High Temperature Nickel Alloys
Nickel is the fifth most common element on Earth, but until the last century its commercial use was limited due to the difficulty of mining and refining. The development of jet engines was an important catalyst for the development of nickel-based alloys that are able to maintain high strength at high temperatures.
Nickel has a melting point of 1453 degrees Celsius, which is much higher than metals such as copper (1084 degrees Celsius) and aluminum (660 degrees Celsius), but much lower than metals such as tungsten (3400 degrees Celsius). However, high temperature performance is not only about the melting point of the metal, otherwise iron (1150 degrees Celsius) or steel (1400 degrees Celsius) would be more widely used. Another property of nickel and nickel alloys is their ability to form a thick and stable passive oxide layer when heated, which protects them from further corrosion, which allows them to operate at high temperatures. This oxide layer can be up to several microns thick, depending on the temperature and environment the metal is exposed to. Nickel alloys are also resistant to carburization, which is the presence of carbon at high temperatures, such as during the cracking of gases in various chemical processing or refining operations.
Strengthening Mechanisms
Depending on the specific alloy, its high-temperature strength is maintained by solid solution strengthening or precipitation strengthening. The mechanism of solid solution strengthening is the addition of atoms of alloying elements to the nickel's crystal lattice. This perturbation of the crystal structure makes deformation more difficult by slowing or preventing the movement of "dislocations." Elements such as molybdenum are added to Inconel 625 (Alloy 625, UNS N06625, 2.4856) to increase strength.
In precipitation strengthening, small amounts of niobium, titanium, and aluminum are combined with nickel to form intermetallic precipitates. These precipitates are formed during the final heat treatment, also known as aging. These precipitates also slow the movement of dislocations in the crystal structure, which increases strength and toughness. At higher temperatures, this mechanism also reduces the likelihood of creep, which is well utilized in aerospace applications. Nickel alloys that utilize precipitation strengthening include Inconel 718 (Alloy 718, UNS N07718, 2.4668), Inconel 725 (Alloy 725, UNS N07725), Incoloy 925 (Alloy 925, UNS N09925), and Monel K-500 (Alloy K-500, UNS N05500, 2.4375).
Alloys such as Inconel 718 and Inconel 625 retain most of their mechanical properties at temperatures up to 650 degrees Celsius (1200 degrees Fahrenheit), but can selectively be used at temperatures approaching 1000 degrees Celsius (1800 degrees Fahrenheit).
Because nickel alloys readily with many other metals, it can improve corrosion resistance and other physical properties in addition to strength. Chromium and molybdenum are often added to enhance corrosion and oxidation resistance, with molybdenum in particular being considered to improve pitting resistance. Copper is also used in Incoloy 825 alloy to enhance its resistance to reducing acids such as sulfuric, phosphoric, and hydrochloric acids.
