Titanium Alloys — Family Index

1. At a glance

Titanium exhibits an allotropic transformation at 882.5 °C (the β-transus of pure Ti):

  • α phase — hexagonal close-packed (HCP), stable below 882.5 °C in pure Ti. Lower ductility, higher creep resistance, weldable, not heat-treatable to high strength.
  • β phase — body-centred cubic (BCC), stable above 882.5 °C in pure Ti. Higher ductility, deeper hardenability, heat-treatable.
  • α + β mixed — produced by alloying with both α-stabilisers (Al, O, N, C, Sn, Zr is neutral) and β-stabilisers (V, Mo, Nb, Ta, Fe, Cr, Cu, Ni, W, Mn, Si).

Alloy class taxonomy:

Classβ-stabiliser content (Mo equiv.)Heat-treatableTypical examples
CP (commercially pure)0NoGrades 1–4, 7, 11, 12
αlowNo (anneal only)Ti-5Al-2.5Sn
Near-α1–2 % Mo eq.LimitedTi-6242, IMI 834
α-β4–10 % Mo eq.Yes (STA)Ti-6Al-4V, Ti-6-6-2
Metastable β10–15 % Mo eq.Yes (STA)Ti-10-2-3, Ti-5553, Ti-15-3
β (stable)> 30 % Mo eq.No (cold-work only)Beta-C, Ti-40

Headline numbers:

  • Density: 4.51 g/cm³ (~57 % of steel, ~165 % of aluminium).
  • Modulus: 100–115 GPa (~half of steel).
  • Yield: 170 MPa (CP Grade 1) to 1100 MPa (β STA forgings, e.g. Ti-5553).
  • Strength-to-weight: best of any common structural metal in the 200–500 °C range.
  • Coefficient of thermal expansion: 8.6 × 10⁻⁶/K (~half of austenitic stainless).
  • Thermal conductivity: 6.7 W/m·K (very low — drives machining behaviour).
  • Excellent passive-oxide corrosion resistance in oxidising/neutral media; vulnerable in reducing acids and dry chlorine.

2. CP grades (commercially pure)

All CP grades are single-phase α. Strength rises with interstitial O + N + Fe; ductility falls.

ASTM B265 gradeComposition (nominal)Yield (MPa)UTS (MPa)Elong (%)Notes
Grade 1unalloyed, O ≤ 0.1817024030Highest formability, deep-draw, explosion clad
Grade 2unalloyed, O ≤ 0.2527534528The common one — heat-exchanger, chem plant, seawater
Grade 3unalloyed, O ≤ 0.3538045025Pressure vessels, mid-strength
Grade 4unalloyed, O ≤ 0.4048055020Highest-strength CP; surgical, airframe
Grade 7Ti + 0.12–0.25 Pd27534528Grade 2 chemistry + Pd → crevice-corrosion resistance
Grade 11Ti + 0.12–0.25 Pd17024030Grade 1 chemistry + Pd
Grade 12Ti-0.3Mo-0.8Ni34548018Improved CR + weldability, mildly reducing acids
Grade 17Ti + 0.04–0.08 Pd27534528Lean-Pd cost-reduced Grade 7
Grade 27Ti + 0.04–0.08 Pd38045025Lean-Pd cost-reduced equivalent of Grade 16/9-class

Grade 2 is the workhorse for chemical-process plant, desalination, geothermal, marine condenser tubing. Grade 7 is the answer for crevice corrosion in hot brines (where Grade 2 may fail by crevice attack above ~70 °C).

3. Alpha and near-α alloys

α and near-α retain α-phase stability up to high temperature → used where creep + weldability matter more than peak strength.

