The Role and Grades of Alloying Elements

Nickel-based alloys are the most widely used and strongest class of high-temperature alloys. The addition of a relatively large amount of Ni acts as a stabilizer for the ferroferrite phase, allowing nickel-based alloys to maintain their free-cell (FCC) structure while dissolving more of other alloying elements, and maintaining good structural stability and material plasticity. Cr, Mo, and Al, on the other hand, provide antioxidant and corrosion-resistant properties, as well as some strengthening effects. Strengthening of nickel-based alloys can be categorized based on the mechanism of element action:

(1) Solid solution strengthening elements, such as W, Mo, Co, Cr, and V, strengthen the material by creating localized lattice strain in the Ni-Fe matrix due to the difference in atomic radii between these elements and the substrate.

(2) Precipitation strengthening elements, such as Al, Ti, Nb, and Ta, can form integrated and ordered A3B-type intermetallic compounds, such as Ni3(Al,Ti) strengthening phases (γ’), effectively strengthening the alloy and achieving higher high-temperature strength than iron-based and cobalt-based alloys.

(3) Grain boundary strengthening elements, such as B, Zr, Mg, and rare earth elements, enhance the high-temperature properties of the alloy. Generally, nickel-based alloy grades are named by the manufacturer. For example, Ni-Cu alloys are also known as Monel alloys, commonly such as Monel 400 and K-500. Ni-Cr alloys are generally called Inconel alloys, which are common nickel-based heat-resistant alloys mainly used in oxidizing media, commonly such as Inconel 600 and 625. If a higher amount of Fe is added to an Inconel alloy to replace Ni, it becomes an Incoloy alloy. Its high-temperature resistance is not as good as nickel-based precipitation-hardening alloys, but it is cheaper and can be used in components of jet engines operating at lower temperatures, as well as petrochemical plant reactors, such as Incoloy 800H and 825. If precipitation-strengthening elements such as Ti, Al, and Nb are added to Inconel and Incoloy, it becomes a precipitation-hardening (iron) nickel-based alloy. It retains good mechanical strength and corrosion resistance at high temperatures and is often used in jet engine components, such as Inconel 718 and Incoloy A-286. Ni-Cr-Mo(-W)(-Cu) alloys are called Hastelloy alloys, with Ni-Cr-Mo primarily used under reducing media corrosion conditions. Representative Hastelloy grades include C-276 and C-2000. A comparison of major nickel-based alloy grades and added elements can be found in Figure 2.