Overview

Dispersion strengthened superalloy is a newly developed superalloy. It has the strength that ordinary superalloys can't reach at high temperature. In this article, we will introduce in detail all aspects of dispersion strengthened alloys.

What is Dispersion Strengthening

What is Dispersion Strengthening

Dispersion strengthening refers to a method of strengthening the alloy by adding some very stable substances (called dispersive particles) to the alloy and distributing these substances evenly in the alloy. The alloys strengthened by this method are called dispersion strengthened alloys.

Why Dispersion Strengthening is Needed

In the previous article, we mentioned that general alloys have two strengthening methods: solid solution strengthening and precipitation strengthening. Among them, precipitation strengthening is the strengthening achieved by precipitating the precipitate phase from the alloy. This strengthening method can make the alloy have higher strength.

However, as the alloy's service temperature rises, the precipitated phase will accumulate and grow, and the strengthening effect will be weakened. When the temperature reaches the dissolution temperature of the precipitated phase, the precipitated phase will re-dissolve into the alloy matrix, completely losing the strengthening effect.

Figure 1
Precipitation and Dissolution of Precipitation Phase

In contrast, dispersion enhancement does not have this disadvantage. The dispersive phase is a substance with relatively stable properties, and is artificially added from the outside, and will not be solid-dissolved into the alloy matrix. This makes this strengthening method will not fail at high temperatures.

Figure 2
Strength Comparison between Precipitation Strengthening and Dispersion Strengthening

In aircraft engines, the operating temperature of some parts makes traditional superalloys difficult to handle, and dispersion-strengthened alloys are designed for these applications.

Combination of Dispersion Strengthening and Precipitation Strengthening

Adding the dispersion strengthening phase to the precipitation strengthening superalloy can make the alloy have both the effects of precipitation strengthening and dispersion strengthening. In different temperature ranges, different strengthening methods play their respective advantages.

The temperature range of precipitation strengthening is below 0.55-0.60 Tm (Tm is the melting point of the alloy). The temperature range of dispersion strengthening is above 0.70Tm. In the range of 0.60Tm to 0.70Tm, the two strengthening methods work at the same time.

Disadvantages of Dispersion Strengthened Alloys

The plasticity of dispersion strengthened alloy is poor, so its product form is limited. In addition, the price of dispersion-strengthened alloys is generally higher.

Ideal Dispersive Particle

The dispersed particles in the dispersion strengthened alloy need to have a higher melting point and a lower diffusion rate. Generally speaking, oxides, carbides, nitrides, and borides can all be used as the dispersed phase. Among them, metal oxides most meet the requirements. This is because the thermal stability of the oxide is the highest. In oxides, the common dispersed phases are Al2O3, ThO2, and Y2O3. Their melting points are above 2000°C.

In the early days, the dispersion phase of ThO2 was most used. But ThO2 is radioactive and harmful to humans.

Recently, the widely used dispersed phase is Y2O3 (yttrium oxide). This material has no radioactive hazard, and it can form composite oxides (3Y2O3·5Al2O3) with excess oxygen and aluminum in the alloy. This composite oxide is more stable than ThO2, and can further improve high-temperature strength.

Unlike precipitation strengthening, the number of dispersed phases needs to be controlled. If the number of dispersed phases is too large, the sintering performance of the alloy will be significantly reduced. If the number of diffuse phases is too small, it will not have the effect of strengthening. The number of dispersed phases in most dispersion-strengthened alloys is between 0.5% and 2%.

Characteristics of Dispersion Strengthened Alloys

High High-temperature Strength

The greatest mechanical performance characteristic of the dispersion strengthened superalloy is its high high-temperature strength.

First of all, the common dispersion strengthening phase Y2O3 has a high melting point, high hardness, is insoluble in the alloy #matrix, hardly grows, and is very stable at high temperatures. This is the basis for it to function at high temperatures.

Secondly, studies have shown that there is mutual attraction between the oxide and the matrix, which will restrain the atoms and hinder the movement of dislocations.

