Titanium and Titanium Alloys
Titanium is an important metallic element with the atomic number 22 in the periodic table (4th period, group VB), possessing a silvery-white metallic luster, a density of 4.507g/cm³, a melting point of 1660°C, and a boiling point of 3287°C. It has several excellent properties including a high melting point, low density, high strength, and good ductility, making it an important industrial material.
Titanium alloys are new types of alloys prepared based on titanium, with the addition of one or more other elements. Titanium is highly reactive, and it can interact with most elements to form continuous solid solutions, limited solid solutions, metallic compounds, covalent compounds, and ionic compounds. Alloying elements can optimize the properties of titanium by changing its polymorphic transformation points and phase compositions.
Titanium has two allotropes, namely the α-phase and the β-phase, with a polymorphic transformation point of about 882°C. When the temperature is below the transformation temperature, titanium has a hexagonal close-packed structure, known as α (α-Ti); when the temperature is above the transformation temperature, titanium has a body-centered cubic structure, known as β titanium (β-Ti).
Different Structures of Titanium Alloys
Titanium alloys have two basic phases, α-phase and β-phase, and the properties of titanium alloys largely depend on the properties of the α-phase and β-phase themselves, as well as their morphology, size, distribution, and proportion in the alloy. The strength of the β-phase is higher than that of the α-phase, and it has more slip systems, making it easier to undergo plastic deformation. High-strength titanium alloys are usually based on the β-phase. The heat resistance and creep resistance of the α-phase are better than those of the β-phase, and high-temperature titanium alloys are usually α-alloys and near-α alloys.
The basic microstructure of titanium alloys is composed of α-Ti-based α-solid solutions and β-Ti-based β-solid solutions. The matrix of α-alloys, near-α alloys, and many α+β alloys is α-solid solution, while the matrix of β-alloys is the β-phase. The microstructure of titanium alloys mainly depends on the alloy composition, deformation process, and heat treatment, with typical structures being lamellar (Widmanstätten) structure, basketweave structure, equiaxed structure, and duplex structure.
