Overview

Alloy refers to a metal material composed of multiple metal elements. Different proportions of metal elements are mixed to form different alloys. Conventional alloys are made by mixing and solidifying molten liquid metals. This process is called smelting.

Different alloys have different performances. On the one hand, this is because the composition of different alloys is different. On the other hand, different alloys often present different microscopic structures. These structures are also called alloy phases. Superalloys have complex chemical compositions, so they also have a wide variety of alloy phases inside. In this article, we will introduce in detail the different superalloy phases and their effects on superalloys.

What is Crystal Structure of Metal?

Before introducing the alloy phase, we must first understand the crystal structure of metals.

First of all, we must know that metals are a kind of crystal. Crystal refers to the material structure in which atoms are arranged according to certain rules. Since metals are crystals, the arrangement of their internal atoms is regular, and this regularity is called the crystal structure of metals. Common metal crystal structures are: body-centered cubic structure (BCC), face-centered cubic structure (FCC) and hexagonal close-packed structure (HCP).

Body-centered cubic structure is a cubic-shaped structure. Atoms are distributed at each vertex and geometric center of the cube. Iron at room temperature is a typical body-centered cubic structure. The following figure is a schematic diagram of the body-centered cubic structure:

Face-centered cubic structure is also a cubic-shaped structure. The difference is that its atoms are distributed at the vertices of the cube and the center of each face. The face-centered cubic structure is a structure that can exist stably at any temperature. The crystal of nickel is a stable face-centered cubic structure. This is why nickel is contained in superalloys. The following is a schematic diagram of the face-centered cubic structure:

The hexagonal close-packed structure is a more complex crystal structure. It presents a hexagonal arrangement. Cobalt is a typical hexagonal close-packed structure. This structure helps the alloy's wear resistance. Therefore, some wear-resistant alloys (such as Satellite Alloy) are cobalt-based alloys. The following is a schematic diagram of the hexagonal close-packed structure:

What is Alloy Phase?

Alloy phase refers to the various microstructures inside the alloy.

Since pure metals are elementary substance, their crystal structures are often simpler and multiple structures will not exist at the same time.

However, the composition of alloys is very complex. They not only contain metal components, but also some non-metallic trace elements. The combination of various elements forms a variety of crystal structures. These different structures are called different alloy phases. Different alloy phases can exist in an alloy at the same time.

Metal Crystal

Alloy Crystal

Alloy phases are generally produced during the cooling of the alloy. During smelting, the alloy is liquid at first. At this time, the atoms in the alloy are irregular. As the liquid alloy cools, the alloy gradually becomes solid. In this process, the atoms in the alloy begin to gather and form a certain arrangement rule. This process is called crystallization. During the crystallization process of superalloys, crystals with different structures are often produced at the same time. These different structures are called different phases. The process of producing these phases is called liquid phase transition.

In addition, alloys will also produce new phases during heat treatment. During heat treatment, the alloy is heated to a higher temperature and cooled at a certain rate. During the cooling process, the structure of the alloy will be reorganized. This process is called recrystallization. During the recrystallization process, new phases are sometimes produced. This phenomenon is called solid phase transition. The type and quantity of the phases that are finally precipitated during recrystallization are closely related to the heat treatment temperature and cooling rate.

The type of phase has a great influence on the performance of superalloys. Below, we will introduce the different types of phases in superalloys and their effects on superalloys.

Common Superalloy Phases

The chemical composition of superalloys is almost the most complex of all alloys. Therefore, the alloy phases in superalloys are also diverse. Among these alloy phases, some phases are helpful to the performance of the alloy, and they are called strengthening phases. Other phases have a negative effect on the alloy. Therefore, they are called harmful phases.

On the other hand, from the perspective of crystal structure, superalloy phases are divided into geometric close-packed phases and topological close-packed phases. The crystal structure of geometric close-packed phases is relatively simple. They are generally body-centered cubic structure (BCC), face-centered cubic structure (FCC) and hexagonal close-packed structure (HCP). The structure of topological close-packed phases is more complex.

Geometric Close-packed Phase

Most of the phases in superalloys are geometric close-packed phases. They have simple structures. There are both strengthening phases and harmful phases in these phases. Let's introduce the common geometric close-packed phases in superalloys.

γ Phase

γ phase is the matrix phase in superalloys and is a face-centered cubic structure. It is also the most numerous phase in superalloys, accounting for at least 30% of superalloys. The key to the formation of γ phase is nickel. Nickel maintains a face-centered cubic structure at any temperature. Therefore, it is indispensable in superalloys.

γ phase is crucial in superalloys because it can dissolve a large amount of alloying elements. This solid solution mechanism greatly improves the strength of the alloy. At the same time, the face-centered cubic structure also ensures that the superalloy has very good high-temperature resistance. At the same time, it also helps the alloy's resistance to oxidation and hot corrosion.

γ' Phase

γ' phase is the most important strengthening phase in superalloys. Its chemical formula is Ni3Al. As can be seen from the chemical formula, the composition of γ' phase is nickel and aluminum. The following is a schematic diagram of the structure of γ' phase:

γ' phase often exists in precipitation-strengthened superalloys. It can greatly improve the strength of the alloy. Below are typical examples of this type of alloy, you can see that they all contain aluminum.

The γ' phase can also dissolve other metal elements such as titanium, niobium, tantalum and tungsten. These elements can further enhance the strengthening effect of the γ' phase.

