Titanium alloy is an alloy based on titanium with other elements added. Titanium has two homogeneous heterocrystals: α titanium with tight hexagonal structure below 882℃ and β titanium with body-centered cubic structure above 882℃. The alloying elements can be divided into three categories according to their effects on the phase transition temperature: 1. The elements that stabilize the α phase and increase the phase transition temperature are the α-stable elements, including aluminum, carbon, oxygen and nitrogen. Among them, aluminum is the main alloy element of titanium alloy. It can improve the strength at room temperature and high temperature, reduce the specific gravity and increase the elastic modulus of the alloy. (2) β stable phase and phase transition temperature drop are β stable elements, which can be divided into isomorphic and eutectoid. The former includes molybdenum, niobium and vanadium, while the latter includes chromium, manganese, copper, iron and silicon. (3) Neutral elements such as zirconium and tin have little effect on the phase transition temperature.
Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloys. The high solubility of oxygen and nitrogen in the α phase strengthens titanium alloy obviously, but reduces its plasticity. It is generally prescribed that the oxygen and nitrogen content in titanium should be lower than 0.15-0.2% and 0.04-0.05%, respectively. The solubility of hydrogen in the alpha phase is very small. Excess hydrogen dissolved in titanium creates hydrides, which make the alloy brittle. Typically, the hydrogen content of titanium alloys is kept below 0.015%. The dissolution of hydrogen in titanium is reversible.
Titanium alloy can be divided into heat resistant alloy, high strength alloy, corrosion resistant alloy (Ti-Mo, Ti-Pd alloy, etc.), low temperature alloy and special function alloy (Ti-Fe hydrogen storage material and Ti-Ni memory alloy). The composition and properties of typical alloys are listed in the table.
Different phase compositions and structures can be obtained by adjusting the heat treatment process. It is generally believed that fine equiaxed structures have better plasticity, thermal stability and fatigue strength. The acicular structure has high endurance strength, creep strength and fracture toughness. Equiaxed acicular hybrid structure has better comprehensive properties.
