For buyers and design engineers, the titanium-alloy decision is governed by four metallurgical classes — Commercially Pure, alpha, near-alpha, alpha-beta and beta — each with a distinct strength, temperature and formability envelope defined in standards such as ASTM B265, B348, B381, B367, B861 and AMS 4911 [S3].
SourceBySpec's encyclopedic reference entry for Titanium Alloy consolidates composition tables, mechanical-property bands and the OEM-grade callouts that show up in RFQs across chemical, marine, medical and aerospace work. eFunda's titanium-alloy database lists 30+ grades and cross-references them to UTS, yield, elongation, density and modulus [S3]; any sensible selection starts there before going to mill stock.
Selection Criteria: The Six Gates That Drive the Decision
Engineers who skip straight to a grade (Ti-6Al-4V, Ti-Grade 2, Ti-10V-2Fe-3Al) without fixing the envelope end up over-specifying cost. The six gates, in order, are: (1) operating temperature band, (2) required UTS/yield, (3) corrosion regime (chloride, reducing acid, sour H2S, seawater), (4) formability / weldability, (5) density-sensitive application, and (6) cost-per-kg relative to Aluminum Alloy and Nickel Alloy substitutes. [S1]
ASTM B265 plate and B348 bar are the two most cross-referenced product forms; B381 covers forgings, B367 covers castings, and B861/B862 cover seamless and welded tube — so a "titanium" RFQ must name the product form before the grade [S3]. Temperature alone eliminates most candidates: CP-Ti and alpha grades are reliable to ~315 °C, near-alpha pushes to ~425 °C, alpha-beta to ~315-400 °C depending on the grade, and metastable-beta alloys to ~300 °C in aged condition but with far higher strength [S3].
Class-by-Class Comparison Across the Four Decision Axes
Commercially Pure (CP) grades 1-4 trade strength for formability and corrosion resistance — Grade 1 is the most formable (~170 MPa UTS, ~30% elongation), Grade 4 is the strongest CP (~550 MPa UTS) [S3]. They are the default pick for chemical processing heat exchangers, anodes and medical implants where biocompatibility and corrosion resistance dominate.
Alpha alloys (Ti-5Al-2.5Sn) keep strength at cryogenic and elevated temperatures; near-alpha (Ti-6Al-2Sn-4Zr-2Mo, Ti-1100) push high-temperature service into the 425-540 °C window — the canonical jet-engine and compressor-case band. Alpha-beta alloys, with Ti-6Al-4V (Grade 5) as the workhorse, are the most produced titanium composition worldwide: ~895 MPa UTS, ~828 MPa yield, ~10% elongation, 4.43 g/cm³ density [S3]. Beta alloys (Ti-10V-2Fe-3Al, Ti-15V-3Cr-3Al-3Sn) double the strength-to-weight ratio but at higher density (~4.65-4.85 g/cm³) and cost premium of 3-5× over Ti-6Al-4V in mill form.
When the design is density-limited (aerospace structure, Formula 1, defense), Ti-6Al-4V's 4.43 g/cm³ against aluminum's 2.7 g/cm³ and Alloy Steel's 7.85 g/cm³ is the core trade-off: 60% lighter than steel for roughly 4-7× the raw-material cost, with the strength and fatigue envelope to justify it [S3]. Sour-service (NACE MR0175) and chloride pitting both push toward CP-Ti Grade 2, Grade 7 (Ti-0.15Pd) or Grade 12 (Ti-0.3Mo-0.8Ni) — the Pd and Ni additions target reducing-acid and crevice corrosion respectively [S3].
Who Should Specify Titanium — and Who Should Not
Titanium is the right call when the application combines three of these four: density-sensitive weight budget, corrosion exposure (chloride, seawater, wet HCl, nitric acid, oxidizing chlorides), operating temperature below ~540 °C, and biocompatibility / non-magnetic requirements. Marine propeller shafts, seawater-cooled heat exchangers, pressure-vessel linings in chlor-alkali, aerospace structural fittings, and orthopedic implants all sit in that band [S3].
It is the wrong call when the duty is bulk structural (a crawler crane boom or a truck-crane chassis — go to alloy steel HSLA grades), or when the temperature exceeds 540 °C sustained — beyond that point, nickel alloys such as Inconel 718 or Waspaloy win on creep-rupture life. Titanium is also a poor pick for sliding wear (galling is severe without surface treatment) and for parts needing very high hardness without heat treatment — alpha and CP grades cannot be hardened by quench-tempering like steel [S3].