  • Ti-5Al-2.5Sn (Grade 6) — true α. Cryogenic tankage (LH₂ pressure vessels — Atlas Centaur), steam-turbine blades. ELI variant for cryogenic toughness.
  • Ti-3Al-2.5V (Grade 9) — near-α (small β content). Hydraulic-tubing standard on commercial airframes (cold-pilgered seamless tube). 620 MPa yield cold-worked.
  • Ti-6Al-2Sn-4Zr-2Mo (Ti-6242 / Ti-6242S with Si) — near-α. Engine compressor discs and blades to 540 °C; Si addition (0.08–0.1 %) for creep.
  • Ti-8Al-1Mo-1V — high-Al near-α; SR-71 / P&W JT9D fan blades historically. Largely superseded by 6242 due to α₂ embrittlement.
  • IMI 685 (Ti-6Al-5Zr-0.5Mo-0.25Si) — near-α, ≤520 °C compressor.
  • IMI 829 (Ti-5.5Al-3.5Sn-3Zr-1Nb-0.25Mo-0.3Si) — ≤550 °C.
  • IMI 834 (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C) — ≤600 °C, Rolls-Royce Trent compressor discs.
  • Ti-1100 (Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si) — ≤600 °C, US analogue of IMI 834.

4. α-β workhorses

The largest production class. Heat-treatable by solution + age (STA); strength tunable by ageing temperature.

  • Ti-6Al-4V (Grade 5) — the iconic alloy. ~50 % of all titanium produced. Mill-annealed: 830 MPa yield, 900 MPa UTS, 14 % elong. STA: up to ~1100 MPa yield in thin sections. Workhorse for airframe forgings (F-22, F-35, 787 wing-to-body fittings), engine fan blades (CFM56 stages 1–3), gas-turbine cases, total hip stems since the 1970s.
  • Ti-6Al-4V ELI (Grade 23) — extra-low interstitial (O ≤ 0.13, Fe ≤ 0.25). Implant grade + cryogenic. Lower strength, much higher fracture toughness.
  • Ti-6Al-4V STA — solution at 925–955 °C + age 480–595 °C → 1100 MPa yield class. Section-size limited (hardenability shallow).
  • Ti-6Al-6V-2Sn (Grade 24) — higher β content than 6-4. 1100 MPa STA in heavier sections than 6-4. Ordnance, rocket motor cases.
  • Ti-3Al-2.5V (Grade 9) — near-α but typically grouped here for tubing applications. CWSR (cold-worked stress-relieved) tube: 690 MPa yield.
  • Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si (β-CEZ, Ti-17) — α-β with deep hardenability. Engine fan and compressor discs to ~430 °C.
  • Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) — α-β; deeper hardenability than 6-4, used to ~450 °C disc applications.

5. Metastable β and β alloys

High β-stabiliser content → β retained on quench, aged at 480–565 °C to precipitate fine α and reach peak strength. Deep-section hardenable; cold-formable in solution-treated condition; heaviest of the Ti alloys (~4.85 g/cm³).

  • Ti-10V-2Fe-3Al (Ti-10-2-3) — Boeing 777 / 787 main and nose landing-gear truck beams, beams, and uplocks. STA: 1240 MPa UTS, deep-section forgeable. Replaced 300M steel in 777 MLG.
  • Ti-5Al-5V-5Mo-3Cr (Ti-5553, VSMPO TC18) — A350 main landing gear, large fittings. STA: 1250 MPa UTS in heavy section. Slightly higher modulus than 10-2-3.
  • Ti-15V-3Cr-3Sn-3Al (Ti-15-3) — cold-rollable sheet and spring alloy. STA: 1000 MPa UTS. Honeycomb skins, springs.
  • Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C, Grade 19/20) — coil and rod springs, downhole oilfield tubulars, fasteners. STA: 1240 MPa UTS.
  • Ti-15Mo (ASTM F2066) — medical β; high-strength orthopaedic.
  • Ti-13Nb-13Zr (ASTM F1713) — medical near-β; modulus ~79 GPa (closer to bone — reduces stress-shielding).
  • Ti-12Mo-6Zr-2Fe (TMZF, ASTM F1813) — medical β; modulus ~74–85 GPa.
  • Ti-35Nb-7Zr-5Ta (TNZT) — research-grade ultra-low-modulus (~55 GPa).
  • Timetal 21S (Ti-15Mo-3Nb-3Al-0.2Si) — high-temperature β for engine ducting / nozzles; oxidation-resistant.

