The melting temperature of nickel is 1453degC, which is far higher than metals such as copper (1084degC) and aluminium (660degC), but far lower than metals such as tungsten (3400degC). The ability to operate at elevated temperatures though is not only linked to a metals melting temperature, otherwise iron (1150degC) or steel (1400degC) would be more widely used. The additional property of nickel and nickel alloys which allows them to operate at elevated temperatures is their ability to form a thick, stable, passivating oxide layer when heated, which protects them from further attack. This oxide layer can be several microns in thickness, dependent upon the temperature and environment to which the metal has been exposed. Nickel alloys can also resist carburisation, where carbon species are present at high temperature, for instance during the cracking of gas in various chemical processing or refinery operations.
The high-temperature strength is retained by solid solution strengthening or precipitation strengthening depending upon the particular alloy. The mechanism of solid solution strengthening works by adding atoms of the alloying element to the crystal lattice of nickel. This disturbance in the crystal structure makes deformation more difficult by slowing down or blocking the movement of ‘dislocations’. Elements such as molybdenum are added in Inconel 625 (Alloy 625, UNS N06625, 2.4856), to improve the level of strength attained.
In precipitation strengthening, small amounts of niobium, titanium and aluminium combine with nickel to form intermetallic precipitates. These precipitates are formed during a final heat treatment process known also as ageing. The precipitates also slow down the movement of dislocations in the crystal structure increase the strength and toughness. At higher temperatures this mechanism also reduces the likelihood for creep to occur, which is exploited in aerospace applications. Nickel alloys that utilise 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 much of their mechanical properties to 650degC (1200degF) but can be selectively used at temperatures approaching 1000degC (1800degF).
As nickel will alloy readily with many other metals, it is possible to improve the level of corrosion resistance and other physical properties besides strength. Chromium and molybdenum are commonly added to enhance resistance to corrosion and oxidation, with molybdenum well understood to improve resistance to pitting corrosion in particular. Copper is also used in Incoloy 825 to enhance resistance to reducing acids such as sulphuric acid, phosphoric acid and hydrochloric acid.
Inconel, Incoloy and Monel are registered trademarks of Special Metals Corporation.