High purity titanium is a kind of metal with low strength and high plasticity. Titanium with the highest purity is prepared by iodide method (i.e. by heating in vacuum and dissociating Ti). Titanium prepared by iodide method contains 0.05% impurities, mainly some metal impurities. Its ultimate strength is C-215-255 MN/m3; yield point is 0.2=120-170MN/m3; elongation is 50-60%; section shrinkage is 70-80%; Brinell hardness is HB1275 MN/m3, impact toughness is more than 250 Joule/cm 3. The elastic properties of titanium prepared by iodide method are as follows: volumetric elastic modulus = MN/m3; standard elastic modulus or Young's modulus 5 = 10.6x10*MN/m2 (= 40xl0s MN/m2); Poisson's coefficient 0.84U33.

Titanium is a transitional element in the group IV subgroup B of Mendeleev periodic table. Titanium has two allotropic forms: Titanium a, which has a dense hexagonal lattice below the polymorphic transformation temperature (882.5 degree C), and cold Titanium, which has a body-centered cubic lattice when the temperature is below the polymorphic transformation temperature but below the melting point. Titanium has a density of 4.51 g/cm 3 at 25 C and cold titanium has a density of 4.32 g/cm 3 at 900 C. The lattice constants of a titanium are as follows: a = 0.2950 nanomicron; C = 0.4683 nanomicron; c/a = 1.587; a-0.3306 nanomicron of cold titanium at 900 C; and a = 0.3282 nanomicron at 25 0. In terms of density, titanium lies between aluminium and iron, and in terms of its distribution in the earth's crust, it ranks fourth among the metals with the largest reserves (next only to aluminium, iron and magnesium). However, titanium iodide has not been used in industry because of its low strength. Industrial pure titanium contains a small amount of impurities such as iron, silicon, hydrogen, oxygen and nitrogen. The presence of these elements, even in small quantities, can greatly increase the strength of titanium and significantly reduce its plasticity at room temperature.

Titanium alloys, the elements involved in the formation of alloys, are classified into three categories according to their effects on the polycrystalline phenomena of titanium:

1. Elements that can improve phase stability. Among various metals, aluminium is a phase stabilizer, which is contained in almost all industrial titanium alloys.

2. Element a, which can improve the phase stability, can be further divided into two sub-categories. Chromium, manganese, iron, nickel, lead, beryllium and cobalt are in the first subgroup. When they are alloyed with titanium, the heterogeneous eutectic decomposition occurs at a relatively low overflow. These elements are called phase-inhomogeneous eutectic stabilizers. Vanadium, molybdenum, niobium, tantalum and tungsten are cautioned in the second category. When they are alloyed with titanium, the phase solution can be maintained at room temperature. These elements are called isomorphic stabilizers of phases.

3. Elements like tin and zirconium which have little effect on stability belong to the third category. Heat treatment does not strengthen titanium alloys. The valuable properties of this alloy lie in its good weldability.