Classification of nickel alloys

Nickel alloys readily with many other metals, including chromium, iron, molybdenum, and copper. This results in various alloys that have excellent resistance to corrosion and high-temperature spalling, excellent high-temperature strength, and other unique properties, such as shape memory and low coefficient of expansion.

Below is a simple classification of the various nickel alloy types.

Wrought Nickel
Pure nickel UNS N02200 is used in the chemical industry for its corrosion resistance, especially to alkalis. It also finds applications for its properties in EMI shielding and sensors.

Nickel-Iron Alloys
These alloys are used as soft magnetic materials, glass-to-metal seals, and materials with specific thermal expansion properties.

Invar (UNS K93600), which contains 36% nickel and the remainder iron, is unique in that its coefficient of thermal expansion is almost zero at room temperature. This makes it valuable in areas where high dimensional stability is required, such as precision measuring instruments and thermostat stems. It can also be used in cryogenic environments due to its extremely low thermal expansion rate.
Alloys with 72-83% nickel have the best soft magnetic properties and are used in transformers, inductors, magnetic amplifiers, magnetic shielding, and storage devices.

Nickel-copper alloys
These alloys are very resistant to alkaline solutions, non-oxidizing salts, and seawater. The best known of these is Alloy 400.

Nickel-molybdenum alloys
These alloys are very resistant to reducing acids in the absence of oxidizing ions (such as iron and copper) or dissolved oxygen. The best known of these is Alloy B-2.

Nickel-chromium alloys
These alloys are characterized by their very high corrosion resistance (anti-scaling) at both room and elevated temperatures, good high-temperature strength, and high electrical resistance.

The alloys are divided into three main categories:
Nickel-chromium (and nickel-chromium-iron) alloys, which have high electrical resistance and are suitable for heating elements, such as Alloy 70-30 (UNS N06008) and Grade C alloys (UNS N06004);
Nickel-chromium alloys (containing iron and other alloying elements), which have good corrosion resistance. The most famous of these are Alloy 600 (UNS N06600) and Alloy 601 (UNS N06601);
Nickel-chromium alloys, which have high-temperature strength and creep resistance and are mostly age-hardenable, such as Alloy X-750 (UNS N07750);
Nickel-chromium-iron alloys
The alloys fall into two main categories:
Nickel-chromium-iron alloys, which have excellent high-temperature strength and resistance to oxidation, carburization, and other types of high-temperature corrosion. The most famous are Alloy 800 (UNS N08800) and its variants 800H (UNS N08810) and 800HT (UNS N08811). (More recently, these alloys have been classified as stainless steels, reflecting their high iron content.)
Nickel-chromium-iron (with molybdenum and copper) alloys, which have excellent corrosion resistance in specific applications. Probably the most famous is Alloy 825 (UNS N08825), which has excellent resistance to sulfuric acid. Alloy G-3 (UNS N06985) has excellent corrosion resistance to commercial phosphoric acid and many complex solutions containing strong oxidizing acids.

Nickel-Chromium-Molybdenum Alloys
These alloys have very strong corrosion resistance, the most famous of which is alloy C-276 (N10276). They have excellent corrosion resistance to reducing acids such as hydrochloric acid and sulfuric acid. Based on this composition, there are several variations that adjust the chromium and molybdenum content and in some cases add copper or tungsten to enhance its corrosion resistance in more oxidizing or reducing conditions. These variations include alloy C-22 (N06022), alloy 59 (N08059), alloy C-2000 (UNS N06200), and alloy 686 (N06686).

Nickel-Chromium-Cobalt Alloys
The addition of cobalt and molybdenum gives Alloy 617 (UNS N06617) solid solution strengthening and high creep rupture strength. Cobalt is added to HR-160 (N12160) to give it excellent resistance to various high temperature corrosions, such as sulfidation and chloride attack, in both reducing and oxidizing atmospheres.

Nickel-Titanium Alloys
55% Nickel-Titanium Alloy (UNS N01555), also known as Nitinol, has shape memory properties. When formed at one temperature and then deformed at a lower temperature, it returns to its original shape when heated again. The transformation temperature can be adjusted by careful control of the composition. Medical devices and specialized connectors are two special applications. The same alloy can also undergo considerable elastic deformation and still return to its original shape (superelastic property). This property has been used in a variety of applications, such as eyeglass frames and shock absorbers to provide earthquake resistance to historic stone buildings.