By Nick Buck, Final Year MEng student, Department of Mechanical Engineering Sciences, University of Surrey.
Inconel 718 is a material of choice for various components of aeroengines, where critically, the material has excellent high temperature properties that enable the increase of engine efficiency. In some engines the alloy makes up almost 50% of the alloys used! Aside from the ability to operate at over 0.7 times its melting temperature, other useful properties include oxidation and corrosion resistance. This makes the alloy important in areas such as turbomachinery components in steam turbines and parts for nuclear power stations.
Inconel 718 is termed a nickel-based superalloy. This means that the main alloy constituent is nickel as shown in the composition in Table 1. The nickel content forms a matrix with an FCC crystal structure in which other alloying additions can dissolve and is denoted γ. The aluminium, titanium, tantalum, and niobium all help create the primary strengthening precipitates. The first three combine with the nickel to form Ni3(Al,Ti,Ta) which is known as γ’. These precipitates have an FCC structure and provide precipitation hardening to the alloy. Niobium and nickel form a second precipitate called γ’’ that also provides precipitation strengthening, this has a BCT crystal structure.
Table 1 Composition ranges of IN718 for aerospace use (Data from Deng, 2018).
A further important phase in the microstructure, are the MC carbides. In Inconel 718, these carbides take the form of (Nb,Ti)C and they are a key strengthener that improves the creep strength of the alloy Substitutional solid solution strengthening is important in addition to the strengthening precipitates. Table 2 summarises all the elements and the roles they play in strengthening the alloy.
Table 2 The role of alloying elements in Inconel 718. Closely based on Deng, 2018 .
|Cr||Solid-solution strengthener, M7C3 and M23C6 carbides former, improves oxidation and hot corrosion resistance.|
|Co||Solid-solution strengthener, raises solvus temperature of γ′ Ni3 (Al,Ti).|
|Al||Strengthening phase γ′, Ni3 (Al,Ti) former, improves oxidation and hot corrosion resistance.|
|Ti||Ti Strengthening phase γ′ Ni3 (Al,Ti) former.|
|Nb||Strengthening phase γ′′, Ni3Nb former, MC and M6C carbides former.|
|Mo||Solid-solution strengthener, MC, M23C6 and M6C carbides former|
|W||Solid-solution strengthener, MC, M23C6 and M6C carbides former.|
|Ta||Solid-solution strengthener, MC carbide former, improves creep properties.|
|C||M(C,N) carbonitrides former, grain-boundary strengthener.|
|B||Grain-boundary strengthener, improve creep properties and rupture strength|
Traditionally IN718 is processed using both casting and wrought techniques. Casting is mainly used to produce turbine blades and allows the simultaneous creation of complex shapes and microstructural control. Advanced methods such as directional solidification have been developed by the aerospace industry to further improve the properties of these parts. In contrast, wrought processing; or forging as it is also known, uses heat and deformation to produce the desired shape. This produces parts that have very good mechanical properties, but it is tricky to produce complex geometries. Powder metallurgy is a third method of manufacturing Inconel 718. This uses the powdered alloy and uses methods such as hot isostatic pressing to produce the part. These parts can be cheaper to make than casting and forging while allowing good microstructural control.
The alloy is hard to machine and causes rapid tool wear. In recent years there has also been development into 3D printing Inconel 718 which addresses this problem. The ability of additive manufacturing to produce a geometrically accurate parts that needs little processing is very desirable.
Although a brief look at one of the most common superalloys used today, this post highlights the unique properties of Inconel 718 and the elements that make up its complex microstructure. With the advances in manufacturing such as with 3d printing, perhaps we will soon see this alloy finding even more widespread use and allow engineers to produce increasingly efficient turbomachinery.