Nickel

Question 1

What are the major phases of Nickel alloy?

Answer 1

• gamma- FCC solid solution of Ni with other elements
• gamma’- develops coherently within the matrix

Question 2

Where does the gamma’ phase occur

Answer 2

• As a coating at grain boundaries
• in the form of nodules
• Around carbides

Question 3

What are Carbides and where do they occur?

Answer 3

Very important 2nd phase. Around grain boundaries and in block-like spherical particles

Question 4

Where does Nickel base alloys derive their strength and temp capability?

Answer 4

The combined influence of the matrix, precipitate phases and carbides

Question 5

Which elements dissolve in the FCC gamma matrix? And what do they do to the solid solution?

Answer 5

Co, Fe, Cr, Mo, W and V. This is due to them being smaller than Ni. makes the slid solution stronger

Question 6

What is Heat treatment?

Answer 6

Where Nickel superalloys derive much of their strength.

1. The alloy temp is increased until alloy is stable with single phase solid solution (gamma)
2. Alloy rapidly cooled to a temperature where 2 or more phases exist (gamma+gamma’)

Question 7

If the precipitation temperature is close to the transition between gamma and gamma+gamma’….

Answer 7

Number of nuclei low

Size of nuclei large

Question 8

If the precipitation temperature moves away from the transition…

Answer 8

Number of nuclei increases

Size of nuclei decreases

Question 9

Issues for Turbine blades

Answer 9

1. High temp creep
2. Thermal fatigue
3. Degradation

Question 10

How are turbine issues addressed?

Answer 10

Single Crystal structure

Question 11

How is a single crystal structure formed?

Answer 11

• Mold is heated above alloy luiquidus temp.
• Heat is extracted rapidly from the bottom so solidifcation moves up through the cast geometry
• Mold slowly removed from the furnace at rate detemined by the temp gradient
• Columnar grains grow preferentially in the <100> direction

Question 12

Issues for Discs

Answer 12

• Overspeed
• Creep deformation

this is due to high stresses and temperatures. These can approach the monotonic tensile strength of the alloy

Question 13

How is Nickel alloy produced using Powder metallurgy?

Answer 13

• Atomize
• Sieve/consolidate
• Extrude
• Forge/heat treat
• Machine

Question 14

List of Ni based alloys

Answer 14

• Nim 75
• Nim 90
• Udimet 500
• Nim 105
• Udimet 700
• Rene 95