Nickel aluminide

Nickel aluminide (Ni₃Al) is an intermetallic alloy of nickel and aluminum with properties similar to both a ceramic and a metal.

There are three materials called nickel aluminide:

• NiAl, CAS number 12003-78-0 (see also Raney nickel)
• NiAl₃, CAS number 12004-71-6
• Ni₃Al, tri-nickel aluminide

An intermetallic compound can be defined as an ordered alloy phase formed between two metallic elements, where an alloy phase is ordered if two or more sublattices are required to describe its atomic structure. The ordered structure exhibits superior elevated-temperature properties because of the long-range ordered superlattice, which reduces dislocation mobility and diffusion processes at elevated temperatures.^[1]

Nickel aluminide is used as a strengthening constituent in high-temperature nickel-base superalloys, however, unalloyed nickel aluminide has a tendency to exhibit brittle fracture and low ductility at ambient temperatures.^[2] Nickel aluminide is unique in that it has very high thermal conductivity combined with high strength at high temperature. These properties, combined with its high strength and low density, make it ideal for special applications like coating blades in gas turbines and jet engines.

In 2005, the most abrasion-resistant material was reportedly created by embedding diamonds in a matrix of nickel aluminide.^[3]

IC-221M

An alloy of Ni₃Al, known as IC-221M, is made up of nickel aluminide combined with several other metals including chromium, molybdenum, zirconium and boron. Adding boron increases the ductility of the alloy by positively altering the grain boundary chemistry and promoting grain refinement. The Hall-Petch parameters for this material were σ_y = 163 MPa and k_y = 8.2 MPaˑcm^1/2.^[4] Boron increases the hardness of bulk Ni₃Al by a similar mechanism.

This alloy is extremely strong for its weight, five times stronger than common SAE 304 stainless steel. Unlike most alloys, IC-221M increases in strength from room temperature up to 800 °C.

The alloy is very resistant to heat and corrosion, and finds use in heat-treating furnaces and other applications where its longer lifespan and reduced corrosion give it an advantage over stainless steel.^[5]

Properties

• Density = 7.16 g/cm³
• Yield Strength = 855 MPa
• Hardness = HRC 12
• Thermal Conductivity Ni₃Al = 28.85 (W/m.K)^[6]
• Thermal Conductivity NiAl = 76 (W/m.K) ^[6]
• Melting Point Ni₃Al = 1668 K
• Melting Point NiAl = 1955 K
• Thermal expansion coefficient = 12.5 (10⁻⁶/K⁻¹)
• Bonding = covalent/metallic
• Electrical resistivity = 32.59 (10⁻⁸Ωm)

References

1. Vinod K. Sikka, Intermetallic-based high-temperature materials, Metals and Ceramics Division, Oak Ridge National Laboratory, U.S. Department of Energy, March 1999
2. "ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys", p. 104
3. Scientists Develop Nickel Aluminide Composite Material that Can Cut Through Cast Iron and Granite
4. Liu, C. T.; White, C. L.; Horton, J. A. (1985). "Effect of boron on grain-boundaries in Ni₃Al". Acta Met. 33 (2): 213–229. doi:10.1016/0001-6160(85)90139-7.
5. Crawford, Gerald (April 2003). "Exotic Alloy Finds Niche". Nickel magazine. Retrieved 2006-12-19.
6. Dey, G. K. (2003). "Physical Metallurgy of Nickel Aluminides" (PDF). Sadhana. 28 (Parts 1 & 2): 247–262. doi:10.1007/bf02717135. Retrieved 2014-03-05.