ti compounds) formed when Al and Ti added to Ni by precipitation hardening heat treatment
• γ’ has the same structure (shape and size) as Ni matrix,
• This coherency between γ (=Nickel) and γ’ is maintained
• γ’ is very stable, difficult to dissolve and has high creep resistance.

Question 14

How to obtain strength in Grain-size refinement

Answer 14

grain-size refinement can be used to increase strength

except at elevated temperatures, where failure is by grain-boundary diffusion-controlled creep mechanism

Question 15

What is the function of the Helix

Answer 15

Helix to select ONLY
one (1) elongated grain
to grow into a single crystal

Question 16

With sketches, describe how a single crystal turbine blade is
fabricated

Answer 16

Done by unidirectional solidification
 The mould containing liquid metal progressively withdrawn from the furnace at a predetermined rate
 A starter block give raise to the chill zone which promotes the solidification of columnar grains
 A helix (spiral) attached between the starter block and turbine blade ensure that only a single crystal grows within the blade
 Blades solidify progressively upwards from the chill as mould is drawn down into water-cooled lower chamber

Question 17

What is a Single Crystal?

Answer 17

A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample with no grain boundaries

Question 18

Advantages of Single Crystals

Answer 18

Relatively high strength at high temperatures
• Low creep at high temperature
• Anisotropic
• Improved corrosion resistance

Question 19

Alloying produce strong, stable microstructure at high temperature using

Answer 19

Solid solution, dispersion, precipitation strengthening.

Question 20

Compare carbide dispersion strengthening and

precipitation hardening in superalloy

Answer 20

Carbide Dispersion Hardening
 The strength obtained from distinct second phase particles
dispersed throughout the materials
 Dispersed particles must be stronger than the matrix
 Add a little C and form TiC, Cr23C6, Mo6C, W6C etc
 Network of fine, stable carbide particles
 Interfere with dislocation movement
 Strengthen grain boundaries
 Prevent grain boundary sliding

Precipitation Hardening
 Strength is obtained from a non-equilibrium structure
produced by a three-step heat treatment
 Ni, Ni-Fe based containing Al & Ti
 Especially compound of Ni with Al, Ti
 Coherent precipitate gamma prime during ageing
 FCC structure coherent with the matrix
 Very stable and difficult to dissolve
 Increase strength and resistance to creep at high temperature

Question 21

Discuss the formation and usefulness of gamma prime

(γ’) in superalloy

Answer 21

-γ’ precipitates (Ni3Al or Ni3Ti compounds) formed when Al and Ti
added to Ni by precipitation hardening heat treatment
 γ’ has same structure (shape and size) as Ni matrix,
 This coherency between γ (=Nickel) and γ’ is maintained
 γ’ is very stable, difficult to dissolve and high creep resistance.