Intermetallic Titanium (y-Titanium Aluminides)

An intermetallic compound does not crystallise in the lattice type of one of its two components. It has its own lattice structure typical for the compound. The quanti­tative ratio of the atoms of both substances corresponds to their amounts in the al­loy composition [BUER01]. The Ti-Al system also forms stable intermetallic compounds [ROMM97]. The phase diagram of the two-material system is shown in Fig. 4-14 [KUMP02, MASS90]. Materials based on these compounds are des­ignated as titanium aluminides. The characteristics of so-called y-titanium alu­minides are derived from the properties of the intermetallic a2-(Ti3Al), packed with maximum hexagonal density, and the y-(TiAl) phase, from both of which the material is composed. Both of these phases can be found in varying phase frac­tions and microstructures, such as duplex or fully lamellar structures.

The duplex structure consists of globular у-grains, a2-particles and lamellar y/a2-grains with approximately equal volumetric portions, Fig. 4-15, right [WANG00]. It is the finest-grained microstructure of TiAl-based alloys and exhib­its good space-temperature ductility [KUMP96, ROMM97, YAMA93]. The duc­tility is dependent on structural homogeneity, but it can also be influenced by fur­ther alloying elements or the amount of impurities [HUAN91, HUAN94, KIM89, KIM91, KIM94, ROMM97, YAMA93].

Fully lamellar (FL) structures (Fig. 4-15, left) consist of parallel-arranged la­mellae of both the tetragonal TiAl phase and the Ti3Al phase, which is packed with maximum hexagonal density [WANG00]. The width of the lamellae varies according to the heat treatment used and further alloying additives between 0.15 und 3 pm, whereby the width of the a2-lamellae is, as a rule, significantly less than that of the y-lamellae [BERG95, ROMM97]. The boundary surfaces of two grains can in this case form a “hooked” structure by means of a “meshing mechanism”, which has a positive effect on creep strength but a generally negative one on mate­rial machinability [WANG99]. These structural types have higher fracture tough­ness, improves high temperature strength properties and a higher endurance limit than duplex structures.