Titanium & Superalloys
Titanium and nickel-based superalloys occupy the high-performance end of the materials spectrum. They're expensive, difficult to process, and absolutely essential where no other material can survive the combination of loads, temperatures, and environments found in jet engines and high-performance structures.
Titanium — The Aerospace Metal
Density: 4.5 g/cm³ — 43% lighter than steel, 57% heavier than aluminum Corrosion resistance: Exceptional — forms a stable TiO₂ passive layer resistant to seawater, acids, and body fluids Melting point: ~1,668°C Crystal structure: HCP (alpha phase below ~882°C), BCC (beta phase above ~882°C)Why Titanium Is Special
Titanium sits in a unique position: nearly as strong as steel at 57% of the weight, with better corrosion resistance than stainless steel and useful temperature capability up to ~380°C. The catch is cost — titanium is 10–20× more expensive than steel and difficult to machine (low thermal conductivity, chemical reactivity with tooling, work hardening).
Alloy Classifications
| Type | Phase | Examples | Characteristics |
|---|---|---|---|
| CP (commercially pure) | α | Grades 1–4 | Increasing strength with grade number. Corrosion applications, chemical processing, biomedical implants. |
| Alpha | α | Ti-5Al-2.5Sn | Good weldability, moderate strength, cryogenic toughness |
| Alpha-Beta | α + β | Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo | Best all-around combination. ~60% of all titanium production. |
| Beta | β | Ti-5553, Ti-10V-2Fe-3Al | Higher strength, better formability, heavier. Landing gear. |
| Near-alpha | α (trace β) | Ti-6242 | High-temp compressor applications up to ~540°C |
Ti-6Al-4V — The Workhorse
Ti-6Al-4V (Grade 5) accounts for roughly 60% of all titanium used in aerospace:
| Property | Annealed | STA (Solution Treated + Aged) |
|---|---|---|
| σy (MPa) | ~880 | ~1,100 |
| σu (MPa) | ~950 | ~1,170 |
| Elongation (%) | ~14 | ~10 |
| E (GPa) | ~110 | ~110 |
| Fracture toughness KIC | ~75 MPa√m | ~55 MPa√m |
Aerospace Titanium Applications
Fan blades & discs — Ti-6Al-4V. Fan blades are the first components hit by ingested birds; titanium's combination of strength, damage tolerance, and fatigue resistance is unmatched (though CFRP is replacing titanium in the newest engines like GE9X). Compressor discs & blades — Ti-6Al-4V (lower stages), Ti-6242 (higher stages up to ~540°C). Up to 1/3 of a jet engine's dry weight is titanium. Airframe bulkheads — F-22 Raptor: 41% titanium by structural weight. Large forged Ti bulkheads connect the wing to the fuselage. Landing gear — Ti-5553 and Ti-10V-2Fe-3Al (beta titanium alloys). Higher strength than Ti-6Al-4V, better section hardenability for thick forgings. Boeing 787: 15% titanium by weight — primarily at interfaces where CFRP meets aluminum (titanium is galvanically compatible with both).Automotive Titanium
Titanium use in automotive is limited by cost, restricted to high-performance applications:
- Exhaust valves: Ti-6Al-4V reduces reciprocating mass, enabling higher RPM (Ferrari, Porsche, BMW M engines)
- Connecting rods: Racing engines (F1, NASCAR)
- Valve springs: Corvette Z06 — titanium retainers reduce valvetrain mass
- Exhaust systems: Aftermarket high-performance (Akrapovič)
- Fasteners: Motorsport — every gram counts
Nickel-Based Superalloys
Superalloys are alloys designed to maintain strength, creep resistance, and oxidation resistance at temperatures above 540°C where conventional steels and titanium degrade. Nickel-based superalloys operate up to 700°C in load-bearing applications and up to 1,100°C+ with cooling and thermal barrier coatings.
Why Nickel?
Nickel has an FCC crystal structure (stable, ductile), a melting point of 1,455°C, and forms a stable, adherent oxide layer. It can dissolve large amounts of alloying elements and supports precipitation of γ' (Ni₃Al) and γ'' (Ni₃Nb) strengthening phases.
Key Superalloy Grades
Inconel 718 — The most widely used superalloy (~50% of all superalloy production by weight).- σy ~1,035 MPa, σu ~1,240 MPa at room temperature
- Service temperature: up to ~650°C (creep-limited)
- Density: 8.2 g/cm³
- Applications: Turbine discs, shafts, casings, rocket engine combustion chambers
- Strengthened by γ'' (Ni₃Nb) precipitates
Single-Crystal Turbine Blades
The hottest section of a jet engine is the high-pressure turbine, where gas temperatures can exceed 1,500°C — far above the melting point of any superalloy (~1,300°C). The solution is a combination of:
- Internal cooling channels — Complex serpentine passages cast into the blade, fed with cooler compressor air
- Thermal barrier coatings (TBC) — ~100–200 μm of yttria-stabilized zirconia (YSZ) ceramic, providing ~150°C temperature drop across the coating
- Single-crystal metallurgy — Eliminating all grain boundaries removes the weakest link for creep failure
Cobalt Superalloys
Cobalt-based alloys (Stellite series) are used where extreme wear and hot corrosion resistance matter more than strength:
- Stellite 6: Valve seats in engines and pumps
- Stellite 21: Dental and orthopedic implants
- L-605 (Haynes 25): Combustion chambers, afterburner components
The Temperature Hierarchy
| Temperature Range | Material | Example Application |
|---|---|---|
| Below 200°C | Steels, aluminum, polymers | Car body, passenger cabin |
| 200–380°C | Titanium alloys | Fan blades, compressor (low stages) |
| 380–540°C | Near-alpha titanium | Compressor (high stages) |
| 540–650°C | Inconel 718 | Turbine discs |
| 650–1,050°C | Cast nickel superalloys | HP turbine blades (with cooling) |
| Above 1,050°C | SiC/SiC CMCs, TBCs | Combustor liners, turbine shrouds |
Key Takeaways
- Ti-6Al-4V is the dominant titanium alloy (60% of production), combining high specific strength with good fatigue and corrosion performance
- Titanium sits between aluminum and steel in density and fills the temperature gap between 200–540°C
- Nickel superalloys (Inconel 718) are the only metallic materials that can carry structural loads above 540°C
- Single-crystal turbine blades + TBC coatings + internal cooling enable operation in gas temperatures above the alloy's melting point
- Cost is the primary barrier to wider titanium adoption in automotive
