What is Nickel

Nickel is a metal chemical element, located in group VIII of the fourth period. Its simple substance is a hard, ductile and ferromagnetic metal that can be highly polished and resistant to corrosion. Nickel is an iron-philic element. The earth's core is mainly composed of iron and nickel. Because this metal does not rust, it is regarded as silver by the indigenous people of Peru. The copper alloy containing nickel is called cupronickel and was used in China in 200 BC. Some of them even extend to Europe.

Physical Properties

Nickel is a silver-white metal with magnetism. It has good plasticity, ductility and medium hardness. It can be highly polished and it is resistant to corrosion. When nickel is dissolved in nitric acid, it turns green.

Chemical Properties

The chemical property of nickel is active, but it is more stable than that of iron. Nickel is difficult to oxidize in the air at room temperature, and it does not react with oxidant solutions including nitric acid. Nickel is insoluble in water. It can form a dense oxide film on the surface in humid air at room temperature, which can prevent the metal from continuing to oxidize. Hydrochloric acid, sulfuric acid, organic acids and alkaline solutions etch nickel very slowly.

Introduction to the Crystal Structure of Nickel

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

The Role of Nickel in Nickel-based Superalloys

The matrix of nickel-based superalloys is nickel. The reason why nickel is chosen as the matrix is that nickel itself has a stable face-centered cubic structure, and its third electronic layer is almost filled (as shown in figure 1), which allows it to dissolve more alloying elements for alloying while still maintaining the stability of the γ austenite phase. As shown in the figure 3, the face-centered cubic (FCC) structure has a higher degree of atomic aggregation (that is, consistency) than the body-centered cubic (BCC) structure, which makes it difficult for atoms to move due to mutual obstruction at high temperatures, so the face-centered cubic structure austenite has good high temperature strength. This is why almost all superalloys are austenitic, and among them, nickel-based superalloys are the most widely used.

The Role of Nickel in Iron-based Superalloys

Iron has the characteristics of isomeric transformation at different temperatures. Iron below 912°C is called α-Fe, which has a body-centered cubic structure. The iron at 912°C to 1394°C is called γ-Fe. At this time, the crystal structure of iron is transformed into a face-centered cubic structure. When it reaches above 1394°C until it melts, the iron becomes δ-Fe again, and the crystal structure at this time is again a body-centered cubic structure. Due to this characteristic of iron, it cannot maintain a face-centered cubic structure at any temperature. As just mentioned, only solid solution with a face-centered cubic structure, that is, austenite, has good high-temperature strength. Therefore, in order to stabilize the austenite structure of iron, it is necessary to add nickel element that can maintain the face-centered cubic structure at any temperature in the iron-based superalloy for alloying. Usually the nickel content in iron-based superalloys needs to reach 25%-50% to transform the ferrite matrix (body-centered cubic) into a stable austenite structure (face-centered cubic). But despite this, the stability of iron-based austenite is still not as good as nickel-based austenite, so iron-based alloys cannot be strengthened by adding too many solid solution strengthening elements and precipitation strengthening elements.

Studies have shown that increasing the nickel content in iron-based alloys from 40% to 50% is beneficial to the improvement of the tensile strength and yield strength of the alloy, but the plasticity is the best when the nickel content is 45%. The effect of nickel content on the alloy durability is as follows Picture:

The Role of Nickel in Cobalt-based Superalloys

Similar to iron, cobalt also has the property of isomeric transformation. Cobalt has a close-packed hexagonal structure at lower temperatures, so 20% nickel must be added to the cobalt-based superalloy to stabilize the austenite structure. Nickel also plays a role in reducing deformation resistance in cobalt-based alloys to improve processing performance. It should be noted that the content of nickel in cast cobalt-based superalloys is usually controlled below 10%, otherwise it will cause a decrease in durability.

Conclusion

In nickel-based superalloys, nickel as an important matrix can dissolve many alloying elements. In iron-based superalloys and cobalt-based superalloys, nickel stabilizes the austenitic structure of the alloy and improves high-temperature performance. In addition, in iron-based superalloys and nickel-based superalloys, nickel can also form precipitation strengthening phases with aluminum, titanium, niobium and other elements to precipitate the alloy.

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2021.3.5