Superalloys - Microstructure

Question 1

What are the two phases on Ni based supperalloys?

Answer 1

Gamma phase - this is solid state Ni so its stable FCC structure.
Gamma prime phase - this is Tri-Nickel Alumnide precipitate , it has an ordered simple cubic structure.

Question 2

What can the addition of Cr, Chromium do for superalloys?

Answer 2

Increase the rate of formation of gamma prime but also increase environmental resistance. Reduces the amount of Al required to produce the same amount of gamma prime.

Question 3

What can the addition of Ti, Titanium do to superalloys?

Answer 3

Increase the strength of the gamma prime phase.

Question 4

What elements can be used to strengthen the gamma, solid state Ni phase?

Answer 4

Co, Cr, Mo, W, Fe

Question 5

What elements can be used to strengthen the gamma prime precipitate phase?

Answer 5

Al, Ti, Ta, Nb, Hf

Question 6

Which elements are used as grain boundary strengtheners?

Answer 6

C, B, Zr

Question 7

How does the grain boundary strengthening work?

Answer 7

The elements partition to the grain boundary. At the GB they can form carbides/borides however the morphology is important

Question 8

What are some detrimental phases that can form?

Answer 8

Topologically closed packed phases can form during service - they degreade mechanical properties and remove Cr from the matrix.
Geometrically closed packed phase - these form at the expense of gamma prime which reduces mechanical properties however can be useful in strengthening grain boundaries.