Like a number of other metals – e.g. Ca, Fe, Co, Zr, Sn, Ce, and Hf – titanium can crystallize in various crystal structures. However, each modification is only stable within particular temperature ranges. The complete transformation from one into another crystal structure is called allotropic transformation; the respective transformation temperature is called the transus temperature. Pure titanium, as well as the majority of titanium alloys, crystallizes at low temperatures in a modified ideally hexagonal close packed structure, called α-titanium. At high temperatures, the body-centered cubic structure is stable and is referred to as ᵦ titanium. The ᵦ -transus temperature for pure titanium is 882±28C. The atomic unit cells of the hexagonal close packed (hcp) α titanium and the body-centered cubic (bcc) ᵦ titanium are schematically shown in Fig. with their most densely packed planes and directions highlighted.
The existence of the two different crystal structures and the corresponding allotropic transformation temperature is of central importance since they are the basis for the large variety of properties achieved by titanium alloys. Both plastic deformation and diffusion rate are closely connected with the respective crystal structure. In addition, the hexagonal crystal lattice causes a distinctive anisotropy of mechanical behavior for the α-titanium. The elastic anisotropy is particularly pronounced. The Young’s modulus of titanium single crystals consistently varies between 145 GPa for a load vertical to the basal plane and only 100 GPa parallel to this plane.
The number of slip systems is determined by the number of slip planes multiplied by the number of slip directions.
The essential features of the three crystal structures pertinent to metals are summarized in Table below. The ease of plastic deformation increases from the hexagonal close packed (hcp) lattice to the body-centered cubic (bcc) to the face-centered cubic (fcc) lattice. This phenomenon also explains the limited plastic deformability of the hcp α-titanium compared to the bcc ᵦ titanium. Generally the number of slip systems which is equivalent to the number of dislocation glide opportunities in a crystal lattice is only 3 for the hcp structure while it is 12 for the bcc lattice. The number of slip systems is determined by the number of slip planes multiplied by the number of slip directions. These planes and directions of highly dense packed atoms are energetically most favorable for plastic deformation.