6. Aerospace alloys grouped by application

Engine — compressor (rotating, hot section approaching turbine):

AlloyService ceilingForm
Ti-6-4315 °CFan blades, LP compressor blades
Ti-6242540 °CHP compressor discs + blades
Ti-6246450 °CDiscs (deeper hardenability than 6-4)
IMI 829550 °CCompressor discs
IMI 834 / Ti-1100600 °CLate-stage HP compressor discs
Ti-17 (5-2-2-4-4)430 °CDiscs (Snecma M88)

Airframe — forgings (high static + fatigue strength, deep sections):

AlloySectionStrengthUse
Ti-6-4thin/mid830 MPa yieldBulkheads, fittings, fasteners
Ti-6-4 STAthin only1100 MPaCritical small forgings
Ti-10-2-3heavy1100–1240 MPa777/787 MLG
Ti-5553heavy1100–1250 MPaA350 MLG, large fittings
Ti-6246mid/heavy1100 MPaDiscs, larger fittings

Airframe — sheet and plate:

AlloyFormUse
Ti-6-4sheet/plateSkins, brackets, firewalls
Ti-15-3sheet (cold-rolled)Skins, honeycomb face sheets
Ti-3-2.5 (Grade 9)tubeHydraulic tubing (3000–5000 psi)
CP Grade 2/4sheetAcoustic panels, ducting

7. Medical / biocompatible alloys

Three biological requirements: passive oxide stability in saline + serum, no V/Ni/Co ion release, modulus matching bone (10–30 GPa cortical) to reduce stress-shielding.

AlloyASTMModulus (GPa)Use
CP Grade 4F67105Dental abutments, dental crowns, plates
Ti-6Al-4V ELIF136114Total hip stems, femoral heads (historical), spinal cages, dental
Ti-6Al-7Nb (Protasul-100)F1295105Hip stems (vanadium-free), bone screws
Ti-13Nb-13ZrF171379Hip stems (lower-modulus generation)
Ti-12Mo-6Zr-2Fe (TMZF)F181374–85Hip stems
Ti-15MoF206678Spinal, orthopaedic
Ti-Ni (Nitinol, ~50 at% Ni)F2063varies (SE)Stents, orthodontic arch wire, guidewires — see titanium-alloys

Surface treatments common in medical: anodised TiO₂ for colour-coding and apatite-affinity; HA (hydroxyapatite) plasma-spray for bone in-growth; SLA (sandblast + acid-etch) for dental implants.

8. Heat treatment

Standard treatments:

  • Mill anneal (MA) — α-β alloys held below β-transus (e.g. 705–790 °C for Ti-6-4), air-cool. Default for plate/bar.
  • Solution treat + age (STA) — solution above or just below β-transus, water- or oil-quench, age 480–595 °C / 4–8 h. Peak strength in α-β and metastable β.
  • Recrystallisation anneal (RA) — 925 °C / 4 h / FC for Ti-6-4 → coarser α, better fracture toughness.
  • Beta anneal (BA) — 30–50 °C above β-transus, slow-cool → lamellar (Widmanstätten) α colony structure. Best fracture toughness, lower fatigue.
  • Stress relief — 480–650 °C / 1–4 h. Standard after welding and machining.
  • Duplex anneal — first hold near β-transus to grow β grain, second below to grow primary α → bi-modal microstructure.

Microstructures (Ti-6-4 reference):

MicrostructureOriginStrengthFatigueToughnessCreep
Equiaxed (α-β annealed)Sub-transus + slow-coolMidBest LCFMidMid
Lamellar (β-annealed)Super-transus + coolMidWorst LCFBestBest
Bi-modal (duplex)Duplex annealHighMid–highMid–highMid
Martensitic α′Super-transus + WQHighestMidLowestLowest
Acicular Widmanstättenβ + air-cool from above transusHighMidHighHigh

Forging is typically α-β (sub-transus) to keep an equiaxed primary-α structure. β-forging (above transus) is used where toughness or creep dominates (turbine discs, large airframe fittings in 10-2-3 or 5553).

