Ni alloys

Question 1

General properties

Answer 1

• crystal structure: FCC
• Density; high
• Melting T: high
• Thermal cond: medium-low
• elastic mod: high

Question 2

Main applications of Ni

Answer 2

• Ni is mainly used in stainless steels
• Batteries
• Ni plating (placcatura)

Question 3

Physical metallurgy of Ni

Answer 3

Some distinctive physica factors male Ni based alloys very useful for elevated temp applications as:
- FCC shows no phase transformations up to melting point
- low creep rate
- Substantial solubility of alloying elements in the Ni matrix.
- Some elements as Al,Ti promote the formation of GAMMA phase
- The creep rate is improved by some boundary strengtheners elements as C, B.
- Cr, Mo form some strong carbides.

Question 4

Microstructure of Ni-based alloys

Answer 4

• GAMMA phase with FCC structure forms a continous matrix phase with a significant concentration of elements in the solid stste solution.
• GAMMA’ phase has precipitates as Al,Ti, Ta.
• GAMMA’’ pahse forms a BCC structure.
• C (up to 0.2%) forms carbides at GBs.
• B can combine with elements such as Cr or Mo to form borides at GAMMA’ phase at GBs.

Question 5

Applications: turbine blades

Answer 5

• Turbine blades are subjected to high temp and centrfugal force.
• Service conditions change along the turbine axis.
• The max temp is at the blade tip.
• The higher the high temp strength of the alloys the better the efficincy of the turbine.

Question 6

Design strategies for very high temp

Answer 6

The main problem with high temp is creep:
- To limit the GBs sliding we can use columnar grains.
- Single crystal structure definitely avoids any concern with GBs.
- Another strategy is to cover the blade with a coating of ceramic material.