Overview

There are many different ways of strengthening superalloys. In the last article on solid solution, we introduced the principle of solid solution strengthening and common solid solution strengthening alloys. In this article, we will continue to introduce precipitation strengthening and precipitation strengthened alloys.

What is Precipitation Strengthening

Metallographic Structure of Nickel Alloy

We have mentioned in many previous articles that the crystal structure of all nickel alloys is a face-centered cubic structure. And nickel alloy is based on nickel element. This face-centered cubic structure composed of nickel as the matrix is called the γ phase. The arrangement of γ phase is shown in the figure below:

Figure 1
Structure of γ Phase

What is Precipitation Strengthening

Among the superalloys, there is a class of alloys called precipitation strengthened superalloys. This alloy will precipitate other strengthening phases different from the γ phase during the heat treatment process.

In the article introducing solid solution, we mentioned that solid solution can be analogous to the dissolution of salt in water, but this dissolution is the dissolution in the solid state. Then the precipitation of the precipitation phase can be analogous to the precipitation of salt crystals during the cooling process of hot brine. Of course, this precipitation is also the precipitation in the solid state.

Why Precipitation Strengthening is Needed

Superalloys are mainly used in high temperature applications. And high-temperature applications are also divided into many types. For example, in an airplane engine, all components must work at high temperatures. However, the functions of the engine's casing and the engine's blades are obviously different.

The engine's casing only needs to have good corrosion resistance at high temperatures and have a certain strength. Under this kind of application, solid solution strengthened superalloys can do the job well.

However, for the blades of the engine, we need a stronger alloy. Because the blade has a large rotation speed during operation, the high rotation speed brings a large centrifugal force. Therefore, precipitation strengthened alloys with superior high-temperature strength have been developed to be used in applications that not only require corrosion resistance, but also resist great stress at high temperatures.

The following table shows the comparison of the tensile strength of some common solid solution strengthened alloys (solution treatment) and precipitation strengthened alloys (ageing treatment).

It can be clearly seen that the strength of all precipitation-strengthened alloys is relatively much higher.

Precipitation Strengthening Phase

As mentioned above, precipitation strengthening is the process of precipitation of other strengthening phases from the γ-phase matrix. The main strengthening phases are γ' and γ" phases.

γ' Phase

The γ' phase is a metallurgical phase composed of Ni3Al, an intermetallic compound. Its structure is also a face-centered cubic crystal. But the composition changed from nickel to Ni3Al. The following figures show the structural differences between the γ phase and the γ' phase.

Figure 2
The Structural Differences Between the γ Phase and the γ' Phase

The γ' phase is the most important precipitation strengthening phase in superalloys, and it can bring a very significant increase in strength to the alloy at high temperatures.

γ" Phase

The γ" phase is a precipitation strengthening phase composed of Ni3Nb. Its structure is body-centered tetragonal crystal. The figure below shows the structure comparison of γ phase, γ' phase and γ" phase.

Figure 3
The Structure Comparison of γ Phase, γ' Phase and γ" Phase

The medium and low temperature strength of the γ" phase is very high, but when the temperature further rises, the strength of the γ" phase will become lower and lower, and the precipitation strengthening effect will become less and less obvious.

The Mechanism of Precipitation Strengthening

Below we will illustrate the three main principles of precipitation strengthening by way of illustrations.

Strengthening Caused by Differences in Lattice Constants

In the previous article, we mentioned that the deformation of the alloy is microscopically the sliding between the crystal layers (also called displacement motion). Both the precipitation strengthening phase γ' phase and the matrix γ phase are face-centered cubic crystals. But their lattice constants (that is, the side length of a single lattice) are different. This hinders the displacement movement of the crystal to a certain extent, improves the ability to resist deformation, that is, increases the strength. As shown below:

Figure 4
Differences In Lattice Constants

In fact, many other elements can also enter the γ' phase. This will further increase the difference between the lattice constant of the γ' phase and the matrix (also called lattice mismatch). This can further increase the strength. The figure below shows the relationship between the degree of lattice mismatch and the hardness of the alloy.

Figure 5
Relationship Between the Degree of Lattice Mismatch and the Hardness

However, in a high-temperature environment, the γ' phase of an alloy with too high a lattice mismatch will become unstable. Therefore, alloys with a smaller degree of mismatch can be selected at high temperatures.

Strengthening Caused by the Bypass Mechanism of the Strengthening Phase

When the intensity of the γ' strengthening phase itself is high, the displacement movement itself becomes difficult. If the matrix needs to continue to move, then it has to bypass the γ' phase. The matrix that bypasses the γ' phase will bend and squeeze. This squeezing will increase the force between atoms and make displacement more difficult.

Figure 6
Bypass Mechanism

Strengthening Caused by the Cutting Mechanism of the Strengthening Phase

In another case, the intensity of the γ' phase itself is not very high. In this case, the matrix can realize displacement movement by cutting the γ' phase. However, the γ' phase is a very stable structure, and a lot of energy needs to be overcome when cutting to destroy this structure. This hinders the occurrence of displacement movement.

