Dr. Dmitri Kopeliovich

There are two crystallographic forms of titanium:

• α-titanium, in which atoms are arranged in Hexagonal Closest Packing (HCP) crystal lattice;

• β-titanium, in which atoms are arranged in Cubic Body Centered (BCC)crystal lattice;

Pure titanium exists in form of α-phase at temperatures above 1621°F (883°C) and in form of β-phase at temperatures below 1621°F (883°C).

The temperature of allotropic transformation of α-titanium to β-titanium is called Beta Transus Temperature.

Alloying elements in titanium alloys may stabilize either α-phase or β-phase of the alloy.

Aluminum (Al), gallium (Ga), Nitrogen (N), Oxygen (O) stabilize α-phase.

Molybdenum (Mo), vanadium (V), tungsten (W), tantalum (Ta), silicon (Si) stabilize β-phase.

Titanium alloys are classified into four groups according to their phase composition:

• Commercially pure and low alloyed titanium alloys

Commercially pure titanium consists of grains of α-phase and dispersed spheroid particles of β-phase. Small amounts of iron, present in the alloys, stabilize β-phase.

Commercially pure titanium has relatively low mechanical strength and good corrosion resistance.

• Titanium alpha and near-alpha alloys

α-alloys consist entirely of α-phase. They contain aluminum as the major alloying element, stabilizing α-phase.

α-alloys have good Fracture Toughness and Creep resistance combined with moderate mechanical strength, which is retained at increased temperatures.

They are easily welded, but their workability in hot state is poor.

Near α-alloys contain small amount of ductile β-phase. Besides α-phase stabilizer (aluminum), near α-alloys are alloyed by 1-2% of β-phase stabilizers (molybdenum, silicon).

Mechanical properties of near α-alloys are similar to those of α-alloys, however due to the presence of β-phase these alloys may be heat-treatable and are forged in hot state.

• Titanium alpha-beta alloys

α-β alloys contain 4-6% of β-phase stabilizers; therefore they consist of a mixture of α and β phases.

α-β alloys are heat-treatable. They have high mechanical strength and good hot formability.

Creep resistance of the alloys is lower, than that of α- and near α-alloys.

• Titanium beta alloys

β-alloys are rich of β-phase. They contain substantial amount of β-phase stabilizers, preventing β-α transformation at high cooling rates of quenching.

β–alloys are heat-treatable to very high strength and have good hot formability.

Ductility and fatigue strength of the alloys in heat-treated conditions are low.

Titanium alloys are designated according to their compositions:

Ti-5Al-2.5Sn identifies titanium alloy, containing 5% of aluminum and 2.5% of tin.

Ti-6Al-4V identifies titanium alloy, containing 6% of aluminum and 4% of vanadium.

In parallel to this designation system other systems for designation titanium alloys exist (ASTM, IMI, military system).

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• Commercially pure and low alloyed titanium alloys
• Titanium alpha and near-alpha alloys
• Titanium alpha-beta alloys
• Titanium beta alloys

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