Overview

Almost all nickel alloys require solid solution treatment, because solid solution has an important role in strengthening the properties of the alloy. This article will introduce the principle and function of solid solution from various aspects.

What is Solid Solution

Crystal Structure of Superalloy

All superalloys have a face-centered cubic crystal structure. The crystal arrangement of this structure is shown in the figure below:

Figure 1
Crystal Arrangement of Single Lattice

Figure 2
Crystal Arrangement

Principle of Solid Solution

From a macro perspective, solid solution is the process of dissolving other metal elements (such as iron, chromium, molybdenum, etc.) into nickel metal. Similar to the process of dissolving salt into water. But just like its name, solid solution is the dissolution of solid substance . Since the movement rate of solid atoms is very slow, the general solid solution requires the metal to be heated to a certain temperature to make the atoms move more freely, so as to achieve the purpose of dissolution.

Among them, the base metal is called a solvent, and other metals are called solutes.

From a microscopic point of view, solid solution is the process by which other metal elements enter into the nickel crystals to form a new face-centered cubic structure. The face-centered cubic crystal after solid solution is known as austenite.

The solid solution process is shown in the figure below:

Figure 3
Solid Solution Process

Solid Solution Strengthening

Strengthening Caused by the Difference in Atomic Radius - Long Range Stress

The strength of the alloy we usually call is actually the ability of the alloy to resist deformation, and the microscopic nature of the deformation is the displacement of atoms. And because the atoms of the metal are arranged according to a certain rule, the displacement inside the metal is realized by sliding between the layers. As shown below:

Figure 4
Sliding Between the Layers

After the solid solution, the difference between the atomic radius of the solute atoms and the nickel will cause lattice distortion, thereby hindering the sliding between the crystal layers. This makes the alloy more difficult to deform and achieves higher strength. As shown below:

Figure 5
Long Range Stress

Strengthening Caused by Forces Between Atoms - Short Range Stress

The repulsive force between atoms increases as the distance between atoms decreases, and the gravitational force increases as the distance between atoms increases. After solid solution, the distance between atoms changes, and more force needs to be overcome during the displacement process. This is another reason why solid solution can strengthen the alloy. The specific principle is as follows:

Figure 6
Short Range Stress

Strengthening Caused by Non-uniform Distribution of Atom - Stacking Fault Energy

After the solid solution, the atoms inside the alloy will be unevenly distributed. In this case, the area where the solute atoms gather will produce defects, which are called stacking faults, and the displaced atoms require more energy to pass through the stacking fault. This effectively prevents the displacement of atoms. Thereby improving the strength of the alloy.

Experiments have shown that the greater the number of electron holes in an atom, the greater the energy needed to overcome stacking faults, and the greater the strength of the alloy.

Figure 7
Stacking Fault Energy

Strengthening Caused by the Difference in Forces Between Atoms - Short Range Order

Different types of atoms have different forces between atoms. In the case of a large number of solute atoms, a short-range order phenomenon will occur where the same solute atoms gather. When passing through the short-range ordered region, other atoms need to overcome the forces of all atoms in this region, which increases the difficulty of displacement. The alloy is strengthened macroscopically.

Figure 8
Short Range Order

Common Solid Solution Strengthening Elements

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

Cobalt

The atomic radius of cobalt is not much different from that of nickel, so the addition of cobalt to the alloy can mainly increase the probability of stacking faults, thereby achieving solid solution strengthening.

Iron

The main solid solution strengthening factor of iron is the difference in radius with nickel atoms, but this strengthening effect is not as obvious as other elements. Adding more iron to the alloy still saves costs.

Chromium

The main function of chromium in superalloys is to increase the corrosion resistance of the alloy. In terms of mechanical properties, chromium can produce stacking faults. In addition, when the content of chromium is 20% ~ 25%, the effect of short-range orderly strengthening can be produced.

Tungsten

The atomic radius of tungsten is very large, which is the main reason why it can strengthen the alloy. At the same time, tungsten can also produce stacking faults.

Molybdenum

The atomic radius of molybdenum is also very large, which allows it to be added to the alloy to effectively prevent displacement and increase the force between atoms. At the same time, molybdenum can also refine the grains of austenite, which can also increase the strength of the alloy.

Niobium

The atomic radius of niobium is larger than that of molybdenum and tungsten, and it can also play a good solid solution strengthening effect. But in general, niobium is more used for precipitation strengthening mechanisms.

Tantalum

Tantalum, like niobium, has a large atomic radius, but it is mainly used for precipitation strengthening. And the price of this element is very high, and the dosage is generally controlled.

Vanadium

The atomic radius of vanadium is larger than that of nickel, and it can refine the grains and strengthen the alloy.

Solution Treatment

Solution treatment is the most important heat treatment method for superalloys. Basically all superalloys must be solution treated.

Purpose of Solution Treatment

• 1. The ultimate goal of solution treatment is to fully dissolve the strengthening phase to achieve the effect of solid solution strengthening.
• 2. Solution treatment can obtain uniform and suitable grain size. Through different solid solution methods, the size of the crystal grains can be controlled to achieve different functions.
• 3. Eliminate segregation and reduce the influence of harmful phases.

Parameters of Solution Treatment

Generally speaking, heat treatment needs to determine three parameters: temperature, time and cooling rate

The temperature of the solid solution should be controlled above the dissolving temperature of the alloy strengthening elements, and at the same time controlled below the initial melting temperature of the alloy. In addition, different alloys also need to confirm the temperature at which the grains begin to grow. If finer grains are required, the solid solution temperature should be maintained below the grain growth temperature.

The solution time is generally 1 ~ 8 hours. The time of solution treatment is closely related to the temperature of solution treatment. If the temperature is above the crystal grain growth temperature, the solution treatment time determines the size of the crystal grains. Otherwise, the solution time has no significant effect on the grain size.

Solid solution cooling methods are divided into furnace cooling, air cooling, oil cooling and water cooling. The cooling rate of the solid solution significantly affects the grain size of the alloy. If a finer grain size is required, oil cooling and water cooling with a faster cooling rate should be selected. Otherwise, furnace cooling and air cooling are used.

Common Solid Solution Strengthened Alloys

Among the common superalloys, all grades of Hastelloy are solid solution strengthened alloys. Therefore, it can be observed that almost all Hastelloy alloys contain a relatively large amount of molybdenum or tungsten. Such as: Hastelloy B-3, Hastelloy C-276.

Among Monel, Inconel and Incoloy alloys, the alloys starting with even numbers are basically solid solution strengthening alloys, such as: Monel 400, Inconel 600, Inconel 625, Incoloy 800, Incoloy 825.

These alloys are all single-phase alloys, except for solid solution strengthening, they can only be strengthened by cold working.

Conclusion

Solid solution is the process of dissolving other metal elements into a specific metal.

The solid solution affects the strength of the superalloy from four aspects (depending on different solid solution elements).

Therefore, all superalloys require solution treatment.

Some alloys can only be strengthened by solution treatment, and these alloys are classified as solid solution strengthened superalloys.

Although solution treatment is a necessary heat treatment for superalloys, there are still some manufacturers who skip solution treatment for higher profits, which results in the performance of the alloy not meeting the standard. AEETHER CO., LIMITED perform solid solution treatment on the alloy in strict accordance with the standard during production. If you have a need for superalloys, please send inquiry to us by E-mail: