5 Titanium and Its Alloys

Titanium is a transition metal that is readily able to form solid solutions with elements whose atoms lie within about 20% of the size of the diameter of titanium atoms. Titanium itself has a high melting temperature, 1678 C, and exists in hexagonal close packed geometry, the so-called ? structure up to 882.5 C, and above this temperature it adopts a body centred cubic structure, the so-called ? structure.74 In alloys, titanium can exist in a variety of structures, based on either pure or near ? or ? forms, or mixtures of ? and ? forms. Alloying elements tend to fall into one of three categories, namely a-stabilisers, such aluminium, oxygen, nitrogen and carbon, ?-stabilisers, generally metals, such as vanadium, iron, nickel and cobalt, and neutral, such as zirconium. This latter group has no effect on the stability of the respective phases of titanium.

For biomedical applications, ? and near- ? alloys are preferred, because their corrosion resistance is superior to that of other types of titanium alloy. Variations in the processing conditions are used to control other aspects of titanium alloys, such as micro-structure, which in turn affects other properties, such as ductility, strength, fatigue resistance and fracture toughness. Table 4.5 gives examples of titanium alloys, their phase structures, and the resulting physical properties.