Overview
Alloys are widely used materials in today's engineering and technology fields. Its excellent properties are often derived from its special crystal structure. Among them, the austenite structure, as an important alloy phase, plays a role that cannot be ignored.
This article will discuss the austenitic structure of the alloy in depth, including its definition, properties and applications in stainless steel, superalloys and other fields. We also introduce the γ, γ' and γ" phases associated with austenite.
Alloy Crystal Structure
To understand the austenitic structure, it is first necessary to understand the crystal structure of the alloy.
Here's a fact that might surprise you: metals are crystals. According to the definition, crystal refers to a material structure in which atoms or molecules are arranged periodically according to certain rules. Metals fit this structure. Therefore, they are undoubtedly a kind of crystal.
In order to show the periodicity of the crystal more intuitively, atoms are often connected by lines in the structure diagram of the crystal. These lines form a regular grid-like structure called a lattice.
In order to describe the lattice more simply, a smallest unit is often taken out of the lattice to represent the structure of the entire lattice. This smallest unit is called a unit cell.
Crystals can be divided into 7 crystal systems and 14 lattice types according to their internal structure. They represent different arrangements of atoms in the crystal. The three most common lattice types in metals are as follows:
(FCC)
(BCC)
(HCP)
What is Austenite?
Before I formally introduce austenite, please allow me to introduce another concept: solid solution.
Solid solution, as the name suggests, is the dissolution of a solid state. In an alloy, the most dominant element is called the matrix. Just like salt dissolves in water, other metallic elements can also dissolve in the matrix. The only difference is that metals exist in solid form. Therefore, the phenomenon that metal dissolves other elements is called solid solution.
From a microscopic point of view, the process of solid solution is that other atoms replace the original atoms in the metal crystal (or directly enter the internal gap of the crystal). Solid solution achieves the effect of changing the properties of metals in this way.
Knowing what is crystal structure and solid solution, we can give the definition of austenite: austenite is the face-centered cubic structure after solid solution. In other words, austenite has two properties:
It is worth noting that the crystal phase of the austenitic structure is often represented by the Greek letter γ. Now that we know what austenite is, let's introduce the properties of austenite:
Austenitic Properties
Good Plasticity
Austenite has very good plasticity. Whether at room temperature or high temperature, austenitic metals can change their shape and form different products through common processing methods. At the same time, austenitic metals also have a certain strength, and it can be increased by cold working. Through heat treatment such as annealing, austenitic metals can be restored to a state of high plasticity.
Therefore, the properties of austenite are easier to control. In practical applications, the good plasticity of austenite allows it to work stably in a wide range of temperatures for a long time, which is why the most commonly used stainless steels are all austenitic stainless steels.
Paramagnetic
Most austenitic alloys are paramagnetic, which means they can be rendered nonmagnetic. This property allows austenitic alloys to work well in some environments with strong magnetic fields. Some alloys, such as Monel 401, are widely used in the electronic components industry, which benefits from the non-magnetic nature of these austenitic alloys.
401
Si
C
Mn
Fe
Cu
Ni
However, not all austenitic alloys are non-magnetic. It depends on the chemical composition of the alloy. For example, Monel 400 has obvious magnetic properties at room temperature. Therefore, it is recommended to fully understand the specific alloy before practical application.
400
Si
C
Mn
Fe
Cu
Ni
Large Coefficient of Linear Expansion
The third characteristic of austenitic alloys is their large coefficient of linear expansion. This metric measures how much a metal expands in volume as it heats up. In other words, austenitic alloys expand more as the temperature increases. This property of austenitic alloys is very suitable for some instrument components that use thermal expansion to change their state.
However, not all austenitic alloys have a large coefficient of linear expansion. There are some low-expansion austenitic alloys (such as Incoloy 909), which have specific iron and nickel contents, and can achieve low linear expansion coefficients in a wide temperature range by adding cobalt.
909
Si
Mn
Al
Ti
Cu
Co
Nb
Fe
Cr
Ni
Good High Temperature Performance
Another important property of austenitic alloy is its excellent high temperature performance. At high temperatures, the face-centered cubic structure can maintain better structural stability, which ensures the strength of austenitic alloys at high temperatures. This is why almost all superalloys are austenitic alloys.