Figure 7
Cutting Mechanism

Common Precipitation Strengthening Elements

Here are some common precipitation strengthening elements and their main reasons for strengthening.

Aluminum

Aluminum is the most important element in precipitation strengthening, and it is also the basic element in the γ' phase. This is why all precipitation strengthened superalloys contain aluminum.

In superalloys, about 20% of aluminum will enter the γ matrix to play a solid solution strengthening effect. The remaining 80% will form Ni3Al for precipitation strengthening.

The aluminum content in the alloy also significantly affects the properties of the alloy. The more aluminum content, the more γ' precipitation phase in the alloy, and the higher the strength of the alloy. However, if the aluminum content is too high, it will lead to the precipitation of harmful phases, which will adversely affect the performance of the alloy. The figure below illustrates the effect of aluminum content on alloy properties.

Figure 8
Effect of Aluminum Content on Alloy Properties

Titanium

In the superalloy, about 10% of titanium has a solid solution strengthening effect, and the remaining 90% has a precipitation strengthening effect. In the γ' phase, the titanium atom can act as the position of the aluminum atom. Therefore, an increase in the content of titanium can also increase the amount of γ' precipitation strengthening phase. The figure below shows the effect of titanium content on the yield strength of the alloy.

In addition, the ratio of titanium to aluminum has a significant effect on the lattice mismatch between the γ phase and the γ' phase. The figure below shows the influence of the change in the ratio of titanium to aluminum on the lattice mismatch.

Figure 9
Influence of the Change in the Ratio of Titanium to Aluminum on the Lattice Mismatch

Niobium

About 90% of niobium in superalloys also enters the γ' strengthening phase. Its addition can increase the number of γ' phases and at the same time increase the binding energy in the γ' phase crystals, making the precipitation phase more difficult to be cut. Achieved higher strength.

When the niobium content in the superalloy is further increased to more than 4%, niobium can not only increase the number of γ' phases, but also generate γ" phases with Ni3Nb as the component, which further improves the strength of the alloy.

Tantalum

Like niobium, about 90% of tantalum can enter the γ' phase. At the same time, tantalum can also play a role in improving the stability of the γ' phase.

The Role of Integrated Elements

Other elements in the alloy can also enter the γ' phase to participate in precipitation strengthening.

Titanium, niobium, tantalum, hafnium, and vanadium preferentially enter the γ' phase to participate in precipitation strengthening, and a small part of it enters into the γ phase to participate in solid solution strengthening.

Cobalt, chromium, and molybdenum preferentially enter the γ phase to participate in solid solution strengthening, and a small part of it enters into the γ' phase to participate in precipitation strengthening.

About 50% of tungsten is involved in solid solution strengthening, and 50% is involved in precipitation strengthening.

Aging Treatment

Purpose of Aging Treatment

The purpose of the aging treatment is to further precipitate fine γ' phase or γ" phase on the basis of the previous heat treatment, so as to increase the main strengthening phase, further increase the strength of the alloy, and achieve the best strengthening effect.

Preconditions for Aging Treatment

The aging treatment must first undergo a solution treatment. In the previous article, we mentioned that the purpose of solution treatment is to fully dissolve the alloy, obtain a suitable grain size, and reduce harmful phases. Therefore, solution treatment is the most basic heat treatment process for all alloys.

In addition, some alloys require intermediate heat treatment after solution treatment. The purpose of this step is to reduce the γ' phase with larger crystal grain size precipitated in the solid solution stage.

Parameters of Aging Treatment

The temperature of the aging treatment is generally the use temperature of the alloy. This is because after the aging treatment at this temperature, the alloy will not precipitate new phases at this temperature, thus maintaining stable performance during work. If the aging temperature is higher than the service temperature, new strengthening phases will be precipitated during the use of the alloy, which will change the properties of the alloy. And if the aging temperature is lower than the service temperature, the strengthening process will cause the strengthening phase to accumulate and grow, thereby reducing the strength of the alloy.

The aging treatment time is generally 16 hours, but some alloys will use 24 hours or 32 hours.

The cooling method of aging treatment is generally air cooling, and the heating rate is not very strict.

Common Precipitation Strengthened Alloys

Among the commonly used superalloys, the Hastelloy series is completely solid solution strengthened, so there is no precipitation strengthened alloy.

In Monel, Inconel and Incoloy alloys, generally speaking, the alloys whose names begin with an odd number are precipitation strengthened alloys. Such as: Monel K-500, Inconel 718, Inconel X-750, Incoloy 909.

However, there are exceptions to this situation. For example, Incoloy A-286 is also a precipitation strengthened alloy, while Incoloy 926 is not a precipitation strengthened alloy.

Conclusion

Precipitation strengthening is a strengthening method that is different from solid solution strengthening. It has a better strengthening effect than solid solution strengthening and can enable alloys to be used in applications with higher stress. Precipitation strengthening is mainly achieved by adding aluminum to the alloy and precipitate strengthening phase through aging treatment.

AEETHER CO., LIMITED produce high-quality precipitation strengthened superalloys. And the aging treatment will be carried out in accordance with the standard. If you have any requirements, you can send inquiry to our E-mail: