Superalloys - Strengthening and creep behaviour

Question 1

What are the three mechanisms for strengthening superalloys?

Answer 1

• Grain size - smaller grains = strength
• Solid solution strengthening of gamma phase - hinders dislocations
• Precipitation hardening from the gamma prime phase.

Question 2

What are the 5 precipitate hardening methods for super alloys?

Answer 2

• Increase gamma prime volume fraction
• Solid solution strengthen gamma prime
• order strengthening (increases anti phase boundary energy)
• precipitate size (smaller size with variation)
• coherancy strengthening (lattice misfit increases strength)

Question 3

What are the two categories of creep?

Answer 3

Dislocation creep (at high T and stress)
Diffusional creep (across grain boundaries or bulk transfer)

Question 4

What is dislocation creep and it’s characteristics?

Answer 4

Dislocation creep is the movement of dislocations aided by vacancy diffusion. This is a steady state process and the dislocation density remains constant.

Question 5

What is diffusional creep and it’s characteristics?

Answer 5

Deformation casued by the migration of vacancies.
Split up into two types:
Coble Creep - flow of vacancies between grain boundaries from tension to compression.
Nabarro-Herring Creep - Bulk diffusion of vacancies.

Question 6

How can superalloys be creep strengthened?

Answer 6

Most normal strength mechanisms work with the EXCEPTION of grain refinement due to Coble creep.

Question 7

What is the general rule with Coble creep?

Answer 7

The smaller the grains the higher the creep strain rate.