Principles of Titanium Alloys

Titanium alloy is an alloy based on titanium and composed of other elements. Titanium has two types of isomorphic crystals: closely packed hexagonal structure below 882 ℃ α Titanium, body centered cubic above 882 ℃ β Titanium.
Alloy elements can be divided into three categories based on their impact on phase transition temperature:
① Stable α The elements that increase the phase transition temperature are α Stable elements include aluminum, carbon, oxygen, and nitrogen. Aluminum is the main alloying element in titanium alloys, which has a significant effect on improving the strength of the alloy at room and high temperatures, reducing the specific gravity, and increasing the elastic modulus.
② Stable β The elements that reduce the phase transition temperature are β Stable elements can be divided into two types: isomorphic and eutectoid. The former includes molybdenum, niobium, vanadium, etc; The latter includes chromium, manganese, copper, iron, silicon, etc.
③ The elements that have little effect 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 in α There is a high solubility in the phase, which has a significant strengthening effect on titanium alloys, but it reduces plasticity. The content of oxygen and nitrogen in titanium is usually specified to be below 0.15-0.2% and 0.04-0.05%, respectively. Hydrogen in α The solubility in the phase is very low, and excessive hydrogen dissolved in titanium alloys can produce hydrides, 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.