The Principle Of Titanium Alloys

Titanium alloys are alloys composed of titanium as a base and other elements. Titanium exists in two allotropic forms: α-titanium with a close-packed hexagonal structure below 882℃, and β-titanium with a body-centered cubic structure above 882℃.

Alloying elements can be classified into three categories based on their influence on the phase transformation temperature:

① Elements that stabilize the α phase and increase the phase transformation temperature are called α-stabilizing elements, including aluminum, carbon, oxygen, and nitrogen. Aluminum is the main alloying element in titanium alloys, significantly improving the alloy's strength at both room and high temperatures, reducing its specific gravity, and increasing its elastic modulus.

② Elements that stabilize the β phase and decrease the phase transformation temperature are called β-stabilizing elements, which can be further divided into isomorphous and eutectoid types. The former includes molybdenum, niobium, and vanadium; the latter includes chromium, manganese, copper, iron, and silicon.

③ Elements that have little effect on the phase transformation temperature are called neutral elements, including zirconium and tin.

Oxygen, nitrogen, carbon, and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have high solubility in the α phase, significantly strengthening titanium alloys but reducing their plasticity. The oxygen and nitrogen contents in titanium are typically specified to be below 0.15–0.2% and 0.04–0.05%, respectively. Hydrogen has very low solubility in the α phase; excessive hydrogen dissolution in titanium alloys will form hydrides, making the alloy brittle. The hydrogen content in titanium alloys is usually controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.