Spec Bands: Reading the Mechanical-Property Tables Correctly
The same sheet highlights that weldment strength drops 10-20% versus parent metal in the as-welded condition unless a post-weld stress relief is specified — buyers who omit the heat-treatment callout on the PO routinely receive mill-annealed stock that fails the UTS spec on the first mechanical test.
The tolerance band on thickness for cold-rolled sheet is ±0.05 mm below 0.5 mm and ±0.10 mm between 0.5-1.0 mm — small, but enough to bust press-brake bend radii if a buyer assumes "titanium sheet tolerances match stainless."
Real Use Cases Mapped to the Class
Seawater reverse-osmosis pressure vessels: Grade 2 CP-Ti shells and Grade 7 (Ti-Pd) tube sheets — the Pd addition handles the crevice-corrosion band between tube and tubesheet. Aerospace: Ti-6Al-4V for structural fittings, near-alpha Ti-6Al-2Sn-4Zr-2Mo for compressor discs and blades exposed to 400-500 °C, beta C (Ti-10V-2Fe-3Al) for landing-gear forgings on large airframes [S3]. Medical: CP Grade 1-4 for dental implants and plates, Ti-6Al-4V ELI (Extra Low Interstitial) for femoral stems where fracture toughness and fatigue life govern.
Industrial chemical: Grade 7 (Ti-0.15Pd) and Grade 12 (Ti-0.3Mo-0.8Ni) for chloride and reducing-acid service; nitrate and organic-acid service is typically Grade 2. The Pd-bearing grades carry a 2-3× raw-material premium over Grade 2 and are the most frequently mis-specified item — buyers default to Grade 5 in chloride service and then see crevice attack inside six months. For pressure transmitters and flow meters wetted parts in chemical and offshore service, Grade 2 or Grade 7 are the standard corrosion-resistant alloy callouts [S3].
Limitations, Failure Modes and Sourcing Constraints
Titanium is unforgiving of three things: hydrogen pickup (embrittlement threshold ~150-200 ppm in CP-Ti, lower in alpha-beta), oxygen contamination during welding (requires inert-gas backing on the root side — purge oxygen below 50 ppm is the typical limit), and galvanic coupling to less-noble metals (Ti is cathodic — pair it to copper-nickel or 316L and the other metal corrodes preferentially). Vacuum die-casting and squeeze-casting operations where titanium is a die or shot-sleeve candidate also run into galling and need surface treatments — buyers comparing vacuum die casting machine tool steels against titanium-faced inserts should test galling resistance in-process, not on the bench. [S2]
Lead time is the other constraint. Grade 2 sheet and plate from stock distributors in standard thicknesses (0.5-25 mm) is available in 2-4 weeks; Grade 5 plate under 12 mm is similar; near-alpha and beta mill products, plus all forged shapes in small batches, push to 12-20 weeks from a mill. The U.S. mill base (TIMET, ATI, Howmet) and Russian (VSMPO-AVISMA) supply chain dominates; secondary processing and certification (AS9100 for aerospace, EN 10204 3.1/3.2 for industrial) adds 3-6 weeks on top. Buyers who do not pre-qualify a mill and lock in a heat-treatment condition pay the 12-20 week cycle twice [S3].
Standards Checklist Before You Sign the PO
Lock down the product form standard (B265 plate, B348 bar, B381 forging, B367 casting, B861 tube), the grade per ASTM or AMS, the heat-treatment condition (mill-annealed, solution-treated-and-aged, beta-annealed), the dimensional and tolerance callout (B265 Table 5 for plate thickness and width), the test report type (EN 10204 3.1 minimum, 3.2 for sour-service or aerospace), and the traceability requirement (heat number, lot, country of melt). For sour-service hydrogen sulfide exposure, NACE MR0175 governs the environmental cracking envelope; for medical, ASTM F136 (Ti-6Al-4V ELI) and F67 (CP) are the governing standards [S3].
The cheapest way to lose money on titanium is to treat the PO like a stainless-steel PO. It is not one. Density is 56% of steel, modulus is half, cost is 4-7×, lead time is 3-10×, and the formability / weldability / corrosion envelope is class-specific, not grade-specific. Pick the class first, the grade second, the product form third, and the mill fourth — and document the heat-treatment and tolerance bands in the PO. Everything else is negotiable; those four are not.