Alpha Titanium Alloys: Structure, Properties, Heat Treatment

Alpha titanium alloys contain mainly alpha phase and are classified as fully alpha or near-alpha. With little or no beta phase, they cannot be strengthened by conventional heat treatment; solid-solution and grain refinement are the main strengthening mechanisms. Al, O, Zr, Sn, and Si are important additions. Aluminum is generally kept below 6% to limit brittle ordered alpha2 (Ti3Al), oxygen provides strong interstitial strengthening but reduces ductility, and near-alpha alloys use silicon to improve creep resistance.

Commercially Pure Titanium

Commercially Pure Titanium (CP Titanium) is commonly organized internationally as ASTM Grades 1–4, but these are presented only as a general framework rather than one-to-one replacements for Chinese grades.

Fully Alpha Microstructures

Fully alpha alloys offer weldability, corrosion resistance, ductility, notch toughness, and a ductile-to-brittle transition temperature as low as −253°C. Equiaxed grains result from alpha-beta working followed by recrystallization annealing. Strength follows grain-size refinement; for the lowest-oxygen CP grade, the source relationship is yield strength = 231 MPa + 10.5d^−1/2. Rapid cooling from beta produces acicular or lath alpha-prime martensite, while slow cooling produces coarse lamellar colonies. Lamellar structures trade lower tensile and fatigue strength and ductility for better fracture toughness and creep resistance.

Ti-5Al-2.5Sn combines high cryogenic strength and toughness and is used for liquid-nitrogen storage near −253°C.

Near-Alpha Alloys

Near-alpha alloys provide creep and oxidation resistance and strength up to about 600°C for gas-turbine compressor service. Examples include Ti-1100, IMI 834, and BT18y. Small Mo, Nb, or V additions improve forgeability through limited beta phase; Zr and Sn strengthen alpha, while Si, C, and rare-earth additions can improve creep.

Forging and heat treatment in the alpha-beta field can produce a bimodal structure of primary equiaxed alpha plus transformed beta containing secondary lamellar alpha. This structure balances creep, fracture toughness, and fatigue.

Faster cooling after beta solution treatment creates thinner alpha laths, higher strength, and lower ductility. Increasing beta content may improve creep in IMI 834 but reduce fatigue performance. Aging or long exposure can precipitate ordered alpha2 and Ti5Si3 silicides, increasing room- and high-temperature strength while reducing ductility.