Plastic Behaviour of TiAl Crystals Containing a Single Set of Lamellae at High Temperatures

Abstract

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 a2 plates in lamellae act as an effective barrier to the motion of dislocations, and the plastic behaviour of 1126-type dislocations in the a2 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 y matrix. Cracks nucleate often at the interface between the a2 plate and y matrix and/or at thick a2 plate. Lack of activation of pyramidal slip in the a2 plate makes the accommodation of the concentrated stress at the interface more difficult. Addition of vanadium, which is thought to activate thepyramidal slip, was effective in improving the ductility. © 1992, The Iron and Steel Institute of Japan. All rights reserved.