Superalloys, or high-performance alloys, are non-ferrous alloys that exhibit outstanding strength and surface stability at high temperatures. Their ability to operate safely at a high fraction of their melting point (up to 85% of their melting points (Tm) expressed in degrees Kelvin, 0.85) is their key characteristics. Superalloys are generally used at temperatures above 540 °C (1000 °F), as at these temperatures ordinary steel and titanium alloys are losing their strengths, also corrosion is common in steels at this temperature. At high temperatures, superalloys retain mechanical strength, resistance to thermal creep deformation, surface stability, and resistance to corrosion or oxidation. Some nickel-based superalloys can withstand temperatures beyond 1200°C, depending on the composition of the alloy. Superalloys are often cast as a single crystal, while grain boundaries may provide strength, they decrease creep resistance.
They were initially developed for use in aircraft piston engine turbosuperchargers. Today, the most common application is in aircraft turbine components, which must withstand exposure to severely oxidizing environments and high temperatures for reasonable time periods. Current applications include:
- Aircraft gas turbines
- Steam turbine power plants
- Medical applications
- Space vehicles and rocket engines
- Heat-treating equipment
- Nuclear power plants
Inconel 718 – Nickel-based Superalloy
In general, Inconel is a registered trademark of Special Metals for a family of austenitic nickel-chromium-based superalloys. Inconel 718 is a nickel-based superalloy that possesses high strength properties and resistance to elevated temperatures. It also demonstrates remarkable protection against corrosion and oxidation. Inconel’s high temperature strength is developed by solid solution strengthening or precipitation hardening, depending on the alloy. Inconel 718 is composed of 55% nickel, 21% chromium, 6% iron, and small amounts of manganese, carbon, and copper.
Common uses of superalloys are in the aerospace and some other high-technology industries. With the combination of corrosion resistance and material strength in the face of extreme heat, this kind of superalloy works well in the nuclear industry. Some nuclear plants use nickel-based superalloys for the reactor core, control rod, and similar parts. In nuclear industry, especially low-cobalt superalloys (due to possible activation of cobalt-59) are used. Some of structural parts of nuclear fuel assemblies, such as top and bottom nozzle, may be produced from superalloys such as Inconel. Spacing grids are usually made of a corrosion-resistant material with low absorption cross section for thermal neutrons, usually zirconium alloy (~ 0.18 × 10–24 cm2). First and last spacing grid may be also made of low-cobalt Inconel, which is a superalloy well suited for service in extreme environments subjected to pressure and heat.
Strength of Superalloys – Inconel 718
In mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. Strength of materials basically considers the relationship between the external loads applied to a material and the resulting deformation or change in material dimensions. Strength of a material is its ability to withstand this applied load without failure or plastic deformation.
Ultimate Tensile Strength
Ultimate tensile strength of superalloy – Inconel 718 depends on heat treatment process, but it is about 1200 MPa.
The ultimate tensile strength is the maximum on the engineering stress-strain curve. This corresponds to the maximum stress that can be sustained by a structure in tension. Ultimate tensile strength is often shortened to “tensile strength” or even to “the ultimate.” If this stress is applied and maintained, fracture will result. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. The stress-strain curve contains no higher stress than the ultimate strength. Even though deformations can continue to increase, the stress usually decreases after the ultimate strength has been achieved. It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material. Ultimate tensile strengths vary from 50 MPa for an aluminum to as high as 3000 MPa for very high-strength steels.
Yield Strength
Yield strength of superalloy – Inconel 718 depends on heat treatment process, but it is about 1030 MPa.
The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behaviour termed a yield point phenomenon. Yield strengths vary from 35 MPa for a low-strength aluminum to greater than 1400 MPa for very high-strength steels.
Young’s Modulus of Elasticity
Young’s modulus of elasticity of superalloy – Inconel 718 is 200 GPa.
The Young’s modulus of elasticity is the elastic modulus for tensile and compressive stress in the linear elasticity regime of a uniaxial deformation and is usually assessed by tensile tests. Up to a limiting stress, a body will be able to recover its dimensions on removal of the load. The applied stresses cause the atoms in a crystal to move from their equilibrium position. All the atoms are displaced the same amount and still maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions and no permanent deformation occurs. According to the Hooke’s law, the stress is proportional to the strain (in the elastic region), and the slope is Young’s modulus. Young’s modulus is equal to the longitudinal stress divided by the strain.
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