1992 Volume 32 Issue 12 Pages 1339-1347
Compression tests were performed on TiAl crystals containing unidirectionally aligned lamellae based on the function of temperature and the angle (φ) between the loading axis and the lamellar planes in order to determine the effect of lamellar structure on plastic behaviour. For specimens with φ=90°, the yield stress increased anomalously with increasing temperature and reached a maximum peak around 500°C. This phenomenon was particularly noticeable in the specimens containing fine and homogeneously distributed lamellae. Thin α2 plates in lamellae act as an effective barrier to the motion of dislocations, and the plastic behaviour of <1126>-type dislocations in the α2 plates may be responsible for the anomalous strengthening. In the cases of φ=45 and 0°, the weak temperature dependence of yield stress was mainly due to the mode of deformation in the γ matrix. Cracks nucleate often at the interface between the α2 plate and γ matrix and/or at thick α2 plate. Lack of activation of pyramidal slip in the α2 plate makes the accommodation of the concentrated stress at the interface more difficult. Addition of vanadium, which is thought to activate the pyramidal slip, was effective in improving the ductility.
