What is Chromium

Chromium is a metal element, its chemical symbol is Cr, atomic number is 24, and belongs to group VIB in the periodic table of elements. The name of the element comes from Greek. Its original meaning is "color", that’s because chromium compounds have colors. The simple substance of chromium is steel gray metal, which is the hardest metal in nature. Natural chromium in free form is extremely rare. It mainly exists in chromium lead ore. According to its content in the earth's crust, chromium is one of the most widely distributed elements. It is more than cobalt, nickel, molybdenum, and tungsten previously discovered. This may be because the natural compounds of chromium are very stable, not easily soluble in water, and it is difficult to reduce.

Physical Properties

Chromium is a silver-white shiny metal. Pure chromium is ductile. Chromium containing impurities is hard and brittle. Chromium is soluble in strong alkaline solutions and has high corrosion resistance. It oxidizes slowly in the air even in the state of red heat. Chromium is insoluble in water. It can protect metal by plating on it.

Chemical Properties

Chromium can slowly dissolve in dilute hydrochloric acid and dilute sulfuric acid to form a blue solution. However, chromium is not soluble in concentrated nitric acid. It is passive because of the dense oxide film formed on its surface. At high temperatures, chromium can directly combine with halogens, sulfur, nitrogen, carbon, etc.

Introduction to the Crystal Structure of Chromium

The crystal structure of chromium is a body-centered cubic structure. Unlike iron, chromium has no allotropic transformation, and it can maintain a stable body-centered cubic structure at any temperature. The structure is shown in the figure 2:

The Role of Chromium in Superalloys

Chromium is an indispensable alloying element in superalloys. Almost all superalloys contain the metallic element chromium. Chromium plays a role in corrosion resistance and high temperature strengthening in superalloys.

Antioxidant Effect of Chromium

A very important role of chromium in the superalloy matrix is to form a Cr2O3 oxide film, so that the superalloy parts have good oxidation and corrosion resistance. Although the matrix composition of various superalloys is different and the difference in alloying elements is also large, chromium still has a significant effect on improving the oxidation resistance. The higher the chromium content, the better the oxidation resistance.

It can be seen from the figure that as the chromium content continues to increase, the maximum corrosion depth of the alloy in the hot corrosion environment continues to decrease, and the hot corrosion resistance continues to improve. So chromium is a very important anti-oxidation and anti-hot corrosion element. As long as the chromium content in the alloy is greater than 15%, even if the aluminum content is less than 5%, a dense and adherent Cr2O3 protective film can be formed on the surface of the superalloy. Therefore, nickel-based superalloys are actually nickel-chromium alloys, cobalt-based superalloys are actually cobalt-nickel-chromium or cobalt-chromium alloys, and iron-based superalloys are actually iron-nickel-chromium alloys.

Solid Solution Strengthening Effect of Chromium

In precipitation-strengthened nickel-based and iron-based superalloys, about one-tenth of the added chromium element enters the γ' phase (that is, the precipitation strengthening phase), and a small amount of chromium forms carbides, and most of the rest are dissolved in γ solid solution (solid solution strengthening phase). Due to the difference between the radii of chromium and nickel atoms, the chromium element in the γ matrix of the superalloy will cause lattice distortion and produce elastic stress field strengthening, which will increase the strength of γ solid solution and play a role of solid solution strengthening (as shown in the figure 4). At the same time, because chromium atoms have a body-centered cubic structure and nickel atoms have a face-centered cubic structure (as shown in the figure 5), Chromium also reduces the stacking fault energy of solid solution, which significantly improves the high-temperature durability.

Precipitation Strengthening Effect of Chromium

The chromium element dissolved in γ solid solution can also form a series of carbides with carbon. The carbon content in nickel-based and iron-based alloys is generally 0.05%-0.20%. Carbon and the active refractory metal elements titanium, tantalum, hafnium, niobium, etc. form MC-type carbides. They are decomposed and formed low carbon and high chromium compounds M23C6 or M6C during heat treatment or use. The compounds are mainly distributed in grain boundaries. In cobalt-based superalloys, the chromium content is 10%-30%. With the different ratio of chromium to carbon, chromium forms a series of carbides, which plays a role of precipitation strengthening. Carbides are the main strengthening phase of cobalt-based superalloys, so the carbon content in cobalt-based superalloys is relatively high, reaching 0.25%-1.0%. The chromium-rich carbides in cobalt-based superalloys include M3C2, M7C3 and M23C6.

Short-range Orderly Strengthening Effect of Chromium

In some nickel-based superalloys, when the chromium content is 20%-25%, such as Hastelloy X and Inconel 625 alloys, chromium atoms can also cause short-range orderly strengthening in the solid solution.

Conclusion

Chromium is an important strengthening element in superalloys, which brings excellent corrosion resistance to superalloys. In nickel-based superalloys and iron-based superalloys, chromium plays a role of solid solution strengthening. In cobalt-based superalloys, chromium plays a role of precipitation strengthening. Therefore, chromium is present in almost all superalloys.

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2021.03.23