Slip and twinning in TiAl PST crystals

Deformation structures were examined in Ti52Al48 PST single crystals orientated for in-plane shear (soft orientation). It was found that all twins are ordinary twins. No super-twins were observed. It was also found that out-of-plane deformation (slip and twinning) plays an important role in the deformation of these samples. Furthermore, the critical resolved shear stress for ordinary slip is slightly lower than that for ordinary twinning in our crystals. Since a combination of in-plane ordinary twinning and in-plane ordinary dislocation slip cannot produce compatible strains in all domains of a given layer in the PST structure, we suspect that the out-of-plane strains are necessary to relieve the incompatibilities which are not relieved by in-plane super-dislocations.