A nickel alloy is a metal that contains a percentage of nickel in its elemental makeup. Nickel is primarily alloyed with chromium, copper, iron, titanium, and molybdenum. For example, Inconel® has excellent corrosion, oxidation, and high-temperature resistance. Most nickel alloys exhibit good corrosion, oxidation, and high-temperature strength properties, with some exceptions. Nickel-iron alloys do not have the same levels of corrosion and oxidation resistance. Nickel alloys are often used in extreme working environments, such as those encountered in the aerospace, chemical processing, and petroleum industries but can also be used in electrical and electronics applications.

This article will describe what a nickel alloy is, where it is used, it's characteristics and physical properties, as well as the different types of nickel alloys and their uses. 

What Is a Nickel Alloy?

The term nickel alloy refers to a metal that has nickel as one of its primary elements. Some types of nickel alloys are referred to as superalloys because of their superior oxidation and creep resistance, allowing them to be used at temperatures of more than half their melting points. Nickel alloys can be machined and welded but tend to pose some processing difficulties, as some alloys will work harden during machining, and their high melting points can make them difficult to weld. 

How Nickel Alloys Are Made

Nickel alloys are typically made from a mixture of various metals and nickel. Although not all metals can be effectively combined with nickel. Some of the most common elements that can be alloyed with nickel are iron (Fe), chromium (Cr), aluminum (Al), molybdenum (Mo), copper (Cu), cobalt (Co), and titanium (Ti). These elements can be combined to produce alloys with different properties. For example, nickel, iron, molybdenum, and chromium alloys, such as stainless steel Type 316, have excellent corrosion resistance. 

Nickel alloys are made with the same process used for most other metal alloys. The alloying elements must be chosen and their ratios must be confirmed. Next, the elements are all melted together in an arc furnace, for example. During smelting, the alloys are also purified. The nickel alloy is then cast into ingots after which it is formed using cold or hot working techniques. 

Characteristics of Nickel Alloys?

Listed below are some common characteristics of nickel alloys:

1. Corrosion Resistance: Nickel alloys have excellent resistance to corrosion and oxidation, making them ideal for use in harsh environments.
2. High Strength: Nickel alloys can have high strength. For that reason, they are especially beneficial where high strength and toughness are needed at elevated temperatures.
3. Heat Resistance: Nickel alloys have excellent heat resistance and can continuously withstand temperatures as high as 980 °C, in the case of Inconel® 718.
4. Ductility: Nickel alloys are highly ductile, and as such are very tough materials, able to withstand impacts without breaking suddenly. 
5. Magnetic Properties: Nickel alloys, especially those with very high levels of nickel have excellent soft magnetic properties, which means they can be easily magnetized and demagnetized. Nickel alloys can be said to have low coercivity (a measure of the difficulty of demagnetizing a magnetic material).
6. Weldability: Nickel alloys can be welded using the MIG (metal inert gas) or TIG (tungsten inert gas) processes. They can also be joined using MMA (manual metal arc) welding. Nickel alloys can be difficult to weld, and issues such as hot or warm cracking and porosity are common.
7. Thermal Conductivity: Nickel alloys do not have particularly good thermal conductivity. However, low thermal conductivity is often desired in high-temperature applications to limit heat transfer to other components. However, nickel-copper alloys can be used if thermally conductive behavior is required.
8. Electrical Conductivity: Nickel alloys have lower electrical conductivity than pure copper or silver. Nickel copper alloys do, however, have improved electrical conductivity along with increased corrosion resistance. 
9. Low Thermal Expansion Coefficient: Nickel alloys typically have low thermal expansion coefficients. This makes them ideal for precision applications that must maintain dimensional stability over a range of temperatures. 
10. Color: The color of a nickel alloy depends entirely on its specific composition. Natural nickel has a silver-white appearance, and nickel alloys will have a similar color depending on their nickel content. Electroless nickel coatings can have a golden-brown appearance due to the presence of phosphorus in the coating. 

In general, it may be difficult to differentiate nickel alloys from other metals which also have a metallic appearance. Nickel alloys can have a silver-white appearance, but this is highly dependent on the surface finish and composition of the alloy. A rough surface will give a dull appearance, whereas a smooth surface may appear reflective.

Different Types of Nickel Alloys

There is a wide range of nickel alloys available. Some of the common grades are listed below:

1. Inconel®

Inconel® is a trade name for a type of nickel alloy that contains chromium, molybdenum, and iron as its main alloying elements. It is known for its oxidation resistance, excellent performance in a high-temperature environment, and mechanical strength. Inconel® can be difficult to cold form due to its rapid work-hardening behavior. Typical alloys are Inconel 625, 718, and 600. 

2. Monel®

Monel® is a trade name for a group of copper-based nickel alloys. Monel has high strength and excellent saltwater resistance, as well as the ability to perform well in low-temperature environments, i.e there is not a large reduction in ductility and there is an increase in hardness and strength. Monel® tends to work harden during cold forming, making it difficult to process. Typical alloys are Monel® 400, R-405, and K-500. 

3. Hastelloy®

Hastelloy® is the trade name for a highly corrosion-resistant alloy of either a nickel-chromium alloy or a nickel-molybdenum alloy. In addition, Hastelloy® also has good thermal stability and is used in the aerospace and chemical processing industries. Typical alloys include C-22 and C-276.

4. Nickel-Copper Alloys

Nickel-copper alloys have excellent saltwater corrosion and fouling resistance. For that reason, they are used extensively in the marine industry. Iron, tin, and manganese are sometimes added to further improve corrosion resistance. The copper content can be anywhere from 55 to 91.5%, with nickel being the next largest alloying element. The other alloying elements make up a small percentage of the total weight. Monel® is considered a nickel-copper alloy. 

5. Nickel-Iron Alloys

Nickel-iron alloys have good ductility, low thermal expansion, and corrosion resistance. These alloys also have soft magnetic properties. These alloys are used in applications where low thermal expansion is desired like sensitive scientific equipment or where soft magnetic properties are beneficial, for example in transformers. A common trade name for a nickel-iron alloy is Invar®. 

6. Nickel-Chromium Alloys

Nickel chromium alloys have excellent corrosion and oxidation resistance while also maintaining their strength at elevated temperatures. They also have high electrical resistivity. This makes nickel-chromium alloys especially fit for application as heater elements, such as those found in ovens or toasters, for example. Nickel chromium alloys can have a melting point of over 1300 °C. These alloys typically have a high nickel content, 70%.

7. Nickel-Titanium Alloys

Nickel-titanium alloys have excellent corrosion resistance and good mechanical strength. However, the most interesting property is that they are a type of shape memory alloy (SMA). They can be formed into a specific shape above a certain high temperature, called the transition temperature. When cool, the part can be deformed to a certain degree, but if it is heated back up to its transition temperature, it will resume its original shape. The exact temperature at which this change takes place can be adjusted by changing the composition of the alloy. These alloys are especially useful in medical applications. For example, stents can be inserted in their compressed form but will expand to their final shape once in the body. Nickel-titanium alloys are also known by the name Nitinol®. 

8. Nickel-Molybdenum Alloys

Nickel molybdenum alloys have excellent corrosion and oxidation resistance at high temperatures. They also maintain their mechanical properties at elevated temperatures. A common nickel-molybdenum alloy is also known as Hastelloy®. This nickel alloy is often used for heat exchangers, pumps, and reactor pressure vessels.