There are two kinds of homogeneous heterocrystals of alloying element titanium: titanium with a close-packed hexagonal α structure below 882 °C, and β titanium with body-centered cubic structure above 882 °C.
According to their effect on the phase transition temperature
Alloying elements can be divided into three categories based on their effect on the phase transition temperature:
(1) The elements that stabilize the α phase and increase the phase transition temperature are α stable elements, including aluminum, carbon, oxygen and nitrogen. Among them, aluminum is the main alloying element of titanium alloy, which has obvious effects on improving the strength of the alloy at room temperature and high temperature, reducing the specific gravity and increasing the elastic modulus.
(2) The elements that stabilize the β phase and reduce the phase transition temperature are β stable elements, which can be divided into two types: isomorphic and eutectic type. The former has molybdenum, niobium, vanadium, etc., and the latter has chromium, manganese, copper, iron, silicon, etc.
(3) The elements that have little influence on the phase transition temperature are neutral elements, such as zirconium and tin.
Oxygen, nitrogen, carbon, and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have a large solubility in the α phase, which has a significant strengthening effect on titanium alloys, but decreases the plasticity. Generally, the content of oxygen and nitrogen in titanium is specified to be below 0.15~0.2% and 0.04~0.05% respectively. Hydrogen has little solubility in the α phase, and dissolving too much hydrogen in titanium alloys will produce hydrides, which make the alloy brittle. Generally, the hydrogen content in titanium alloys is controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed with vacuum annealing.
According to the composition of the phase
Titanium alloys can be divided into three categories according to the composition of the phase: α alloys, (α+β) alloys and β alloys, which are represented by TA, TC and TB in China.
(1) The α alloy contains a certain amount of elements with stable α phases, and is mainly composed of α phases in the equilibrium state. α alloy has a small specific gravity, good thermal strength, good weldability and excellent corrosion resistance, and the disadvantage is that it has low strength at room temperature, and is usually used as a heat-resistant material and corrosion-resistant material. α alloys are generally divided into all-α alloys (TA7), near-α alloys (Ti-8Al-1Mo-1V), and α alloys with small compounds (Ti-2.5Cu).
(2) (α+β) The alloy contains a certain amount of elements of stable α phase and β phase, and the structure of the alloy in the equilibrium state is α phase and β phase. (α+β) alloy has medium strength and can be strengthened by heat treatment, but the welding performance is poor. (α+ β) alloys are widely used, among which the output of Ti-6Al-4V alloy accounts for more than half of all titanium materials.
(3) The β alloy contains a large number of elements that stabilize the β phase, which can retain all the high-temperature β phases to room temperature. β alloys are generally divided into heat-treatable β alloys (sub-stable β alloys and near-sub-stable β alloys) and heat-stable β alloys. Heat-treatable β alloy has excellent plasticity in the quenched state, and can make the tensile strength reach 130~140kgf/mm2 through aging treatment. β alloys are usually used as high-strength and high-toughness materials. The disadvantages are large proportion, high cost, poor welding performance, and difficult cutting processing.