Effect of strain rate on twinning and room temperature ductility of TiAl with fine equiaxed microstructure

The aim of the present work is to study the effect of strain rate on the twinning development and the ductility of stoichiometric gamma -titanium aluminide with fine equiaxed microstructure which provides a transition from planar slip to homogeneous one and makes the greatest ductility available. With varying the strain rate, the TiAl ductility is determined by the intensity of the twinning developemnt. At low strain rates a large number of grains (approx50% at epsilon =3.3x10 exp -4 s exp -1 ) is not twinned that, according to Mizes criterion (taking into account the limited number of slip systems in TiAl), leads to disturbance of the compatibility of deformation between the grains and to low ductility. On the contrary, at high strain rates the twinning intensity is found to be excessive. As a consequence even at elongation of delta =2.6% the effective grain size becomes insufficient for further twinning. This worsens the compatibility of deformation between the fragments and restricts the ductility of TiAl. Meanwhile, at the optimum strain-rate interval, the twinning in TiAl apparently develops by the most favorable way. On the one hand, it supports the compatibility of deformation between the grains and, consequently, the slip development. On the other hand, due to above mentioned features (development of twinning predominantly by one system) "self-stopping" of twinning occurs not so fast as it takes place at high strain rates. As a result the elongation of delta =4-6.9%, rather high for polycrystals based on gamma -TiAl, is reached in stoichiometric TiAl with d=8 mu m at epsilon =3.3x10 exp -2 -8.3x10 exp -1 s exp -1 . Thus, the twinning development during deformation determines to a great extent the room temperature ductility in polycrystalline gamma -TiAl. It is caused by the main feature of the room temperature deformation of the intermetallic, namely, by limited number of slip systems. As follows from the present work, in order to obtain the maximum ductility it is important that the twinning must embrace all the grains but develop only by one system and proceed during the deformation process up to failure. Maintenance of this condition may be the key to ductility improvement in polycrystalline alloys based on gamma -TiAl. High ductility of PCT-crystals oriented favorable to the twinning development, points to this too.