9. Processing notes

Melting: double or triple VAR (vacuum arc remelt) from Kroll-process sponge + master-alloy. Critical applications (rotating engine parts) use triple-VAR or VAR-after-EB-CHM (cold-hearth) to remove high-density inclusions (HDI — tungsten carbide from Kroll reactor) and hard-α (oxygen-rich nitride/oxide). Per AMS 2380 / 4928 rev N.

Forging window: 815–980 °C for α-β alloys; β-forging windows are alloy-specific and typically span ~30 °C above transus. Lubricant: glass-frit (Acheson) coatings. Die life is poor — Ti work-hardens against the die.

Machining:

  • Cutting speeds 30–60 m/min (Ti-6-4) vs ~150 for steel.
  • Sharp tools, large positive rake, high feed rate, low speed.
  • Flood coolant (water-soluble) — never run dry. Heat does not leave with the chip; it goes into the tool.
  • Carbide grades K10/K20 or coated; high-pressure (70 bar+) through-spindle coolant for milling.
  • Climb-mill, not conventional. Never dwell.
  • Chip ignition risk on dry grinding; magnesium-like fire response (no water on burning Ti; smother with Class D dry powder).

Welding: GTAW (TIG) with argon back-purge + trailing shield; oxygen pickup above 425 °C embrittles the weld. Acceptable hardness rise via colour: silver/straw OK, blue marginal, grey/white reject. EB and laser also standard. Resistance and friction-stir for some sheet applications.

Forming: SPF (superplastic forming) at 870–925 °C for Ti-6-4 — strain rates ~10⁻⁴ /s, elongations 300–1000 %. Combined SPF/DB (diffusion-bonding) for honeycomb sandwich.

Additive manufacturing:

  • Ti-6-4 LPBF — standard. Build under argon; HIP at 920 °C / 100 MPa / 2 h is mandatory for fatigue-rated parts to close internal porosity.
  • Ti-6-4 EBM — Arcam-process; built at 700 °C in vacuum; no post-stress-relief needed.
  • Ti-6242 LPBF — emerging for engine brackets.
  • CP grades + Beta-C + Ti-5553 — also commercially available.

Galvanic / contamination cautions:

  • No contact with bare steel during heat treatment (Fe diffuses in, lowers β-transus locally, ruins fatigue).
  • No chlorinated cutting fluids — risk of stress-corrosion cracking (SCC) in subsequent service.
  • No fluoride-containing solutions, no hot concentrated HCl, no anhydrous methanol, no red fuming nitric acid (RFNA) — pyrophoric reaction with Ti.
  • No Cd plating in contact with Ti above ~230 °C — liquid-metal embrittlement.
  • Avoid mercury, AgCl, lead solder near titanium parts.

10. Comparison table — common alloys

UTS / Yield / Elong are typical room-temperature properties for the common heat-treat condition.