The γ' phase is generally obtained by the aging treatment. This heat treatment process is very slow because the γ' phase needs enough time to precipitate.

γ" Phase

The γ" phase is very similar to the γ' phase. It is also a strengthening phase and can also be obtained by aging treatment. The difference is that the γ" phase is composed of nickel and niobium. Its chemical formula is NiXNb. Below is a schematic diagram of the γ" phase:

The structure of γ" is body-centered tetragonal. You can think of it as a combination of two different face-centered cubic structures. The strengthening effect of the γ" phase is more obvious than that of γ'. However, its strengthening temperature range is narrower than that of γ'. When the temperature reaches a certain level, the strengthening effect of the γ" phase will fail.

η Phase

The chemical formula of the η phase is Ni3Ti. It has a hexagonal close-packed structure. As mentioned above, titanium can enhance the strengthening effect of the γ' phase. Therefore, most precipitation strengthened alloys will add titanium. However, if the titanium content is too high relative to the aluminum content, the γ' phase will become unstable, and the η phase will be formed.

η has both positive and negative effects on the properties of the alloy. In some specific alloys (such as Incoloy A-286), a small amount of η phase is conducive to grain refinement. It gives the alloy good comprehensive properties.

However, in most cases, η phase is a harmful phase. It will reduce the plasticity, instantaneous tensile properties and endurance of the alloy.

β Phase

The chemical formula of β phase is NiAl. It has a body-centered cubic structure. When the aluminum content in the alloy is too much, β phase will precipitate. Therefore, it is also a phase that will appear in precipitation-strengthened alloys.

β phase is a harmful phase. It easily leads to the formation of cracks. At the same time, it reduces the tensile strength and plasticity of the alloy.

α Phase

α phase has a face-centered cubic structure, and its chemical formula is Ni2AlTi. This harmful phase will also reduce the tensile strength and plasticity of the alloy. However, it does not appear in most superalloys.

δ Phase

The chemical formula of δ phase is Ni3Nb. It is transformed from γ" phase. Similar to η phase, a small amount of δ phase can also have the effect of refining grains. However, a large amount of δ phase will also reduce the performance of the alloy.

ε Phase and ε" Phase

ε and ε" phases are mainly found in low expansion superalloys (such as Incoloy 909). Their chemical formula is (NiFeCo)3(NbTi). Low expansion superalloys generally have a large amount of cobalt, and they are also precipitation strengthened alloys. These factors meet the conditions for the formation of ε and ε" phases.

ε phase is a hexagonal close-packed structure. ε" phase is a transition phase in the process of transformation from γ' phase to ε phase, and it still retains the face-centered cubic structure. It is worth noting that the silicon content promotes the formation of ε" phase.

For low expansion superalloys, ε and ε" phases are very important strengthening phases. They improve the notch rupture life of the alloy. However, ε and ε" need to be controlled. If their amount is too much, niobium and titanium in the alloy will be consumed, which will reduce the amount of γ' phase and thus reduce the strengthening effect of the alloy.

Topological Close-packed Phase

The structure of topological close-packed phase is much more complex than that of geometric close-packed phase. Almost all topological close-packed phases are harmful phases. Below, we introduce three common topological close-packed phases.

σ Phase

σ phase is a compound. Each unit cell contains 30 atoms. In iron-based superalloys, σ phase is FeCr type. In nickel-based superalloys, σ phase is (Cr, Mo)(Ni, Co) type. Therefore, when the chromium and molybdenum content in the superalloy is high, σ phase is easier to precipitate.

In superalloys, σ phase will cause crack formation, thereby greatly reducing the strength, plasticity and creep rupture time of the alloy.

Laves Phase

Laves phase is an AB2 type compound. Among them, the atomic diameter of A is larger (such as titanium, niobium, tantalum, molybdenum and tungsten), while the atomic diameter of B is smaller (such as iron, cobalt, nickel). When the content of elements such as chromium, titanium, molybdenum, tungsten, and niobium exceeds the solubility of the alloy matrix, Laves phase is easy to precipitate.

Laves phase can also cause the formation and expansion of cracks. It will significantly reduce the alloy's endurance strength and room temperature plasticity.

μ Phase

μ phase has a rhombic crystal structure. It is commonly found in iron-molybdenum alloys and iron-chromium-molybdenum alloys. Its chemical formula is A7B6 type. Such as Fe7Mo6, Fe7W6, Co7W6, etc. Like the first two phases, μ phase can also cause crack formation.

FAQ

Further Reading

What is Solid Solution? Why do Nickel Alloy / Superalloy need Solution Treatment?

What is Precipitation Strengthening & Aging Treatment. Which Alloys need Aging Treatment?

Fully Understand What is Austenite Structure of Alloy

Conclusion

Alloy phase refers to the various microstructures present in the alloy. Various alloy phases can exist in the same alloy at the same time. The types of alloy phases will also vary according to different alloys. The precipitation of alloy phases is related to the smelting and heat treatment process of the alloy.

In the alloy phase, the strengthening phase can strengthen the performance of the alloy, while the harmful phase will reduce the performance of the alloy. In superalloys, common strengthening phases are γ' phase, γ" phase, ε phase and ε" phase; common harmful phases are η phase, δ phase, σ phase, Laves phase, etc.

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