ASTM B977/B977M-19 covers eight named titanium and titanium alloy ingot grades, with Grade 5 (UNS R56400, Ti-6Al-4V with 6 % aluminium, 4 % vanadium) the dominant aerospace workhorse and Grade 2 (UNS R50400) the commercial-purity default for chemical processing [S2].
Beyond the ASTM grade list, titanium alloys are sorted into three microstructure families — α, α+β and β — defined by the phase stabilisers present and by the β-transus behaviour of the alloy [S4]. For a parallel reference on ferrous equivalents, see the alloy steel classification entry.
ASTM B977 Ingot Grades: The Eight-Line Reference
ASTM B977/B977M-19 names eight explicit composition slots: Grade 1 UNS R50250, Grade 2 UNS R50400, Grade 3 UNS R50550, Grade 4 UNS R50700, Grade 5 UNS R56400 (6 % Al, 4 % V), Grade 6 UNS R54520 (5 % Al, 2.5 % Sn), Grade 7 UNS R52400 (CP plus 0.12–0.25 % Pd), and Grade 9 UNS R56320 [S2]. Grades 1–4 and 7 are unalloyed or Pd-enhanced commercial-purity (CP) titanium; grades 5, 6 and 9 are α+β or near-α alloys in the standard's listing. SAE AMS2486F-2024 is the companion surface-treatment standard tracked by the same standardisation body [S1].
CP Grades 1–4 differ primarily in interstitial oxygen and iron content, which drives a monotonic rise in tensile strength from roughly 240 MPa (Grade 1) toward 550 MPa (Grade 4); Grade 2 sits near 345 MPa minimum UTS and is the workhorse for pressure transmitter diaphragms and chemical-process anodes. Per ASTM B977/B977M-19, Grade 7 (UNS R52400) is unalloyed titanium with a 0.12 to 0.25 % palladium addition.
The Three Microstructure Families: α, α+β, β
α titanium is a single-phase hexagonal close-packed (HCP) structure stable to roughly 880 °C; α-stabilisers are Al, O, N and Sn, with Grade 5 (Ti-6Al-4V) and Grade 6 (Ti-6Al-2.5Sn) sitting at the alloyed end of the family [S4]. α alloys cannot be heat-treated for strength gain — properties come from solid-solution strengthening — but they weld well, retain creep strength above 300 °C, and are non-hardenable in the martensitic sense.
β titanium stabilises the body-centred cubic (BCC) phase at room temperature through additions of V, Mo, Nb, Fe and Cr; β alloys are heat-treatable, reach yield strengths above 1100 MPa in the solution-treated-and-aged (STA) condition, and are the spec choice for landing-gear forgings and spring elements [S4]. α+β alloys (Grade 5 Ti-6Al-4V the canonical example) retain some β phase at room temperature, are heat-treatable, balance formability against strength, and represent the bulk of aerospace mill product by tonnage. The phase-balance split is the basis for the comparison table below.
Criteria-Based Comparison of α, α+β and β Families
Three decision criteria separate the families in spec work: heat-treat response, weldability, and section-size capability. α alloys are non-heat-treatable, weldable, and limited to thin sections and moderate strength; α+β alloys (Ti-6Al-4V in particular) are heat-treatable, weldable with procedure qualification, and produced in billet, plate, and forging up to roughly 150 mm cross-section. β alloys offer the highest strength-to-weight ratio, are heat-treatable to 1100+ MPa yield, but carry restricted weldability and require inert-atmosphere processing to avoid α-case contamination. [S1]
For corrosion-led selection, Pd-bearing Grade 7 outperforms CP grades in reducing environments, while Grade 5 (Ti-6Al-4V) remains the default for fatigue-loaded aerospace structure where strength-to-weight is the governing metric [S2]. The titanium alloy reference page consolidates these criteria in one matrix for spec writers.
Standardisation, Designations and Traceability
Two designation systems coexist: the ASTM/UNS letter-number scheme (R50250 through R56400 in B977) and the aerospace TA-/TB-/TC- legacy nomenclature where TA = α, TB = β, TC = α+β. UNS traceability is mandatory for ASME Section VIII and NACE MR0175 sour-service submissions; the Pd-bearing Grade 7 carries an explicit NACE MR0175 listing for downhole tubular components in H2S-containing brines. [S2]
AMS2486F-2024 governs anodic film production for titanium fasteners and honeycomb-facing surfaces — the surface-treatment standard that closes the loop between B977 ingot chemistry and finished-part performance [S1]. For biomaterial extensions, ISO 5832-2/3 specify Grade 1–4 CP and Ti-6Al-4V bar for surgical implants, and Daido Steel's medical-titanium line lists biocompatibility, non-magnetic MRI compatibility, and a density of 4.5 g/cm³ — half that of nickel-based superalloys referenced on the nickel alloy page [S5].
Failure Modes and Selection Triggers to Avoid
α-case formation in β alloys above ~700 °C in oxygen-bearing atmospheres produces a brittle oxygen-diffused surface layer; mitigation is vacuum or inert-atmosphere heat treatment, or chemical milling of the affected case. Hydrogen embrittlement at sustained loading above 300 °C in moist service dictates a max operating ceiling of roughly 315 °C for CP grades in pressure-vessel service. [S3]
Galvanic coupling against aluminium alloys and carbon steel in seawater stacks cathodically; titanium drives current into the coupled metal, accelerating its corrosion — designers must isolate titanium hardware with nylon or EPDM washers. In hot HCl above 3 % concentration, even Grade 7 requires a chloride-stabilised inhibitor package; CP Grades 1–4 will pit. For complementary ferrous options where galvanic risk dominates, the cast iron advantages and disadvantages reference covers density and damping trade-offs against titanium.
Application Map: Where Each Family Fits
Grade 2 (CP) owns chemical-processing anodes, condenser tubes, and instrument diaphragms; Grade 5 (Ti-6Al-4V) is specified for fan blades, compressor discs, and structural forgings; Grade 9 (Ti-3Al-2.5V) is the tubing choice for hydraulic lines in 300 °C aircraft systems; Grade 23 (Ti-6Al-4V ELI) extends Grade 5 with extra-low interstitials for fracture-critical aerospace and surgical implants. β alloys (Ti-10V-2Fe-3Al, Ti-15V-3Cr-3Al-3Sn) appear in landing-gear forgings and high-strength fasteners where 1100+ MPa yield offsets cost penalty [S2].
Aluminium alloy substitution is common when stiffness-to-weight dominates over temperature; see the aluminum alloy page for the comparable density-driven selection logic. The adjacent silicon steel TCO reference is the right cross-link for designers weighing electrical-steel cores against titanium structural frames in traction-motor assemblies.
Procurement Signals Worth Tracking
Watch for ASTM B977 revision ballots and for AMS2486 surface-treatment addenda that tighten anodic-film thickness tolerances for aerospace fastener lots. B977M-19's UNS R-numbers are stable and remain the most reliable cross-reference for sourcing across ASTM, ASME and NACE documentation trails [S2].