Material
Nickel Alloys Ni
What you use when titanium runs out of temperature. Nickel superalloys hold useful strength above 1000 degrees Celsius, which is why the hot section of every jet engine is made of them.
Overview
Nickel-based superalloys occupy the top of the temperature ladder in structural metals. Above roughly 600 degrees Celsius titanium loses strength and the choice becomes nickel, cobalt, or ceramics. Their strength comes from gamma prime precipitates that remain stable at temperatures where most alloys would soften, and from heavy additions of refractory elements that make them expensive, difficult to melt cleanly and famously hard to machine. A second family, the corrosion resistant alloys such as the Hastelloy and Inconel 625 grades, trades some high temperature capability for resistance to chemical environments that destroy stainless.
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Defense applications
Gas turbine hot section
Turbine blades, vanes, discs, combustors and exhaust structure. Single crystal castings in this family are among the most technically demanding parts made anywhere, and the qualified supplier list is correspondingly short.
Turbine blades and vanes, Turbine discs, Combustor liners, Exhaust nozzles
Rocket propulsion
Combustion chamber structure, injectors, turbopump components and nozzle extensions where the combination of temperature, pressure and hydrogen environment eliminates most alternatives.
Combustion chambers, Turbopump housings, Injector plates, Nozzle extensions
Corrosion and chemical service
Alloy 625, C-276 and similar grades handle seawater, acids and high chloride environments where even duplex stainless fails, at a substantial cost premium.
Seawater piping, Scrubber and exhaust systems, Chemical processing hardware, Submarine seawater valves
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| Inconel 718 | Precipitation hardening nickel-iron-chromium | The most widely used superalloy by a wide margin. Sluggish aging response makes it weldable in a way most gamma prime alloys are not, which is why it dominates fabricated engine structure. | Bar and Rod, Billet, Plate, Sheet, Forgings, Powder | |
| Inconel 625 | Solid solution nickel-chromium-molybdenum | A corrosion resistant workhorse rather than a high temperature structural alloy. Excellent in seawater and chlorides, readily welded, and widely used for cladding. | Plate, Sheet, Bar and Rod, Tube and Pipe, Wire, Powder | |
| Inconel X-750 | Precipitation hardening nickel-chromium | An age hardenable alloy used where spring properties and relaxation resistance are needed at temperature. | Bar and Rod, Wire, Sheet, Forgings | |
| Hastelloy C-276 | Nickel-molybdenum-chromium | Among the most broadly corrosion resistant alloys in production, handling both oxidizing and reducing acids that destroy most other materials. | Plate, Sheet, Bar and Rod, Tube and Pipe | |
| Waspaloy | Gamma prime strengthened nickel | A high temperature disc and fastener alloy holding strength above where 718 falls off, at the cost of much greater difficulty in forging and welding. | Billet, Bar and Rod, Forgings | |
| Hastelloy X | Solid solution nickel-chromium-iron-molybdenum | A high temperature sheet alloy with excellent oxidation resistance and fabricability, long used for combustor and burner hardware. | Sheet, Plate, Bar and Rod, Powder |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 8.2 to 8.9 | g/cm3 | Roughly double titanium; a real weight penalty accepted for temperature capability |
| Melting range | approximately 1290 to 1400 | degrees C | Varies substantially with alloy content |
| Elastic modulus | 200 to 220 | GPa | Comparable to steel |
| Tensile strength, Inconel 718 aged | 1275 typical | MPa | 185 ksi at room temperature; the workhorse aerospace superalloy |
| Maximum service temperature | approximately 1000 to 1100 | degrees C | Well above titanium's 600 degree limit; single crystal blade alloys go higher still |
| Thermal conductivity | 9 to 15 | W/m-K | Very low, which is the main reason these alloys are so hard on cutting tools |
| Coefficient of thermal expansion | 12 to 14 | micrometre/m-K | |
| Machinability | Poor | Work hardens aggressively and holds heat at the cutting edge; expect low speeds and high tool consumption |
Advantages
- Useful strength above 1000 degrees Celsius, far beyond titanium or steel
- Excellent creep and stress rupture resistance at temperature
- Outstanding corrosion resistance in the chemical service grades
- Retains toughness across a very wide temperature range including cryogenic
- Oxidation resistant, forming a protective chromia or alumina scale
- Well established domestic melting and mill product base
Limitations
- Roughly double the density of titanium
- Expensive, driven by nickel, cobalt, rhenium and other refractory additions
- Very poor machinability, with low cutting speeds and high tool consumption
- Requires vacuum induction melting and often vacuum arc remelting for clean aerospace material
- Difficult to weld in the gamma prime strengthened grades, which are prone to strain age cracking
- Long lead times, since melt capacity for aerospace-grade material is limited
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| AMS 5662 | SAE International | Inconel 718 bars, forgings and rings, solution treated |
| AMS 5596 | SAE International | Inconel 718 sheet, strip and plate |
| AMS 5599 | SAE International | Inconel 625 sheet, strip and plate |
| ASTM B443 | ASTM International | Nickel-chromium-molybdenum-columbium alloy plate, sheet and strip |
| ASTM B637 | ASTM International | Precipitation hardening nickel alloy bars, forgings and forging stock for high temperature service |
| AMS 2774 | SAE International | Heat treatment of wrought nickel and cobalt alloy parts |
| DFARS 252.225-7009 | U.S. Department of Defense | Specialty metals restriction. Nickel alloys fall inside the definition |