Figure 4
Attraction Between the Oxide and the Matrix

Anisotropy

Another characteristic of the dispersion strengthened alloy is that the grain structure is very long. This is because the alloy needs to be recrystallized during the production process. The processed alloy grains are long and thin, and the grain boundary density is low, which is conducive to high temperature strength.

Figure 5
Dispersion Strengthened Alloy Grain Shape in Different Directions

For other alloys, the grain size is a measure of the strength of the alloy. For dispersion strengthened alloys, the grain aspect ratio has a greater impact on the strength of the alloy. The length of a single grain of the dispersion strengthened alloy can reach up to 500mm, while the width is only 10mm, and the aspect ratio is up to 50:1.

The elongated grain structure also leads to the anisotropic nature of the alloy. The strength in the direction of grain elongation will be much higher than the strength in other directions. Therefore, the grain direction of the alloy needs to be adjusted according to the actual application.

Oxidation Resistance

The addition of oxides can also significantly improve the oxidation resistance and corrosion resistance of the alloy. Studies have shown that under the condition that the other chemical composition of the alloy remains unchanged. The corrosion resistance of the alloy with Y2O3 is much better. This is mainly because Y2O3 can form a rare-earth-rich oxide layer at the interface between the oxide film and the substrate. The rare earth oxide enriched layer changes the oxidation mechanism of the alloy, prevents the destruction of the oxide film, and increases the adhesion of the oxide film.

Common Dispersion Strengthened Alloys

The current dispersion strengthened alloys on the market are all oxide dispersion strengthened alloys (ODS alloys)

The dispersion strengthened alloys we can provide are Inconel MA754, Inconel MA758 and Incoloy MA956. All these alloys have high high temperature strength.

Production Process of Dispersion Strengthened Alloy

Mechanical Alloying

Ordinary superalloys can be alloyed by smelting. Just melt the different metals and mix them to get the desired alloy. However, the smelting method cannot be used for dispersion strengthened alloys. This is because the oxides will float on the surface of the liquid metal during the smelting process, and there is no guarantee that the oxides can be dispersed in the alloy after solidification.

Dispersion-strengthened alloys need to be manufactured by mechanical alloying. The so-called mechanical alloying is to mix different metal powders and then polish them under a high-speed rotating spherical abrasive tool. The temperature and pressure generated by high-speed rotation will weld different powders together. The alloy produced in this way can ensure maximum uniformity.

Figure 6
Mechanical Alloying Process

Thermomechanical Processing

Since the dispersion-strengthened alloy has very high strength, its forming needs to be carried out at high temperatures. The commonly used forming method is hot extrusion, that is, the heated alloy is placed in a mold for extrusion.

In addition, hot extrusion can make the alloy gain internal energy so that in the subsequent secondary recrystallization treatment, a coarser anisotropic grain structure can be obtained, thereby improving high-temperature mechanical properties.

Secondary Recrystallization Treatment

The secondary recrystallization treatment is to put the hot-extruded ODS alloy into a high-temperature furnace for annealing treatment. Since the energy is stored in the alloy during the hot-extrusion process, this part of the energy will elongate the grains during the recrystallization process.

Inconel MA754 alloy needs to be air-cooled annealed at 1315°C for 1 hour to achieve an aspect ratio of more than 8:1. Incoloy MA956 alloy needs to be air-cooled annealed at a temperature of 1300°C for 1 hour, which can achieve an aspect ratio of more than 50:1.

Conclusion

Dispersion strengthened alloys have higher high-temperature strength than ordinary superalloys, and the addition of oxides can effectively improve the oxidation resistance of the alloy. The dispersion strengthened alloy has very long grains, which can also effectively improve the high-temperature strength. Unlike ordinary alloys, dispersion-strengthened superalloys need to be manufactured by mechanical alloying.

AEETHER CO., LIMITED produce dispersion strengthened alloys including Inconel MA754, Inconel MA758, and Incoloy MA956. Due to the particularity of these materials, the minimum order quantity is 500kg. If you have corresponding requirements, you can send an inquiry to our E-mail: