Inconel 718

Inconel 718
Ascaron · CC BY-SA 3.0 · source
Name	Inconel 718
Othernames	Alloy 718, UNS N07718
Composition	Nickel-chromium-iron with niobium, molybdenum, titanium, aluminium, trace elements
Type	Precipitation-hardened nickel-based superalloy
Density	8.19 g/cm3
Melting point	~1260–1336 °C
Developed	1950s
Primary uses	High-temperature components, gas turbines, rocket engines

Inconel 718 is a precipitation-hardened nickel-chromium-iron superalloy noted for a combination of high strength, creep resistance, and corrosion resistance at elevated temperatures. It was developed in the mid-20th century and has been widely adopted in aerospace, power generation, and chemical processing for components exposed to severe environments. Major industrial users and developers include manufacturers and agencies such as General Electric, Rolls-Royce plc, Pratt & Whitney, NASA, and national laboratories involved in materials research.

Composition and microstructure

The nominal chemistry of the alloy includes nickel, chromium, and iron with alloying additions of niobium, molybdenum, titanium, and aluminium, forming a complex matrix that produces austenitic and intermetallic phases; this composition evolution is documented in metallurgical studies by institutions such as Oak Ridge National Laboratory, Sandia National Laboratories, and MIT. The microstructure is characterized by a face-centered cubic austenitic matrix with strengthening precipitates of gamma-prime and gamma-double-prime, carbides, and sometimes delta phase; influential research groups at Imperial College London, ETH Zurich, and University of Cambridge have published on precipitate morphology. Thermomechanical processing and aging cycles control the distribution of niobium-rich phases, which was a central topic in reports from U.S. Air Force Research Laboratory, Aerospace Corporation, and industrial labs at Alcoa.

Mechanical properties and behavior

Mechanical behavior includes high yield strength, tensile strength, and fatigue resistance up to intermediate-high temperatures, characteristics tested in standards from organizations like ASTM International, ISO, and SAE International. Creep and stress-rupture performance under sustained loading has been the subject of experimental programs by National Institute of Standards and Technology, European Space Agency, and university research groups such as University of Oxford. Fracture mechanics, fatigue crack growth rates, and creep-fatigue interaction have been investigated in collaboration with manufacturers including Honeywell Aerospace and research consortia involving Rolls-Royce plc and Boeing.

Heat treatment and processing

Solution annealing, quenching, and age-hardening treatments produce the desired gamma-double-prime precipitates; process parameters are specified in documents from ASTM International, Aerospace Materials Specifications, and internal vendor data from firms like Goodrich Corporation. Hot working, cold working, and thermomechanical processing routes developed at facilities such as Carnegie Mellon University and Lyon-based research centers affect grain structure and texture, influencing properties required by Airbus and Lockheed Martin for aerospace components. Additive manufacturing and powder metallurgy approaches, explored by Lawrence Livermore National Laboratory and Fraunhofer Society, require tailored heat treatments to control anisotropy and defect populations.

Corrosion and oxidation resistance

The alloy exhibits oxidation resistance due to chromium-enriched scales and nickel matrix stability; comparative studies by Corrosion Science groups at University of Manchester and IMechE assess performance in oxidizing and sulfidizing atmospheres. Resistance to chloride-induced stress corrosion cracking and pitting has been evaluated in chemical processing contexts by companies such as BASF and research programs funded by European Commission initiatives. High-temperature oxidation behavior relevant to gas turbines has been examined by Siemens Energy and Mitsubishi Heavy Industries under simulated combustion environments.

Fabrication and joining methods

Fabrication routes include machining, forging, rolling, and powder consolidation employed by industrial suppliers like Arconic and Timet, with machining studies performed at Cranfield University and University of Sheffield. Welding techniques including electron beam welding, laser welding, plasma welding, and gas tungsten arc welding have been developed by teams at TWI (The Welding Institute), Fraunhofer IWS, and corporate labs at General Electric for rotating engine parts. Challenges such as hot cracking, porosity, and post-weld aging are addressed in qualification programs run by NATO and certification authorities such as EASA.

Applications and industries

Primary applications include turbine disks, shafts, combustion liners, and fasteners in aero-engines used by operators such as British Airways, United Airlines, and militaries that procure engines from Rolls-Royce plc and Pratt & Whitney. Spaceflight hardware including rocket motor cases and thrust chamber components have employed the alloy in programs by SpaceX, NASA, and national space agencies like JAXA. Power generation, oil and gas, and chemical processing industries represented by companies such as General Electric, Schlumberger, and Shell plc apply the alloy where high-temperature strength and corrosion resistance are critical.

Standards and specifications

Specifications and designations are provided by standards bodies and agencies including ASTM International, SAE International, AMS (Aerospace Material Specifications), and national standards organizations such as BSI and DIN. Military and aerospace procurement documents from MIL‑STD series and qualification procedures from EASA and FAA reference heat treatment, mechanical testing, and traceability requirements. Manufacturer material data sheets and procurement contracts for components used by Rolls-Royce plc and Pratt & Whitney cite these standards for certification and quality assurance.

Category:Nickel alloys Category:Superalloys