How to Obtain Austenitic Structure?
So what exactly determines whether an alloy has an austenitic structure?
Now, we know that austenitic alloys need to have a face centered cubic (FCC) structure. Therefore, we need to further know which metal elements have the FCC structure.
The most common metal is iron, and iron metal does have an FCC structure. Unfortunately, iron has allotropic properties. That is to say, iron can only maintain the FCC structure in the temperature range of 912°C to 1394°C. At other temperatures, iron becomes a body-centered cubic (BCC) structure.
At this point, we urgently need a metal element that can guarantee an FCC structure at any temperature. This element is nickel.
As long as the nickel content of the alloy is above 9%, it can maintain the FCC structure. Therefore, it is only necessary to add more than 9% nickel to iron to make the alloy maintain an austenitic structure at any temperature. This is why all stainless steels and superalloys contain nickel.
In addition, if the manganese content in the alloy is above 13%, the austenite structure can also be maintained. However, since manganese has some negative effects on alloys, it is rarely used for alloying.
What are γ, γ' and γ" Phases?
We mentioned above that austenite (or, face-centered cubic structure) is usually represented by the Greek letter γ. Therefore, most of the metallographic structure of austenite is called γ phase.
There are γ' and γ" phases in some special stainless steels and superalloys (precipitation strengthened superalloys and precipitation hardening stainless steels). These two phases belong to precipitated phases. They are also FCC structures. However, they are strengthening phases with different components precipitated from the γ phase. They can be obtained by a heat treatment called aging, and greatly increase the strength of the alloy.
The main elements forming the γ' phase are aluminum and titanium. The main elements that form the γ" phase are niobium and tantalum. Both of them can increase the strength of the alloy at high temperatures. However, the effective temperature of the γ" phase is lower than that of the γ' phase. Therefore, for some superalloys (such as Inconel 718), γ' and γ" phases often exist at the same time, which ensures that the alloy has high strength in a large temperature range.
718
Si
C
Al
Ti
Co
Nb
Ta
Mo
Fe
Cr
Ni
FAQ
Besides austenite, what other common alloy structures are there?
For stainless steel, there are two more common structures: ferrite and martensite.
Ferrite, like austenite, is a solid solution metal structure. The difference is that austenite has a face-centered cubic (FCC) structure and ferrite has a body-centered cubic (BCC) structure. Ferritic stainless steels are magnetic and less ductile than austenitic stainless steels. Therefore, its application range is narrower than that of austenitic stainless steel.
Martensite is a supersaturated solid solution. Unlike austenitic stainless steels, martensitic stainless steels generally have a very low nickel content. Therefore, it cannot maintain the FCC structure at room temperature. Martensite is often obtained by rapid cooling (quenching) of austenite at high temperature. Its purpose is to prevent the carbon dissolved in the high-temperature austenite from being precipitated and remain in the BCC structure at room temperature. The martensitic structure has high strength but high brittleness. It is more suitable for some high-intensity applications.
Is the corrosion resistance of stainless steel and superalloys related to austenite?
Austenite is not directly related to corrosion resistance. However, since austenitic alloys often contain a certain amount of nickel. These nickel elements can bring certain corrosion resistance properties to the alloy. Nevertheless, the corrosion resistance in alloys is mainly derived from corrosion-resistant elements such as copper, chromium, and molybdenum.
What is super austenitic stainless steel?
Super austenitic stainless steel is a better material than general austenitic stainless steel. However, in terms of crystal structure, they all belong to austenite. Super austenitic stainless steels tend to have higher nickel, chromium, and molybdenum contents. The addition of these elements gives them more advantages in strength, corrosion resistance and plasticity. Of course, the price of super austenitic stainless steel is also higher.
Further Reading
Conclusion
Austenite is an important metal crystal structure. Its wide application in stainless steel, superalloys and other fields proves its excellent ability in improving the properties of materials. Through an in-depth understanding of the austenite structure and its related phases, people can better design and manufacture alloy materials that meet specific needs, and promote the progress of modern engineering and technology.
We produce a wide range of austenitic superalloys. If you have corresponding purchasing needs, please do not hesitate to contact us.