AlloyClassUTS (MPa)Yield (MPa)Elong (%)T_service (°C)Typical use
CP Grade 1α (CP)24017030300Deep-draw, explosion clad
CP Grade 2α (CP)34527528300Chem plant, seawater
CP Grade 3α (CP)45038025300Pressure vessel
CP Grade 4α (CP)55048020300Surgical, airframe
Grade 7α (CP+Pd)34527528300Crevice-corrosion service
Grade 12α (CP+MoNi)48034518300Mild reducing acids
Ti-5Al-2.5Sn (Gr 6)α86080016480Cryogenic, steam turbine
Ti-5Al-2.5Sn ELIα7607301720 KLH₂ tankage
Ti-3Al-2.5V (Gr 9)near-α69062015300Hydraulic tube CWSR
Ti-8Al-1Mo-1Vnear-α100095015455(Legacy) engine
Ti-6242near-α101095014540Compressor disc/blade
IMI 685near-α99088013520Compressor disc
IMI 829near-α100090013550Compressor disc
IMI 834near-α105095012600Compressor disc
Ti-1100near-α101091012600Compressor disc
Ti-6Al-4V (Gr 5) MAα-β95088014400Workhorse
Ti-6Al-4V STAα-β1170110010400Fasteners, fittings
Ti-6Al-4V ELI (Gr 23)α-β8607951520 K / implantHip, cryogenic
Ti-6Al-6V-2Sn (Gr 24)α-β1170110010315Heavier sections than 6-4
Ti-6246α-β1170110010450Disc, heavy forging
Ti-17 (5-2-2-4-4)α-β1180109012430Engine disc (M88)
Ti-10-2-3meta-β124011708315777 MLG
Ti-5553meta-β125011708315A350 MLG, large fittings
Ti-15-3meta-β100096510315Sheet, springs
Beta-C (Gr 19/20)meta-β124011708315Springs, oilfield
Ti-6Al-7Nbα-β90080015implantV-free hip stem
Ti-13Nb-13Zrnear-β97083015implantLow-modulus hip
Ti-15Moβ87076021implantSpinal
Timetal 21Sβ1240117010540Engine duct, nozzle

Densities all 4.4–4.9 g/cm³ (Ti-10-2-3 + Beta-C heaviest at ~4.85; CP lightest at 4.51).

11. Selection heuristics

  • Aircraft fastener, high-strength → Ti-6Al-4V forged + STA (AMS 4967, AS3239).
  • Engine compressor blade ≤ 540 °C → Ti-6242S (with Si).
  • Engine compressor disc ≤ 600 °C → IMI 834 or Ti-1100.
  • Fan blade, large → Ti-6Al-4V (hollow, with SPF/DB).
  • Orthopaedic implant, classical → Ti-6Al-4V ELI (F136) or Ti-6Al-7Nb (V-free, F1295).
  • Orthopaedic implant, lower modulus to reduce stress-shielding → Ti-13Nb-13Zr or Ti-12Mo-6Zr-2Fe.
  • Coil and torsion springs → Beta-C STA (Grade 19/20) or Ti-15-3.
  • Sheet, cold-formable, age-hardenable → Ti-15-3.
  • Heavy-section landing-gear forging → Ti-10-2-3 (Boeing legacy) or Ti-5553 (Airbus).
  • Hydraulic tubing, airframe (3000–5000 psi) → Ti-3Al-2.5V (Grade 9) CWSR per AMS 4945.
  • Corrosive chem-plant heat exchanger → CP Grade 2 (ASTM B265, ASTM B338 tubing).
  • Hot brine with crevice risk (geothermal, seawater desalination > 70 °C) → Grade 7 or Grade 27 (Pd-containing).
  • Cryogenic pressure vessel (LH₂, LOX) → Ti-5Al-2.5Sn ELI.
  • Surgical instrument body → CP Grade 4 or Ti-6-4 (anodised colour for size-coding).
  • AM aerospace bracket → Ti-6-4 LPBF + HIP (HIP per AMS 2774 / ASTM F3001).
  • Hot oxidation-resistant nozzle / exhaust skin → Timetal 21S.

