Overview
Nickel-based superalloys are a special class of metal materials with excellent corrosion resistance and high-temperature strength characteristics in high-temperature environments, and are widely used in aerospace, energy, chemical, petroleum and other fields. It is one of the indispensable materials in the modern industrial field.
In the previous article, we introduced the characteristics of iron-based superalloys in detail. In this article, we will introduce the composition, properties and differences between nickel-based superalloys and iron-based superalloys.
What is Nickel-based Superalloy
Nickel-based superalloy is an alloy with nickel as the matrix (nickel accounts for the largest proportion in the alloy). On the basis of nickel, it will also add many other metal elements for alloying. Nickel-based superalloys are the most widely used of all classifications of superalloys. It has better corrosion resistance and high temperature strength than iron-based superalloys. In many applications it cannot be replaced.
First of all, the most important component in nickel-based superalloys is nickel. Nickel maintains the austenitic structure well at all temperatures. Therefore it is very suitable as a matrix for superalloys. Its existence ensures the high-temperature basic performance of the alloy and brings good corrosion resistance to the alloy.
In addition, since the oxidized nickel cannot protect the alloy well, almost all nickel-based superalloys need to add 10% ~ 25% chromium to ensure the oxidation resistance of the alloy. At the same time, chromium can also play a good role in solid solution strengthening. A typical nickel-chromium alloy is Inconel 600.
600
Si
C
Mn
Cu
Fe
Cr
Ni
There is also a large part of nickel-based superalloys that will add a certain amount of molybdenum to further improve the reduction resistance of the alloy. At the same time, molybdenum is also a very effective solid solution strengthening element.
Operating Temperature
The structure of nickel itself is very stable at high temperature. Moreover, compared with iron-based superalloys, nickel-based superalloys have more alloying elements. This allows nickel-based superalloys to perform better at higher temperatures. In the previous article, we mentioned that the service temperature of iron-based superalloys is 600~800°C. And nickel-based alloys can work well even in environments above 800°C.
Solid Solution Strengthened Nickel-Based Alloys
The main principle of solid solution strengthening is to use the atomic radius difference between the strengthening element and the basic element to expand the lattice to achieve strengthening. The principle of solid solution strengthening is introduced in detail below:
What is Solid Solution? Why do Nickel Alloy / Superalloy need Solution Treatment?
Nickel-based superalloys have more alloying elements than iron-based superalloys. Among them, molybdenum, tungsten and cobalt are the most commonly used solid solution strengthening elements with a large proportion of components.
Molybdenum is the most common solid solution strengthening element in nickel-based superalloys. About 15% molybdenum is added to the alloy to form a nickel-chromium-molybdenum alloy. A typical nickel-chromium-molybdenum alloy is Hastelloy C-276. These alloys often have good comprehensive corrosion resistance and certain strength.
C-276
Mn
Co
V
W
Fe
Mo
Cr
Ni
About 15% cobalt will be added to some nickel-based alloys to form nickel-chromium-cobalt alloys. Cobalt achieves solid solution strengthening of the alloy in another way. It can effectively improve the high temperature strength of the alloy. A typical nickel-chromium-cobalt alloy is Inconel 617.
617
Si
C
Mn
Al
Ti
Cu
Co
Mo
Fe
Cr
Ni
In individual alloys, about 10% of tungsten is added to form nickel-chromium-tungsten alloys. But in more cases, tungsten is only used as a minor solid solution strengthening element. It accounts for no more than 5%.
In addition, other common solid solution strengthening elements are: niobium, tantalum, vanadium, etc. Most of these elements can be found in groups V, VI, and VIII of the periodic table.
Compared with iron-based superalloys, nickel-based superalloys tend to have higher high-temperature strength. The high temperature strength of solid solution strengthening nickel-based alloys is related to the amount of solid solution strengthening elements. The higher the content of tungsten, molybdenum, cobalt, niobium and other elements, the higher the strength of the nickel-based superalloy.
Precipitation Strengthened Nickel-based Alloy
The principle of precipitation strengthening is to allow the precipitation strengthening elements in the alloy to precipitate to form a precipitate phase to achieve the effect of hindering the displacement of the alloy crystal. The precipitation phase can be divided into γ' phase and γ" phase. These two strengthening phases act on superalloys at different temperatures respectively. The following article introduces the principle of precipitation strengthening in detail:
What is Precipitation Strengthening & Aging Treatment. Which Alloys need Aging Treatment?
Compared with iron-based superalloys, precipitation strengthened nickel-based superalloys will be strengthened by adding more precipitation strengthening elements. For some high-performance nickel-based alloys, the total amount of aluminum and titanium can even reach 10%. As we said in the previous article, the proportion of precipitation strengthening phase (γ’ phase) in iron-based superalloys can only reach up to 20%. While the γ' phase of nickel-based superalloys can reach up to 65%.
50% of the niobium element in the nickel-based superalloy plays the role of solid solution strengthening, and 30% plays the role of precipitation strengthening phase. In addition, in some nickel-based superalloys containing more iron (such as Inconel 718), niobium can also generate γ’’ phase to improve the medium temperature strength of the alloy. In addition to niobium, 80% of tantalum also plays a precipitation strengthening role in nickel-based superalloys.
718
Si
C
Al
Ti
Co
Nb
Ta
Mo
Fe
Cr
Ni
Most of the precipitation strengthening elements of nickel-based superalloys can be found in groups III, IV, and V of the periodic table.
Because the precipitation strengthening phase in nickel-based superalloys accounts for a large proportion, its high-temperature strength is also much greater than that of iron-based superalloys. The higher the content of aluminum, titanium, niobium and tantalum in the alloy, the better the mechanical properties of the precipitation strengthened nickel-based superalloy.
In addition, the structural stability of nickel-based superalloys is far better than that of iron-based superalloys. During aging treatment and high temperature service, nickel-based superalloys are less likely to precipitate harmful phases. On the one hand, it ensures that the amount of precipitation strengthening phase is not reduced, and on the other hand, it prevents the adverse effect of harmful phase on mechanical properties.
Nickel-based superalloys add chromium, molybdenum, cobalt, aluminum, titanium and other elements on the basis of nickel for solid solution strengthening and precipitation strengthening. Its corrosion resistance and strength are better than iron-based superalloys. At the same time, nickel-based superalloys are suitable for working at higher temperatures.
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