Titanium alloy classification and comparison of common titanium and titanium alloy grades

In China, the naming scheme of titanium alloy materials is usually separated in three categories:

·         α-type titanium alloys (including a nearly α-type alloy) are designated with TA

·         β-type titanium alloys (including a nearly β-type alloy) are designated with TB

·         α+ β-type titanium alloys are designated with TC

① The elements that stabilize α phase and increase phase transition temperature are α-stable elements, such as aluminum, carbon, oxygen, and nitrogen. Aluminum is the main alloying element of titanium alloys, which has a noticeable effect on improving the strength of the titanium alloy at room temperatures and high temperatures, reducing specific gravity, and increasing elastic modulus.

② The elements that stabilize the β phase and reduce the phase transition temperatures are categorized as β-stable elements, which can be divided into the isomorphous type and eutectoid type. The isomorphous types contain molybdenum, niobium, vanadium etc., while eutectoid types contain elements like chromium, manganese, copper, iron, silicon, etc.

③ Elements with little effect on the temperature of phase transition are categorized as neutral alloying elements, such as zirconium and tin.

Common titanium alloy grades are compared with UNS codes as following

Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have high solubility in α phase, which can strengthen the titanium alloy significantly, but decreases the plasticity. The content of oxygen and nitrogen in titanium is generally defined to be less than 0.15 ~ 0.2% and 0.04 ~ 0.05%, respectively.

The solubility of hydrogen in the α phase is very small, and too much hydrogen dissolved in titanium alloys will produce hydride, making the alloy brittle. Usually, the hydrogen content in titanium alloys is controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.

American standard Grades 1~38 compared to UNS code

The most common used titanium alloy composition is Ti-6Al-4V, which according to the American standard is Grade 5 and according to the Chinese designation TC4. In 1994 the National standard GB/T3620.1-2007 was updated and removed 2 titanium grades while adding 54 new grades. The new total number of titanium and titanium alloy grades is 76.

After the material development of the last three "five-year plans", a new generation of aircraft backbone titanium alloy material with Chinese developed characteristics has begun to take shape. The middle strength and high damage tolerance titanium alloy TC4-DT independently developed in China has the same nominal composition as TC4, but the oxygen content is reduced and the fracture toughness is improved.

New materials such as Ti45NB (wire material), TA18 (pipe material), TB8 (sheet material, wire material, forgings) and TC21 (forgings) have also been well used. By combining the existing TC1/TC2 (GR1 / GR2 plate), TC4(GR5) (forgings, plates, wires) and ZTC4 cast titanium alloy, a complete set of material which are of low strength and high plasticity, medium strength and high plasticity, high strength and high plasticity, ultra-high strength titanium alloy and cast titanium alloy has been formed.