Structure and Deformation of Al-Rich TiAl Single Crystals

Abstract

TiAl possesses a wide composition range extending primarily on the Al-rich side and its mechanical properties strongly depend on aluminum content. The TiAl phase in equilibrium with the Ti3Al phase in TiAl/Ti3Al two-phase alloys has been found to be highly deformable [1–3]. However, the tensile ductility of the TiAl phase is quickly reduced to zero when aluminum content exceeds 53 at % Al [4]. Since the TiAl/(TiAl+Ti3Al) phase boundary is believed to be located at about 50 at% Al at room temperature, the drastic change in deformability occurs in a narrow composition range from 50 to 53 at% Al. To clarify the deformation behavior and mechanisms of Al-rich single-phase TiAl alloys, some basic studies have been done using single crystals [5–13]. In Al-rich alloys, ordinary slip on 111<110], superlattice slip on {111]<101] and twinning on 111<112] can be operative depending on crystal orientation, similarly to the TiAl phase in TiAl/Ti3Al two-phase alloys. Some other slip systems can be operative, but they are observed only for very limited orientations and at temperatures higher than 1000°C. In view of the major deformation modes, no significant difference exists between the TiAl phase in two-phase TiAl alloys and the Al-rich TiAl phase. However, a remarkable difference exists in the relative ease of the three major deformation modes between the two TiAl phases with different Al contents. The relative ease of the three deformation modes in the former TiAl phase is given as $$T_{W} > S_{O} > S_{S}\;\;\;\;(1)$$ while in the latter TiAl phase it is given as $$S_{S} >> S_{O} >>> T_{W}\;\;\;\;(2)$$ , where Tw, So and Ss are the ease of twinning, ordinary slip and superlattice slip, and >, ≫ and ⋙ respectively indicate a small, a medium and a large difference in ease between the corresponding two deformation modes [3,12].