4: Nickel Alloys

Introduction to Nickel Alloys

• Nickel alloys are dense metals with a wide range of strengths.
• Pure nickel alloys are softer compared to nickel superalloys, which have high strength.
• On a specific strength basis, nickel alloys perform similarly to steels but not as well as aluminum, titanium, or magnesium alloys.

Usage of Nickel in Engineering

• Approximately 60% of produced nickel is used in stainless steels, primarily austenitic stainless steels.
• About 10% is utilized for alloying in alloy steels, 10% in nickel plating, and 13% in nickel-based alloys.

Corrosion Resistance and Strength

• Nickel alloys are crucial in environments needing corrosion resistance and high temperature stability.
• Key benefits include:
  • Moisture Resistance: Effective against neutral and seawater.
  • Chemical Resistance:
  • Pure nickel is resistant to caustics and high pH at elevated temperatures.
  • Specific nickel alloys vary in acid resistance.
  • Oxidation Resistance:
  • Nickel superalloys can withstand oxidation up to $1,100$ °C without significant mass loss.

Applications of Nickel Alloys

• Common applications include:
  • Aircraft gas turbines
  • Steam turbines
  • Reciprocating engines
  • Medical devices (dentistry, prosthetics)
  • Space vehicles
  • Heat treating equipment
  • Nuclear power control systems
  • Chemical processing (including pollution control)
  • Pulp and paper mills
• Nickel alloys are categorized by trade names instead of standard designations.

Classification of Nickel Alloys

• Nickel alloys fall into four main classes:
  1. Corrosion Resistant Alloys
  2. Super Alloys
  3. Electrical and Magnetic Alloys
  4. Thermal Expansion Alloys
• Superalloys: Strong and creep-resistant at high temperatures, resistant to oxidation and corrosion.

Specific Grades of Nickel Alloys

Commercially Pure Nickels

• Common grades: Nickel 200 and Nickel 201.

• Properties:

  • Excellent resistance to caustics and reducing environments.
  • Ferromagnetic when annealed; magnetic properties decrease with cold work.
  • Ductile with high work hardening capacity.

• Strength:

  • Yield strength: $20$ MPa
  • Ultimate tensile strength: $460$ MPa
  • Good formability, weldability, and challenging machinability due to work hardening characteristics.

DuraNickel 301

• A low alloy nickel similar to 8,000 series aluminums, designed for increased strength.
• Alloying elements: Aluminum and titanium for precipitation hardening.
• Properties:
• Cold finishing yields higher strength than hot finishing.
• Ultimate tensile strength can reach up to $1.2$ GPa with lower yield strength at high temperatures.

Monels

• Mix of nickel and copper (single-phase alloys) with varying electrical and magnetic properties based on composition.

• Excellent corrosion resistance in seawater and extreme environments.

• Often used for fasteners and marine applications:

  • Common grades: Monel 400 and Monel K-500.

• Monel 400:

  • Yield stress: $240$ MPa with ductility at room temperature.
  • Good weldability but variable magnetic properties.
  • Maintains corrosion resistance in diverse environments including acids and alkalis.

• Monel K-500:

  • Precipitation hardenable with higher yield stress: up to $790$ MPa.
  • Significant fatigue resistance suitable for fasteners in seawater.
  • Welding should be done before aging treatments.

Conclusion

• Understanding the various grades and types of nickel alloys is crucial for selecting appropriate materials for engineering applications, particularly in extreme environments.
• Each class and specific alloy has unique properties that cater to specific industrial needs, making nickel alloys versatile and valuable in multiple fields.