12. Failure modes

  • Hydrogen embrittlement — β-stabilised alloys absorb H from cathodic cleaning, plating bath, or acid pickling. Threshold ~100–150 ppm H for Ti-6-4; lower for β alloys. Manifests as delayed cracking. Remedy: vacuum anneal at 700 °C / 4 h post-pickle.
  • Hard-α inclusions — local high-O/N regions from sponge or scrap, brittle and crack on first overstress. Drove the Sioux City DC-10 disc rupture (1989) → triple-VAR + sonic inspection mandated for rotating engine grade.
  • Fines / turnings ignition — chips, dust, and grinding swarf are pyrophoric in air; metal-fire hazard. Wet-collect and store under water; never let dust accumulate.
  • Salt-water SCC — high-strength α-β alloys (Ti-8-1-1, Ti-6-4 STA) crack under load in NaCl above 290 °C. Service-temp guidance: do not solution-treat-and-age Ti-6-4 for parts exposed to hot salt water.
  • Galling / self-mating thread seizure — Ti against Ti or Ti against austenitic stainless cold-welds. Mitigation: Ag-plate or Cu-plate on threads, MoS₂-grease (with caution — sulphur SCC at high T), or Cetyl wax.
  • Fretting fatigue — Ti fan blade roots / dovetails. Mitigation: shot-peen + DLC or CuNiIn coatings.
  • Liquid-metal embrittlement — Cd > 230 °C, Hg, Ag at 850 °C+. Keep Ti away from cadmium-plated tooling and fasteners.
  • Pyrophoric reaction with chlorine — dry Cl₂ and Ti ignite spontaneously above ~30 °C. Wet Cl₂ (≥ 1 % H₂O) is safe.
  • Oxygen-enriched fires — burns in pure O₂ at modest pressure. Avoid Ti in LOX systems or use only the cryogenic ELI grade with controlled surface finish.

13. Vendors

CompanyCountrySpecialty
VSMPO-AVISMARULargest Ti producer worldwide; mill + forgings; ingot for 787, A350, A380
TIMET (Berkshire Hathaway)USMill products, ingot, plate, sheet, tube
ATI (Allegheny Technologies)USMill products, melt, ingot, forging stock
Howmet AerospaceUSEngine + airframe forgings (ex-Arconic Forgings)
PCC Structurals (Berkshire Hathaway)USInvestment castings (Ti-6-4 engine cases, airframe fittings)
Aubert & DuvalFRForgings, special alloys
Tokushu Kinzoku / Kobe Steel / Nippon SteelJPMill products + medical
Western Superconducting Technologies (WST)CNIngot, mill products, forging stock
BAOTI / PangangCNCP grades, sponge
OSAKA Titanium / Toho TitaniumJPKroll sponge feedstock

14. Cross-references

15. Citations

  • ASM Handbook Vol. 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials (1990), titanium section.
  • ASM Handbook Vol. 3: Alloy Phase Diagrams — Ti-Al, Ti-V, Ti-Mo binaries and ternaries.
  • ASM Handbook Vol. 4: Heat Treating — titanium chapter (STA windows, β-transus tables).
  • Donachie, M.J., Titanium: A Technical Guide, 2nd ed., ASM International, 2000.
  • Lütjering, G. and Williams, J.C., Titanium, 2nd ed., Springer Engineering Materials and Processes, 2007.
  • Boyer, R., Welsch, G. and Collings, E.W. (eds.), Materials Properties Handbook: Titanium Alloys, ASM, 1994.
  • ASTM B265 — Standard Specification for Titanium and Titanium Alloy Strip, Sheet, and Plate.
  • ASTM B348 — Bars and Billets.
  • ASTM B338 — Seamless and Welded Tubes for Condensers and Heat Exchangers.
  • ASTM F67 — CP titanium for surgical implant applications.
  • ASTM F136 — Ti-6Al-4V ELI for surgical implants.
  • ASTM F1295 — Ti-6Al-7Nb for surgical implants.
  • ASTM F1713 — Ti-13Nb-13Zr for surgical implants.
  • ASTM F2066 — Ti-15Mo for surgical implants.
  • AMS 4911 — Ti-6Al-4V annealed sheet, strip, and plate.
  • AMS 4928 — Ti-6Al-4V annealed bar, wire, forgings, ring.
  • AMS 4945 — Ti-3Al-2.5V seamless hydraulic tubing.
  • AMS 4967 — Ti-6Al-4V STA bar and forgings.
  • AMS 2380 — Approval and control of premium-quality titanium alloys (triple-VAR for rotating-grade).
  • AMS 2774 — Heat treatment, wrought titanium alloy parts.
  • ASTM F3001 — Additive manufacturing, Ti-6Al-4V ELI with powder-bed fusion.

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