nickel alloys

Question 1

differences between nickel and iron

Answer 1

Lot of similar properties with iron But nickel is an FCC crystal structure and iron BCC this makes a difference, FCC is more workable and efficiently at high temperature and this lead to higher creep resistance

Question 2

Main fields of application for Ni and related alloys

Answer 2

• Ni is mainly found in stainless steels as alloying element
• Batteries (e.g. Ni-Cd) and Ni-plating
• high-temperature applications and in corrosive environments

Question 3

why are Ni-base alloys very useful for elevated temperature applications

Answer 3

• FCC crystal structure shows no phase transformation up to melting point and their thermally-activated processes controlling creep deformation are slow (→lower creep rate)
• we can create a lot of extended solid solutions (substantial solubility of alloying elements in the Ni matrix (gamma-phase))

Question 4

effects of Al, Ti, Nb, Ta in nickel alloys

Answer 4

They have greater atomic radius and promote the formation of the gamma’-phase

Question 5

effects of C, B and Zr in nickel alloys

Answer 5

they are Grain boundary strengtheners, they increase the creep performance preventing GB sliding

Question 6

effects of Cr, Mo, W, Nb, Ta and titanium in nickel alloys

Answer 6

they are strong carbide formers, Cr and Mo also promote the formation of borides

Question 7

Microstructure of Ni-based alloys

Answer 7

a) The gamma phase (FCC structure): it forms a continuous matrix phase with significant concentrations of elements in solid solution
b) The gamma’ phase: usually as precipitates in coherent form with the gamma-matrix and rich in elements such as Al, Ti and Ta. If Nb is present, the ordered gamma’’ phase (BCC) forms preferentially
c) Carbon, often present at concentrations up to 0.2 wt%, combines with reactive elements such as Ti, Ta and Hf to form carbides, typically at GBs.
d) B can combine with elements such as Cr or Mo to form borides, which reside on the gamma-grain boundaries.

Question 7

typical nickel alloys applications

Answer 7

• turbine blades for jet engines
• Materials for turbine blades are subjected to heavy thermal and mechanical loads in corrosive environments
• When we reach combustion stage temperatures increase very much (1800°C)
• in blades in the front part of the engine we use titanium not nickel because there temperature is not that high
• the principal stress is given by centrifugal force
• the temperature is the highest on tip of blades
• higher the high-temperature strength of the alloys, the better the efficiency of the turbine
• Columnar grains limit GB sliding under longitudinal loading
• Single crystal structure definitely avoids any concern with GBs (no grain coarsening, no GB sliding, no GB precipitation, ….)
• to keep the working temperature of blades as low as possible we use ceramic coatings as «thermal barriers» or to protect from corrosion attack (TBC= thermal barrier coating)
• Nickel alloys are very expensive, we use them just for very special applications (già nickel è costoso poi alloy con altri elementi particolari ancora di più)