Case Study: Processing for creep resistance

Question 1

Outline the three different processing strategies used to produce creep resistant turbine blades.

Answer 1

1: Use of a creep-resistant alloy
2: Control of grain size during casting
3: Blade cooling

Question 2

Outline the use of Nickel-based superalloys for creep resistant materials (S1)

Answer 2

Nickel based alloys have a high melting temperature so can withstand high temperature applications.
High alloying content gives solid solution and precipitation hardening which reduces dislocation creep.
Increased dislocation resistance (through alloying) reduces the power law creep regime and moves the operating point to the diffusional flow regime significantly reducing strain rate.

Question 3

Explain why grain size is controlled during casting of creep resistant materials (S2)

Answer 3

The use of superalloys moves the operating point to the diffusional creep regime. The rate of diffusion creep is influenced by grain size and grain boundary area, so this must be minimised during casting.
Blades can be cast with only columnar grains (no chill zone or equiaxial grains) which maximises the diffusion distance. (Directional Solidification)
A further improvement is the eliminating grain boundaries entirely by casting the blade as a single crystal.

Question 4

Explain blade cooling and how it is used to in creep-resistant materials (S3)

Answer 4

One of the consequences of further alloying is the reduction in melting point (Melting point of Ni-Superalloy < Ni)
There are two ways to achieve blade cooling in order to ensure the the operating temp does not exceed 70% of the melting temperature:
Cool the blade from the inside
Add a Zirconia (Zr02) certain thermal barrier coating which acts as an insulating layer between hot gases and the Ni super alloy.