Fracture toughness of a lamellar orientation-controlled TiAl-based alloy processed by either one-step or two-step compression at high temperature

In this work, the effects of lamellae orientation and the distribution of fine grains and fine lamellar colonies on the fracture toughness of orientation-controlled Ti-43 mol%Al alloy were examined. Fracture toughness was determined by three-point bending tests following the introduction of a chevron notch, and thermomechanical processing to control lamellar orientation consisted of high temperature compression in the α single-phase and (α + γ) two-phase regions. Materials processed via one-step compression in the α single-phase region exhibited tilting of the lamellae interface by approximately 20-30° relative to the compression plane. Further compression in the (α + γ) two-phase region (termed two-step compression) arranged the lamellae interface nearly parallel to the compression plane. Fine grains were also observed around the orientation-controlled lamellar colonies, and were converted into fine lamellar colonies during heat treatment following processing in the (α + γ) two-phase region. The proportion of fine grains decreased with an increase in the true strain rate during compression in the (α + γ) two-phase region. Specimens processed by one-step compression had fracture toughness values lower than those of two-step compression specimens. In addition, specimens processed using two-step compression and exhibiting a crack arrester orientation had higher fracture toughness values than those with a crack divider orientation. The fine grains and colonies formed at the lamellar colony boundaries were found not to affect the fracture toughness of specimens with either of these crack orientations. However, decreased quantities of fine grains and colonies may increase the toughness of specimens for which crack propagation occurs in conjunction with crack arrester orientation.