Inconel 718 Superalloy
Inconel 718 is one of the most widely used nickel-based superalloys in high-performance engineering, renowned for its exceptional strength at elevated temperatures, excellent corrosion resistance, and superior creep resistance. It is a precipitation-hardening alloy designed to maintain structural integrity in extreme environments, making it indispensable in aerospace, energy, and industrial applications where reliability under heat and stress is critical.
Basic Information
Definition: Inconel 718 is a nickel-chromium-iron superalloy strengthened by precipitates of niobium (columbium) and titanium carbides/intermetallic phases. Its unique microstructure allows it to retain high strength at temperatures up to 650°C (1200°F) and resist oxidation and corrosion in aggressive environments.
Corresponding Standards: Conforms to international specifications such as ASTM B637 (bars/wire), ASTM B670 (forgings), and AMS 5662 (aerospace grade). It is designated as UNS N07718 and Werkstoff Nr. 2.4668.
Key Characteristics: High-temperature strength, excellent creep resistance, good corrosion resistance in oxidizing and reducing environments, and weldability with minimal post-weld heat treatment.
Chemical Composition
Inconel 718’s composition is precisely engineered to balance strength, corrosion resistance, and fabricability:
Element Content Range Role in the Alloy
Nickel (Ni) 50.0–55.0% Base element; stabilizes the austenitic matrix and provides high-temperature stability.
Chromium (Cr) 17.0–21.0% Forms a protective oxide layer, enhancing oxidation and corrosion resistance.
Iron (Fe) 余量 (Balance) Improves ductility and reduces material cost while supporting alloy stability.
Niobium (Nb) + Tantalum (Ta) 4.75–5.50% Key strengthening elements; form γ” (Ni₃Nb) and γ’ (Ni₃(Al,Ti)) precipitates for high-temperature strength.
Molybdenum (Mo) 2.80–3.30% Enhances strength and creep resistance; improves corrosion resistance in reducing environments.
Titanium (Ti) 0.65–1.15% Aids in precipitation hardening by forming γ’ phases; enhances high-temperature strength.
Aluminum (Al) 0.20–0.80% Promotes γ’ precipitate formation, contributing to strength and oxidation resistance.
Carbon (C) ≤0.08% Controls carbide formation; improves grain boundary strength.
Manganese (Mn) ≤0.35% Aids in deoxidation; controlled to avoid reducing toughness.
Silicon (Si) ≤0.35% Acts as a deoxidizer; excessive content can reduce corrosion resistance.
Phosphorus (P) ≤0.015% Strictly controlled to prevent grain boundary brittleness.
Sulfur (S) ≤0.015% Minimized to avoid hot cracking during welding and forging.
Physical Properties
Density: Approximately 8.19 g/cm³.
Melting Point: 1260–1320°C (2300–2400°F).
Thermal Conductivity: ~11.4 W/(m·K) at 20°C; increases to ~19 W/(m·K) at 650°C.
Coefficient of Thermal Expansion: ~11.7×10⁻⁶/°C (20–100°C); ~14.1×10⁻⁶/°C (20–650°C).
Elastic Modulus: ~204 GPa at 20°C; ~186 GPa at 650°C.
Magnetic Properties: Non-magnetic in the annealed and aged conditions.
Mechanical Properties
Inconel 718’s mechanical properties are optimized through precipitation hardening, with exceptional performance at both ambient and elevated temperatures:
Typical Properties (Aged Condition)
Property Value (Room Temperature) Value (650°C)
Tensile Strength ≥1240 MPa ≥965 MPa
Yield Strength (0.2% offset) ≥1035 MPa ≥895 MPa
Elongation ≥12% ≥15%
Hardness 38–45 HRC ~35 HRC
Creep Resistance: Excellent—resists deformation under long-term stress at high temperatures (e.g., <0.2% creep strain after 1000 hours at 650°C under 620 MPa stress). Impact Toughness: ~20–30 J (Charpy V-notch) at room temperature; retains toughness at cryogenic temperatures. Fatigue Strength: ~620 MPa (10⁷ cycles at room temperature), making it suitable for cyclic load applications like turbine blades. Heat Treatment Inconel 718’s strength is achieved through a two-step aging process to precipitate strengthening phases: Solution Annealing Purpose: Dissolve alloying elements uniformly and prepare for precipitation. Temperature: 950–1050°C (1740–1920°F), held for 1–2 hours. Cooling: Air cooling or water quenching to retain a supersaturated solid solution. Aging First Step (Intermediate Aging): 720°C (1330°F) for 8 hours, followed by furnace cooling to 620°C (1150°F) over 2 hours. Second Step (Final Aging): Hold at 620°C (1150°F) for 8 hours, then air cooling. Result: Forms fine γ'' (Ni₃Nb) and γ' (Ni₃(Al,Ti)) precipitates, maximizing strength and creep resistance. Processing Performance Weldability: Excellent compared to other nickel-based superalloys. It can be welded using GTAW (TIG), GMAW (MIG), and electron beam welding. Post-weld aging (without solution annealing) restores strength, reducing production complexity. Machinability: Moderate to challenging due to high strength and work-hardening tendency. Requires carbide tools, low cutting speeds, and ample coolant to prevent tool wear and overheating. Forging & Forming: Can be hot-forged at 980–1120°C (1800–2050°F) to shape complex components. Cold forming is possible but limited by high strength; intermediate annealing may be needed. Castability: Available as castings (e.g., investment cast turbine parts) with properties similar to wrought forms. Application Fields Inconel 718 is the material of choice for high-temperature, high-stress applications across industries: Aerospace & Defense: Gas turbine engine components (blades, disks, casings, and fasteners) in jet engines. Rocket motor casings, spacecraft structural parts, and missile components. Energy Industry: Gas turbine blades and vanes in power generation plants. Oil and gas downhole tools, wellhead components, and subsea equipment (resists corrosive brines). Industrial Engineering: High-temperature fasteners, valves, and heat exchangers in chemical processing. Nuclear reactor components (resists radiation and high-temperature coolant environments). Automotive: Turbocharger wheels and exhaust components in high-performance engines. In summary, Inconel 718’s unmatched combination of high-temperature strength, corrosion resistance, and fabricability has solidified its role as a critical material in advanced engineering. Its reliability in extreme environments makes it a staple in industries where failure is not an option.