Titanium metallography distinguishes alpha, alpha-beta, and beta alloy families by phase content, grain size, morphology, and distribution. This guide explains equiaxed, lamellar, acicular, and Widmanstatten alpha; transitional omega phase; and stable, metastable, and near-beta structures. It also shows how annealing above phase-transition temperatures, solution treatment, quenching, furnace or air cooling, and aging alter representative microstructures. Internationalized alloy names are applied consistently to the text and image placeholders while the approximate relationship for TC21 is retained explicitly. The result is a practical phase-identification reference for metallographic review.
Titanium and titanium alloys combine low density, high specific strength, corrosion resistance, elevated-temperature capability, nonmagnetic behavior, and nontoxicity. They are widely used in aerospace, marine engineering, petroleum, chemical processing, light industry, metallurgy, machinery, medicine, and energy.
Titanium alloys are commonly divided into alpha, alpha-beta, and beta alloys. Their basic phases are comparatively simple, but processing conditions can produce many morphologies and distributions as well as transitional and impurity phases.
Commercially pure titanium (CP titanium) belongs to the alpha family. Other alpha alloys generally contain about 6% aluminum plus small amounts of neutral elements and are almost entirely alpha after annealing. This collective classification includes CP titanium and alpha titanium alloys. Near-alpha alloys additionally contain no more than about 4% beta-stabilizing elements; examples include Ti-8Al-1Mo-1V (Ti-811) and Ti-4Al-2V.
Alpha titanium alloys consist almost entirely of alpha phase, although traces of beta may appear when iron or manganese is relatively high. Alpha occurs in two principal forms: equiaxed and lamellar or acicular. Working and annealing in the single-alpha region produces equiaxed alpha grains similar to those in CP titanium.
If annealing exceeds the alpha/alpha-beta transition, approximately 955°C, or the alpha-beta/beta transition, approximately 1040°C, high-temperature beta transforms during subsequent furnace or air cooling into lamellar or acicular alpha with a Widmanstatten character. Furnace cooling produces larger, rounded plates; air cooling produces finer, sharper needles. Compared with equiaxed alpha, these morphologies have little effect on tensile strength but reduce ductility.
Alpha-beta alloys contain aluminum at less than 6% by mass together with varying beta stabilizers and neutral elements. After annealing, they contain different proportions of alpha and beta. They generally weld well and can be strengthened by heat treatment, but their cold formability and cold-workability are poor. Examples include Ti-6Al-4V, Ti-6.5Al-3.5Mo-1.5Zr-0.3Si, and TC21 (approximately comparable to Ti-62222).
Ti-6Al-4V is the most widely used alpha-beta titanium alloy. It is commonly hot-worked and annealed, but it can also be quenched and aged. Its alpha-to-beta ratio and the morphology, size, and distribution of both phases are highly sensitive to hot-working conditions.
During quenching or isothermal treatment, alpha precipitation from beta may be accompanied by transitional omega phase, a nonequilibrium submicroscopic phase formed through nucleation and growth.
Beta alloys are subdivided into stable-beta, metastable-beta, and near-beta alloys. Important metallographic variables include beta-grain size, the amount and morphology of primary and secondary alpha, and alpha distribution along grain boundaries.
Stable-beta alloys contain enough beta stabilizers to suppress martensitic transformation during quenching to room temperature. Their annealed and quenched matrices are equiaxed beta, and high-temperature creep can form twins. They have relatively low room-temperature strength, good cold formability, and good corrosion resistance in reducing media.
Metastable-beta alloys contain more than the critical concentration of beta stabilizers, with a molybdenum equivalent of about 10%, no more than 3% aluminum, and small amounts of neutral elements. Solution treatment in the beta field leaves an almost entirely metastable-beta structure; aging precipitates alpha. These alloys combine good cold-workability with high aged strength. Representative compositions include Ti-5Mo-5V-8Cr-3Al, Ti-10Mo-8V-1Fe-3.5Al, and Ti-15V-3Cr-3Sn-3Al (Ti-15-3).
Near-beta alloys contain beta stabilizers near the critical concentration together with neutral elements and aluminum. Beta-field solution treatment produces abundant metastable beta and other metastable alpha or omega constituents; after aging, alpha and beta predominate. These alloys are well suited to forgings and provide a strong strength-toughness balance. Ti-10V-2Fe-3Al (Ti-10-2-3) is a representative alloy.
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